/**
 * @experimental
 * @since 3.18.0
 */
import * as Data from "./Data.js";
import * as Equal from "./Equal.js";
import { dual } from "./Function.js";
import * as Hash from "./Hash.js";
import { format, NodeInspectSymbol } from "./Inspectable.js";
import * as Option from "./Option.js";
import { pipeArguments } from "./Pipeable.js";
/**
 * Safely get a value from a Map, returning an Option.
 * Uses explicit key presence check with map.has() for better safety.
 * @internal
 */
const getMapSafe = (map, key) => {
  if (map.has(key)) {
    return Option.some(map.get(key));
  }
  return Option.none();
};
/**
 * Unique identifier for Graph instances.
 *
 * @since 3.18.0
 * @category symbol
 */
export const TypeId = "~effect/Graph";
/**
 * Edge data containing source, target, and user data.
 *
 * @since 3.18.0
 * @category models
 */
export class Edge extends Data.Class {}
// =============================================================================
// Proto Objects
// =============================================================================
/** @internal */
const ProtoGraph = {
  [TypeId]: TypeId,
  [Symbol.iterator]() {
    return this.nodes[Symbol.iterator]();
  },
  [NodeInspectSymbol]() {
    return this.toJSON();
  },
  [Equal.symbol](that) {
    if (isGraph(that)) {
      if (this.nodes.size !== that.nodes.size || this.edges.size !== that.edges.size || this.type !== that.type) {
        return false;
      }
      // Compare nodes
      for (const [nodeIndex, nodeData] of this.nodes) {
        if (!that.nodes.has(nodeIndex)) {
          return false;
        }
        const otherNodeData = that.nodes.get(nodeIndex);
        if (!Equal.equals(nodeData, otherNodeData)) {
          return false;
        }
      }
      // Compare edges
      for (const [edgeIndex, edgeData] of this.edges) {
        if (!that.edges.has(edgeIndex)) {
          return false;
        }
        const otherEdge = that.edges.get(edgeIndex);
        if (!Equal.equals(edgeData, otherEdge)) {
          return false;
        }
      }
      return true;
    }
    return false;
  },
  [Hash.symbol]() {
    let hash = Hash.string("Graph");
    hash = hash ^ Hash.string(this.type);
    hash = hash ^ Hash.number(this.nodes.size);
    hash = hash ^ Hash.number(this.edges.size);
    for (const [nodeIndex, nodeData] of this.nodes) {
      hash = hash ^ Hash.hash(nodeIndex) + Hash.hash(nodeData);
    }
    for (const [edgeIndex, edgeData] of this.edges) {
      hash = hash ^ Hash.hash(edgeIndex) + Hash.hash(edgeData);
    }
    return hash;
  },
  toJSON() {
    return {
      _id: "Graph",
      nodeCount: this.nodes.size,
      edgeCount: this.edges.size,
      type: this.type
    };
  },
  toString() {
    return format(this);
  },
  pipe() {
    return pipeArguments(this, arguments);
  }
};
// =============================================================================
// Constructors
// =============================================================================
/** @internal */
export const isGraph = u => typeof u === "object" && u !== null && TypeId in u;
/**
 * Creates a directed graph, optionally with initial mutations.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * // Directed graph with initial nodes and edges
 * const graph = Graph.directed<string, string>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   const c = Graph.addNode(mutable, "C")
 *   Graph.addEdge(mutable, a, b, "A->B")
 *   Graph.addEdge(mutable, b, c, "B->C")
 * })
 * ```
 *
 * @since 3.18.0
 * @category constructors
 */
export const directed = mutate => {
  const graph = Object.create(ProtoGraph);
  graph.type = "directed";
  graph.nodes = new Map();
  graph.edges = new Map();
  graph.adjacency = new Map();
  graph.reverseAdjacency = new Map();
  graph.nextNodeIndex = 0;
  graph.nextEdgeIndex = 0;
  graph.isAcyclic = Option.some(true);
  graph.mutable = false;
  if (mutate) {
    const mutable = beginMutation(graph);
    mutate(mutable);
    return endMutation(mutable);
  }
  return graph;
};
/**
 * Creates an undirected graph, optionally with initial mutations.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * // Undirected graph with initial nodes and edges
 * const graph = Graph.undirected<string, string>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   const c = Graph.addNode(mutable, "C")
 *   Graph.addEdge(mutable, a, b, "A-B")
 *   Graph.addEdge(mutable, b, c, "B-C")
 * })
 * ```
 *
 * @since 3.18.0
 * @category constructors
 */
export const undirected = mutate => {
  const graph = Object.create(ProtoGraph);
  graph.type = "undirected";
  graph.nodes = new Map();
  graph.edges = new Map();
  graph.adjacency = new Map();
  graph.reverseAdjacency = new Map();
  graph.nextNodeIndex = 0;
  graph.nextEdgeIndex = 0;
  graph.isAcyclic = Option.some(true);
  graph.mutable = false;
  if (mutate) {
    const mutable = beginMutation(graph);
    mutate(mutable);
    return endMutation(mutable);
  }
  return graph;
};
// =============================================================================
// Scoped Mutable API
// =============================================================================
/**
 * Creates a mutable scope for safe graph mutations by copying the data structure.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>()
 * const mutable = Graph.beginMutation(graph)
 * // Now mutable can be safely modified without affecting original graph
 * ```
 *
 * @since 3.18.0
 * @category mutations
 */
export const beginMutation = graph => {
  // Copy adjacency maps with deep cloned arrays
  const adjacency = new Map();
  const reverseAdjacency = new Map();
  for (const [nodeIndex, edges] of graph.adjacency) {
    adjacency.set(nodeIndex, [...edges]);
  }
  for (const [nodeIndex, edges] of graph.reverseAdjacency) {
    reverseAdjacency.set(nodeIndex, [...edges]);
  }
  const mutable = Object.create(ProtoGraph);
  mutable.type = graph.type;
  mutable.nodes = new Map(graph.nodes);
  mutable.edges = new Map(graph.edges);
  mutable.adjacency = adjacency;
  mutable.reverseAdjacency = reverseAdjacency;
  mutable.nextNodeIndex = graph.nextNodeIndex;
  mutable.nextEdgeIndex = graph.nextEdgeIndex;
  mutable.isAcyclic = graph.isAcyclic;
  mutable.mutable = true;
  return mutable;
};
/**
 * Converts a mutable graph back to an immutable graph, ending the mutation scope.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>()
 * const mutable = Graph.beginMutation(graph)
 * // ... perform mutations on mutable ...
 * const newGraph = Graph.endMutation(mutable)
 * ```
 *
 * @since 3.18.0
 * @category mutations
 */
export const endMutation = mutable => {
  const graph = Object.create(ProtoGraph);
  graph.type = mutable.type;
  graph.nodes = new Map(mutable.nodes);
  graph.edges = new Map(mutable.edges);
  graph.adjacency = mutable.adjacency;
  graph.reverseAdjacency = mutable.reverseAdjacency;
  graph.nextNodeIndex = mutable.nextNodeIndex;
  graph.nextEdgeIndex = mutable.nextEdgeIndex;
  graph.isAcyclic = mutable.isAcyclic;
  graph.mutable = false;
  return graph;
};
/**
 * Performs scoped mutations on a graph, automatically managing the mutation lifecycle.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>()
 * const newGraph = Graph.mutate(graph, (mutable) => {
 *   // Safe mutations go here
 *   // mutable gets automatically converted back to immutable
 * })
 * ```
 *
 * @since 3.18.0
 * @category mutations
 */
export const mutate = /*#__PURE__*/dual(2, (graph, f) => {
  const mutable = beginMutation(graph);
  f(mutable);
  return endMutation(mutable);
});
// =============================================================================
// Basic Node Operations
// =============================================================================
/**
 * Adds a new node to a mutable graph and returns its index.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const result = Graph.mutate(Graph.directed<string, number>(), (mutable) => {
 *   const nodeA = Graph.addNode(mutable, "Node A")
 *   const nodeB = Graph.addNode(mutable, "Node B")
 *   console.log(nodeA) // NodeIndex with value 0
 *   console.log(nodeB) // NodeIndex with value 1
 * })
 * ```
 *
 * @since 3.18.0
 * @category mutations
 */
export const addNode = (mutable, data) => {
  const nodeIndex = mutable.nextNodeIndex;
  // Add node data
  mutable.nodes.set(nodeIndex, data);
  // Initialize empty adjacency lists
  mutable.adjacency.set(nodeIndex, []);
  mutable.reverseAdjacency.set(nodeIndex, []);
  // Update graph allocators
  mutable.nextNodeIndex = mutable.nextNodeIndex + 1;
  return nodeIndex;
};
/**
 * Gets the data associated with a node index, if it exists.
 *
 * @example
 * ```ts
 * import { Graph, Option } from "effect"
 *
 * const graph = Graph.mutate(Graph.directed<string, number>(), (mutable) => {
 *   Graph.addNode(mutable, "Node A")
 * })
 *
 * const nodeIndex = 0
 * const nodeData = Graph.getNode(graph, nodeIndex)
 *
 * if (Option.isSome(nodeData)) {
 *   console.log(nodeData.value) // "Node A"
 * }
 * ```
 *
 * @since 3.18.0
 * @category getters
 */
export const getNode = (graph, nodeIndex) => getMapSafe(graph.nodes, nodeIndex);
/**
 * Checks if a node with the given index exists in the graph.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.mutate(Graph.directed<string, number>(), (mutable) => {
 *   Graph.addNode(mutable, "Node A")
 * })
 *
 * const nodeIndex = 0
 * const exists = Graph.hasNode(graph, nodeIndex)
 * console.log(exists) // true
 *
 * const nonExistentIndex = 999
 * const notExists = Graph.hasNode(graph, nonExistentIndex)
 * console.log(notExists) // false
 * ```
 *
 * @since 3.18.0
 * @category getters
 */
export const hasNode = (graph, nodeIndex) => graph.nodes.has(nodeIndex);
/**
 * Returns the number of nodes in the graph.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const emptyGraph = Graph.directed<string, number>()
 * console.log(Graph.nodeCount(emptyGraph)) // 0
 *
 * const graphWithNodes = Graph.mutate(emptyGraph, (mutable) => {
 *   Graph.addNode(mutable, "Node A")
 *   Graph.addNode(mutable, "Node B")
 *   Graph.addNode(mutable, "Node C")
 * })
 *
 * console.log(Graph.nodeCount(graphWithNodes)) // 3
 * ```
 *
 * @since 3.18.0
 * @category getters
 */
export const nodeCount = graph => graph.nodes.size;
/**
 * Finds the first node that matches the given predicate.
 *
 * @example
 * ```ts
 * import { Graph, Option } from "effect"
 *
 * const graph = Graph.mutate(Graph.directed<string, number>(), (mutable) => {
 *   Graph.addNode(mutable, "Node A")
 *   Graph.addNode(mutable, "Node B")
 *   Graph.addNode(mutable, "Node C")
 * })
 *
 * const result = Graph.findNode(graph, (data) => data.startsWith("Node B"))
 * console.log(result) // Option.some(1)
 *
 * const notFound = Graph.findNode(graph, (data) => data === "Node D")
 * console.log(notFound) // Option.none()
 * ```
 *
 * @since 3.18.0
 * @category getters
 */
export const findNode = (graph, predicate) => {
  for (const [index, data] of graph.nodes) {
    if (predicate(data)) {
      return Option.some(index);
    }
  }
  return Option.none();
};
/**
 * Finds all nodes that match the given predicate.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.mutate(Graph.directed<string, number>(), (mutable) => {
 *   Graph.addNode(mutable, "Start A")
 *   Graph.addNode(mutable, "Node B")
 *   Graph.addNode(mutable, "Start C")
 * })
 *
 * const result = Graph.findNodes(graph, (data) => data.startsWith("Start"))
 * console.log(result) // [0, 2]
 *
 * const empty = Graph.findNodes(graph, (data) => data === "Not Found")
 * console.log(empty) // []
 * ```
 *
 * @since 3.18.0
 * @category getters
 */
export const findNodes = (graph, predicate) => {
  const results = [];
  for (const [index, data] of graph.nodes) {
    if (predicate(data)) {
      results.push(index);
    }
  }
  return results;
};
/**
 * Finds the first edge that matches the given predicate.
 *
 * @example
 * ```ts
 * import { Graph, Option } from "effect"
 *
 * const graph = Graph.mutate(Graph.directed<string, number>(), (mutable) => {
 *   const nodeA = Graph.addNode(mutable, "Node A")
 *   const nodeB = Graph.addNode(mutable, "Node B")
 *   const nodeC = Graph.addNode(mutable, "Node C")
 *   Graph.addEdge(mutable, nodeA, nodeB, 10)
 *   Graph.addEdge(mutable, nodeB, nodeC, 20)
 * })
 *
 * const result = Graph.findEdge(graph, (data) => data > 15)
 * console.log(result) // Option.some(1)
 *
 * const notFound = Graph.findEdge(graph, (data) => data > 100)
 * console.log(notFound) // Option.none()
 * ```
 *
 * @since 3.18.0
 * @category getters
 */
export const findEdge = (graph, predicate) => {
  for (const [edgeIndex, edgeData] of graph.edges) {
    if (predicate(edgeData.data, edgeData.source, edgeData.target)) {
      return Option.some(edgeIndex);
    }
  }
  return Option.none();
};
/**
 * Finds all edges that match the given predicate.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.mutate(Graph.directed<string, number>(), (mutable) => {
 *   const nodeA = Graph.addNode(mutable, "Node A")
 *   const nodeB = Graph.addNode(mutable, "Node B")
 *   const nodeC = Graph.addNode(mutable, "Node C")
 *   Graph.addEdge(mutable, nodeA, nodeB, 10)
 *   Graph.addEdge(mutable, nodeB, nodeC, 20)
 *   Graph.addEdge(mutable, nodeC, nodeA, 30)
 * })
 *
 * const result = Graph.findEdges(graph, (data) => data >= 20)
 * console.log(result) // [1, 2]
 *
 * const empty = Graph.findEdges(graph, (data) => data > 100)
 * console.log(empty) // []
 * ```
 *
 * @since 3.18.0
 * @category getters
 */
export const findEdges = (graph, predicate) => {
  const results = [];
  for (const [edgeIndex, edgeData] of graph.edges) {
    if (predicate(edgeData.data, edgeData.source, edgeData.target)) {
      results.push(edgeIndex);
    }
  }
  return results;
};
/**
 * Updates a single node's data by applying a transformation function.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   Graph.addNode(mutable, "Node A")
 *   Graph.addNode(mutable, "Node B")
 *   Graph.updateNode(mutable, 0, (data) => data.toUpperCase())
 * })
 *
 * const nodeData = Graph.getNode(graph, 0)
 * console.log(nodeData) // Option.some("NODE A")
 * ```
 *
 * @since 3.18.0
 * @category transformations
 */
export const updateNode = (mutable, index, f) => {
  if (!mutable.nodes.has(index)) {
    return;
  }
  const currentData = mutable.nodes.get(index);
  const newData = f(currentData);
  mutable.nodes.set(index, newData);
};
/**
 * Updates a single edge's data by applying a transformation function.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const result = Graph.mutate(Graph.directed<string, number>(), (mutable) => {
 *   const nodeA = Graph.addNode(mutable, "Node A")
 *   const nodeB = Graph.addNode(mutable, "Node B")
 *   const edgeIndex = Graph.addEdge(mutable, nodeA, nodeB, 10)
 *   Graph.updateEdge(mutable, edgeIndex, (data) => data * 2)
 * })
 *
 * const edgeData = Graph.getEdge(result, 0)
 * console.log(edgeData) // Option.some({ source: 0, target: 1, data: 20 })
 * ```
 *
 * @since 3.18.0
 * @category mutations
 */
export const updateEdge = (mutable, edgeIndex, f) => {
  if (!mutable.edges.has(edgeIndex)) {
    return;
  }
  const currentEdge = mutable.edges.get(edgeIndex);
  const newData = f(currentEdge.data);
  mutable.edges.set(edgeIndex, {
    ...currentEdge,
    data: newData
  });
};
/**
 * Creates a new graph with transformed node data using the provided mapping function.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   Graph.addNode(mutable, "node a")
 *   Graph.addNode(mutable, "node b")
 *   Graph.addNode(mutable, "node c")
 *   Graph.mapNodes(mutable, (data) => data.toUpperCase())
 * })
 *
 * const nodeData = Graph.getNode(graph, 0)
 * console.log(nodeData) // Option.some("NODE A")
 * ```
 *
 * @since 3.18.0
 * @category transformations
 */
export const mapNodes = (mutable, f) => {
  // Transform existing node data in place
  for (const [index, data] of mutable.nodes) {
    const newData = f(data);
    mutable.nodes.set(index, newData);
  }
};
/**
 * Transforms all edge data in a mutable graph using the provided mapping function.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   const c = Graph.addNode(mutable, "C")
 *   Graph.addEdge(mutable, a, b, 10)
 *   Graph.addEdge(mutable, b, c, 20)
 *   Graph.mapEdges(mutable, (data) => data * 2)
 * })
 *
 * const edgeData = Graph.getEdge(graph, 0)
 * console.log(edgeData) // Option.some({ source: 0, target: 1, data: 20 })
 * ```
 *
 * @since 3.18.0
 * @category transformations
 */
export const mapEdges = (mutable, f) => {
  // Transform existing edge data in place
  for (const [index, edgeData] of mutable.edges) {
    const newData = f(edgeData.data);
    mutable.edges.set(index, {
      ...edgeData,
      data: newData
    });
  }
};
/**
 * Reverses all edge directions in a mutable graph by swapping source and target nodes.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   const c = Graph.addNode(mutable, "C")
 *   Graph.addEdge(mutable, a, b, 1)  // A -> B
 *   Graph.addEdge(mutable, b, c, 2)  // B -> C
 *   Graph.reverse(mutable)           // Now B -> A, C -> B
 * })
 *
 * const edge0 = Graph.getEdge(graph, 0)
 * console.log(edge0) // Option.some({ source: 1, target: 0, data: 1 }) - B -> A
 * ```
 *
 * @since 3.18.0
 * @category transformations
 */
export const reverse = mutable => {
  // Reverse all edges by swapping source and target
  for (const [index, edgeData] of mutable.edges) {
    mutable.edges.set(index, {
      source: edgeData.target,
      target: edgeData.source,
      data: edgeData.data
    });
  }
  // Clear and rebuild adjacency lists with reversed directions
  mutable.adjacency.clear();
  mutable.reverseAdjacency.clear();
  // Rebuild adjacency lists with reversed directions
  for (const [edgeIndex, edgeData] of mutable.edges) {
    // Add to forward adjacency (source -> target)
    const sourceEdges = mutable.adjacency.get(edgeData.source) || [];
    sourceEdges.push(edgeIndex);
    mutable.adjacency.set(edgeData.source, sourceEdges);
    // Add to reverse adjacency (target <- source)
    const targetEdges = mutable.reverseAdjacency.get(edgeData.target) || [];
    targetEdges.push(edgeIndex);
    mutable.reverseAdjacency.set(edgeData.target, targetEdges);
  }
  // Invalidate cycle flag since edge directions changed
  mutable.isAcyclic = Option.none();
};
/**
 * Filters and optionally transforms nodes in a mutable graph using a predicate function.
 * Nodes that return Option.none are removed along with all their connected edges.
 *
 * @example
 * ```ts
 * import { Graph, Option } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   const a = Graph.addNode(mutable, "active")
 *   const b = Graph.addNode(mutable, "inactive")
 *   const c = Graph.addNode(mutable, "active")
 *   Graph.addEdge(mutable, a, b, 1)
 *   Graph.addEdge(mutable, b, c, 2)
 *
 *   // Keep only "active" nodes and transform to uppercase
 *   Graph.filterMapNodes(mutable, (data) =>
 *     data === "active" ? Option.some(data.toUpperCase()) : Option.none()
 *   )
 * })
 *
 * console.log(Graph.nodeCount(graph)) // 2 (only "active" nodes remain)
 * ```
 *
 * @since 3.18.0
 * @category transformations
 */
export const filterMapNodes = (mutable, f) => {
  const nodesToRemove = [];
  // First pass: identify nodes to remove and transform data for nodes to keep
  for (const [index, data] of mutable.nodes) {
    const result = f(data);
    if (Option.isSome(result)) {
      // Transform node data
      mutable.nodes.set(index, result.value);
    } else {
      // Mark for removal
      nodesToRemove.push(index);
    }
  }
  // Second pass: remove filtered out nodes and their edges
  for (const nodeIndex of nodesToRemove) {
    removeNode(mutable, nodeIndex);
  }
};
/**
 * Filters and optionally transforms edges in a mutable graph using a predicate function.
 * Edges that return Option.none are removed from the graph.
 *
 * @example
 * ```ts
 * import { Graph, Option } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   const c = Graph.addNode(mutable, "C")
 *   Graph.addEdge(mutable, a, b, 5)
 *   Graph.addEdge(mutable, b, c, 15)
 *   Graph.addEdge(mutable, c, a, 25)
 *
 *   // Keep only edges with weight >= 10 and double their weight
 *   Graph.filterMapEdges(mutable, (data) =>
 *     data >= 10 ? Option.some(data * 2) : Option.none()
 *   )
 * })
 *
 * console.log(Graph.edgeCount(graph)) // 2 (edges with weight 5 removed)
 * ```
 *
 * @since 3.18.0
 * @category transformations
 */
export const filterMapEdges = (mutable, f) => {
  const edgesToRemove = [];
  // First pass: identify edges to remove and transform data for edges to keep
  for (const [index, edgeData] of mutable.edges) {
    const result = f(edgeData.data);
    if (Option.isSome(result)) {
      // Transform edge data
      mutable.edges.set(index, {
        ...edgeData,
        data: result.value
      });
    } else {
      // Mark for removal
      edgesToRemove.push(index);
    }
  }
  // Second pass: remove filtered out edges
  for (const edgeIndex of edgesToRemove) {
    removeEdge(mutable, edgeIndex);
  }
};
/**
 * Filters nodes by removing those that don't match the predicate.
 * This function modifies the mutable graph in place.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   Graph.addNode(mutable, "active")
 *   Graph.addNode(mutable, "inactive")
 *   Graph.addNode(mutable, "pending")
 *   Graph.addNode(mutable, "active")
 *
 *   // Keep only "active" nodes
 *   Graph.filterNodes(mutable, (data) => data === "active")
 * })
 *
 * console.log(Graph.nodeCount(graph)) // 2 (only "active" nodes remain)
 * ```
 *
 * @since 3.18.0
 * @category transformations
 */
export const filterNodes = (mutable, predicate) => {
  const nodesToRemove = [];
  // Identify nodes to remove
  for (const [index, data] of mutable.nodes) {
    if (!predicate(data)) {
      nodesToRemove.push(index);
    }
  }
  // Remove filtered out nodes (this also removes connected edges)
  for (const nodeIndex of nodesToRemove) {
    removeNode(mutable, nodeIndex);
  }
};
/**
 * Filters edges by removing those that don't match the predicate.
 * This function modifies the mutable graph in place.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   const c = Graph.addNode(mutable, "C")
 *
 *   Graph.addEdge(mutable, a, b, 5)
 *   Graph.addEdge(mutable, b, c, 15)
 *   Graph.addEdge(mutable, c, a, 25)
 *
 *   // Keep only edges with weight >= 10
 *   Graph.filterEdges(mutable, (data) => data >= 10)
 * })
 *
 * console.log(Graph.edgeCount(graph)) // 2 (edge with weight 5 removed)
 * ```
 *
 * @since 3.18.0
 * @category transformations
 */
export const filterEdges = (mutable, predicate) => {
  const edgesToRemove = [];
  // Identify edges to remove
  for (const [index, edgeData] of mutable.edges) {
    if (!predicate(edgeData.data)) {
      edgesToRemove.push(index);
    }
  }
  // Remove filtered out edges
  for (const edgeIndex of edgesToRemove) {
    removeEdge(mutable, edgeIndex);
  }
};
// =============================================================================
// Cycle Flag Management (Internal)
// =============================================================================
/** @internal */
const invalidateCycleFlagOnRemoval = mutable => {
  // Only invalidate if the graph had cycles (removing edges/nodes cannot introduce cycles in acyclic graphs)
  // If already unknown (null) or acyclic (true), no need to change
  if (Option.isSome(mutable.isAcyclic) && mutable.isAcyclic.value === false) {
    mutable.isAcyclic = Option.none();
  }
};
/** @internal */
const invalidateCycleFlagOnAddition = mutable => {
  // Only invalidate if the graph was acyclic (adding edges cannot remove cycles from cyclic graphs)
  // If already unknown (null) or cyclic (false), no need to change
  if (Option.isSome(mutable.isAcyclic) && mutable.isAcyclic.value === true) {
    mutable.isAcyclic = Option.none();
  }
};
// =============================================================================
// Edge Operations
// =============================================================================
/**
 * Adds a new edge to a mutable graph and returns its index.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const result = Graph.mutate(Graph.directed<string, number>(), (mutable) => {
 *   const nodeA = Graph.addNode(mutable, "Node A")
 *   const nodeB = Graph.addNode(mutable, "Node B")
 *   const edge = Graph.addEdge(mutable, nodeA, nodeB, 42)
 *   console.log(edge) // EdgeIndex with value 0
 * })
 * ```
 *
 * @since 3.18.0
 * @category mutations
 */
export const addEdge = (mutable, source, target, data) => {
  // Validate that both nodes exist
  if (!mutable.nodes.has(source)) {
    throw new Error(`Source node ${source} does not exist`);
  }
  if (!mutable.nodes.has(target)) {
    throw new Error(`Target node ${target} does not exist`);
  }
  const edgeIndex = mutable.nextEdgeIndex;
  // Create edge data
  const edgeData = new Edge({
    source,
    target,
    data
  });
  mutable.edges.set(edgeIndex, edgeData);
  // Update adjacency lists
  const sourceAdjacency = getMapSafe(mutable.adjacency, source);
  if (Option.isSome(sourceAdjacency)) {
    sourceAdjacency.value.push(edgeIndex);
  }
  const targetReverseAdjacency = getMapSafe(mutable.reverseAdjacency, target);
  if (Option.isSome(targetReverseAdjacency)) {
    targetReverseAdjacency.value.push(edgeIndex);
  }
  // For undirected graphs, add reverse connections
  if (mutable.type === "undirected") {
    const targetAdjacency = getMapSafe(mutable.adjacency, target);
    if (Option.isSome(targetAdjacency)) {
      targetAdjacency.value.push(edgeIndex);
    }
    const sourceReverseAdjacency = getMapSafe(mutable.reverseAdjacency, source);
    if (Option.isSome(sourceReverseAdjacency)) {
      sourceReverseAdjacency.value.push(edgeIndex);
    }
  }
  // Update allocators
  mutable.nextEdgeIndex = mutable.nextEdgeIndex + 1;
  // Only invalidate cycle flag if the graph was acyclic
  // Adding edges cannot remove cycles from cyclic graphs
  invalidateCycleFlagOnAddition(mutable);
  return edgeIndex;
};
/**
 * Removes a node and all its incident edges from a mutable graph.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const result = Graph.mutate(Graph.directed<string, number>(), (mutable) => {
 *   const nodeA = Graph.addNode(mutable, "Node A")
 *   const nodeB = Graph.addNode(mutable, "Node B")
 *   Graph.addEdge(mutable, nodeA, nodeB, 42)
 *
 *   // Remove nodeA and all edges connected to it
 *   Graph.removeNode(mutable, nodeA)
 * })
 * ```
 *
 * @since 3.18.0
 * @category mutations
 */
export const removeNode = (mutable, nodeIndex) => {
  // Check if node exists
  if (!mutable.nodes.has(nodeIndex)) {
    return; // Node doesn't exist, nothing to remove
  }
  // Collect all incident edges for removal
  const edgesToRemove = [];
  // Get outgoing edges
  const outgoingEdges = getMapSafe(mutable.adjacency, nodeIndex);
  if (Option.isSome(outgoingEdges)) {
    for (const edge of outgoingEdges.value) {
      edgesToRemove.push(edge);
    }
  }
  // Get incoming edges
  const incomingEdges = getMapSafe(mutable.reverseAdjacency, nodeIndex);
  if (Option.isSome(incomingEdges)) {
    for (const edge of incomingEdges.value) {
      edgesToRemove.push(edge);
    }
  }
  // Remove all incident edges
  for (const edgeIndex of edgesToRemove) {
    removeEdgeInternal(mutable, edgeIndex);
  }
  // Remove the node itself
  mutable.nodes.delete(nodeIndex);
  mutable.adjacency.delete(nodeIndex);
  mutable.reverseAdjacency.delete(nodeIndex);
  // Only invalidate cycle flag if the graph wasn't already known to be acyclic
  // Removing nodes cannot introduce cycles in an acyclic graph
  invalidateCycleFlagOnRemoval(mutable);
};
/**
 * Removes an edge from a mutable graph.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const result = Graph.mutate(Graph.directed<string, number>(), (mutable) => {
 *   const nodeA = Graph.addNode(mutable, "Node A")
 *   const nodeB = Graph.addNode(mutable, "Node B")
 *   const edge = Graph.addEdge(mutable, nodeA, nodeB, 42)
 *
 *   // Remove the edge
 *   Graph.removeEdge(mutable, edge)
 * })
 * ```
 *
 * @since 3.18.0
 * @category mutations
 */
export const removeEdge = (mutable, edgeIndex) => {
  const wasRemoved = removeEdgeInternal(mutable, edgeIndex);
  // Only invalidate cycle flag if an edge was actually removed
  // and only if the graph wasn't already known to be acyclic
  if (wasRemoved) {
    invalidateCycleFlagOnRemoval(mutable);
  }
};
/** @internal */
const removeEdgeInternal = (mutable, edgeIndex) => {
  // Get edge data
  const edge = getMapSafe(mutable.edges, edgeIndex);
  if (Option.isNone(edge)) {
    return false; // Edge doesn't exist, no mutation occurred
  }
  const {
    source,
    target
  } = edge.value;
  // Remove from adjacency lists
  const sourceAdjacency = getMapSafe(mutable.adjacency, source);
  if (Option.isSome(sourceAdjacency)) {
    const index = sourceAdjacency.value.indexOf(edgeIndex);
    if (index !== -1) {
      sourceAdjacency.value.splice(index, 1);
    }
  }
  const targetReverseAdjacency = getMapSafe(mutable.reverseAdjacency, target);
  if (Option.isSome(targetReverseAdjacency)) {
    const index = targetReverseAdjacency.value.indexOf(edgeIndex);
    if (index !== -1) {
      targetReverseAdjacency.value.splice(index, 1);
    }
  }
  // For undirected graphs, remove reverse connections
  if (mutable.type === "undirected") {
    const targetAdjacency = getMapSafe(mutable.adjacency, target);
    if (Option.isSome(targetAdjacency)) {
      const index = targetAdjacency.value.indexOf(edgeIndex);
      if (index !== -1) {
        targetAdjacency.value.splice(index, 1);
      }
    }
    const sourceReverseAdjacency = getMapSafe(mutable.reverseAdjacency, source);
    if (Option.isSome(sourceReverseAdjacency)) {
      const index = sourceReverseAdjacency.value.indexOf(edgeIndex);
      if (index !== -1) {
        sourceReverseAdjacency.value.splice(index, 1);
      }
    }
  }
  // Remove edge data
  mutable.edges.delete(edgeIndex);
  return true; // Edge was successfully removed
};
// =============================================================================
// Edge Query Operations
// =============================================================================
/**
 * Gets the edge data associated with an edge index, if it exists.
 *
 * @example
 * ```ts
 * import { Graph, Option } from "effect"
 *
 * const graph = Graph.mutate(Graph.directed<string, number>(), (mutable) => {
 *   const nodeA = Graph.addNode(mutable, "Node A")
 *   const nodeB = Graph.addNode(mutable, "Node B")
 *   Graph.addEdge(mutable, nodeA, nodeB, 42)
 * })
 *
 * const edgeIndex = 0
 * const edgeData = Graph.getEdge(graph, edgeIndex)
 *
 * if (Option.isSome(edgeData)) {
 *   console.log(edgeData.value.data) // 42
 *   console.log(edgeData.value.source) // NodeIndex(0)
 *   console.log(edgeData.value.target) // NodeIndex(1)
 * }
 * ```
 *
 * @since 3.18.0
 * @category getters
 */
export const getEdge = (graph, edgeIndex) => getMapSafe(graph.edges, edgeIndex);
/**
 * Checks if an edge exists between two nodes in the graph.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.mutate(Graph.directed<string, number>(), (mutable) => {
 *   const nodeA = Graph.addNode(mutable, "Node A")
 *   const nodeB = Graph.addNode(mutable, "Node B")
 *   const nodeC = Graph.addNode(mutable, "Node C")
 *   Graph.addEdge(mutable, nodeA, nodeB, 42)
 * })
 *
 * const nodeA = 0
 * const nodeB = 1
 * const nodeC = 2
 *
 * const hasAB = Graph.hasEdge(graph, nodeA, nodeB)
 * console.log(hasAB) // true
 *
 * const hasAC = Graph.hasEdge(graph, nodeA, nodeC)
 * console.log(hasAC) // false
 * ```
 *
 * @since 3.18.0
 * @category getters
 */
export const hasEdge = (graph, source, target) => {
  const adjacencyList = getMapSafe(graph.adjacency, source);
  if (Option.isNone(adjacencyList)) {
    return false;
  }
  // Check if any edge in the adjacency list connects to the target
  for (const edgeIndex of adjacencyList.value) {
    const edge = getMapSafe(graph.edges, edgeIndex);
    if (Option.isSome(edge) && edge.value.target === target) {
      return true;
    }
  }
  return false;
};
/**
 * Returns the number of edges in the graph.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const emptyGraph = Graph.directed<string, number>()
 * console.log(Graph.edgeCount(emptyGraph)) // 0
 *
 * const graphWithEdges = Graph.mutate(emptyGraph, (mutable) => {
 *   const nodeA = Graph.addNode(mutable, "Node A")
 *   const nodeB = Graph.addNode(mutable, "Node B")
 *   const nodeC = Graph.addNode(mutable, "Node C")
 *   Graph.addEdge(mutable, nodeA, nodeB, 1)
 *   Graph.addEdge(mutable, nodeB, nodeC, 2)
 *   Graph.addEdge(mutable, nodeC, nodeA, 3)
 * })
 *
 * console.log(Graph.edgeCount(graphWithEdges)) // 3
 * ```
 *
 * @since 3.18.0
 * @category getters
 */
export const edgeCount = graph => graph.edges.size;
/**
 * Returns the neighboring nodes (targets of outgoing edges) for a given node.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.mutate(Graph.directed<string, number>(), (mutable) => {
 *   const nodeA = Graph.addNode(mutable, "Node A")
 *   const nodeB = Graph.addNode(mutable, "Node B")
 *   const nodeC = Graph.addNode(mutable, "Node C")
 *   Graph.addEdge(mutable, nodeA, nodeB, 1)
 *   Graph.addEdge(mutable, nodeA, nodeC, 2)
 * })
 *
 * const nodeA = 0
 * const nodeB = 1
 * const nodeC = 2
 *
 * const neighborsA = Graph.neighbors(graph, nodeA)
 * console.log(neighborsA) // [NodeIndex(1), NodeIndex(2)]
 *
 * const neighborsB = Graph.neighbors(graph, nodeB)
 * console.log(neighborsB) // []
 * ```
 *
 * @since 3.18.0
 * @category getters
 */
export const neighbors = (graph, nodeIndex) => {
  const adjacencyList = getMapSafe(graph.adjacency, nodeIndex);
  if (Option.isNone(adjacencyList)) {
    return [];
  }
  const result = [];
  for (const edgeIndex of adjacencyList.value) {
    const edge = getMapSafe(graph.edges, edgeIndex);
    if (Option.isSome(edge)) {
      result.push(edge.value.target);
    }
  }
  return result;
};
/**
 * Get neighbors of a node in a specific direction for bidirectional traversal.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, string>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   Graph.addEdge(mutable, a, b, "A->B")
 * })
 *
 * const nodeA = 0
 * const nodeB = 1
 *
 * // Get outgoing neighbors (nodes that nodeA points to)
 * const outgoing = Graph.neighborsDirected(graph, nodeA, "outgoing")
 *
 * // Get incoming neighbors (nodes that point to nodeB)
 * const incoming = Graph.neighborsDirected(graph, nodeB, "incoming")
 * ```
 *
 * @since 3.18.0
 * @category queries
 */
export const neighborsDirected = (graph, nodeIndex, direction) => {
  const adjacencyMap = direction === "incoming" ? graph.reverseAdjacency : graph.adjacency;
  const adjacencyList = getMapSafe(adjacencyMap, nodeIndex);
  if (Option.isNone(adjacencyList)) {
    return [];
  }
  const result = [];
  for (const edgeIndex of adjacencyList.value) {
    const edge = getMapSafe(graph.edges, edgeIndex);
    if (Option.isSome(edge)) {
      // For incoming direction, we want the source node instead of target
      const neighborNode = direction === "incoming" ? edge.value.source : edge.value.target;
      result.push(neighborNode);
    }
  }
  return result;
};
// =============================================================================
// GraphViz Export
// =============================================================================
/**
 * Exports a graph to GraphViz DOT format for visualization.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.mutate(Graph.directed<string, number>(), (mutable) => {
 *   const nodeA = Graph.addNode(mutable, "Node A")
 *   const nodeB = Graph.addNode(mutable, "Node B")
 *   const nodeC = Graph.addNode(mutable, "Node C")
 *   Graph.addEdge(mutable, nodeA, nodeB, 1)
 *   Graph.addEdge(mutable, nodeB, nodeC, 2)
 *   Graph.addEdge(mutable, nodeC, nodeA, 3)
 * })
 *
 * const dot = Graph.toGraphViz(graph)
 * console.log(dot)
 * // digraph G {
 * //   "0" [label="Node A"];
 * //   "1" [label="Node B"];
 * //   "2" [label="Node C"];
 * //   "0" -> "1" [label="1"];
 * //   "1" -> "2" [label="2"];
 * //   "2" -> "0" [label="3"];
 * // }
 * ```
 *
 * @since 3.18.0
 * @category utils
 */
export const toGraphViz = (graph, options) => {
  const {
    edgeLabel = data => String(data),
    graphName = "G",
    nodeLabel = data => String(data)
  } = options ?? {};
  const isDirected = graph.type === "directed";
  const graphType = isDirected ? "digraph" : "graph";
  const edgeOperator = isDirected ? "->" : "--";
  const lines = [];
  lines.push(`${graphType} ${graphName} {`);
  // Add nodes
  for (const [nodeIndex, nodeData] of graph.nodes) {
    const label = nodeLabel(nodeData).replace(/"/g, "\\\"");
    lines.push(`  "${nodeIndex}" [label="${label}"];`);
  }
  // Add edges
  for (const [, edgeData] of graph.edges) {
    const label = edgeLabel(edgeData.data).replace(/"/g, "\\\"");
    lines.push(`  "${edgeData.source}" ${edgeOperator} "${edgeData.target}" [label="${label}"];`);
  }
  lines.push("}");
  return lines.join("\n");
};
// =============================================================================
// =============================================================================
// Graph Structure Analysis Algorithms (Phase 5A)
// =============================================================================
/**
 * Checks if the graph is acyclic (contains no cycles).
 *
 * Uses depth-first search to detect back edges, which indicate cycles.
 * For directed graphs, any back edge creates a cycle. For undirected graphs,
 * a back edge that doesn't go to the immediate parent creates a cycle.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * // Acyclic directed graph (DAG)
 * const dag = Graph.directed<string, string>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   const c = Graph.addNode(mutable, "C")
 *   Graph.addEdge(mutable, a, b, "A->B")
 *   Graph.addEdge(mutable, b, c, "B->C")
 * })
 * console.log(Graph.isAcyclic(dag)) // true
 *
 * // Cyclic directed graph
 * const cyclic = Graph.directed<string, string>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   Graph.addEdge(mutable, a, b, "A->B")
 *   Graph.addEdge(mutable, b, a, "B->A") // Creates cycle
 * })
 * console.log(Graph.isAcyclic(cyclic)) // false
 * ```
 *
 * @since 3.18.0
 * @category algorithms
 */
export const isAcyclic = graph => {
  // Use existing cycle flag if available
  if (Option.isSome(graph.isAcyclic)) {
    return graph.isAcyclic.value;
  }
  // Stack-safe DFS cycle detection using iterative approach
  const visited = new Set();
  const recursionStack = new Set();
  // Get all nodes to handle disconnected components
  for (const startNode of graph.nodes.keys()) {
    if (visited.has(startNode)) {
      continue; // Already processed this component
    }
    // Iterative DFS with explicit stack
    const stack = [[startNode, [], 0, true]];
    while (stack.length > 0) {
      const [node, neighbors, neighborIndex, isFirstVisit] = stack[stack.length - 1];
      // First visit to this node
      if (isFirstVisit) {
        if (recursionStack.has(node)) {
          // Back edge found - cycle detected
          graph.isAcyclic = Option.some(false);
          return false;
        }
        if (visited.has(node)) {
          stack.pop();
          continue;
        }
        visited.add(node);
        recursionStack.add(node);
        // Get neighbors for this node
        const nodeNeighbors = Array.from(neighborsDirected(graph, node, "outgoing"));
        stack[stack.length - 1] = [node, nodeNeighbors, 0, false];
        continue;
      }
      // Process next neighbor
      if (neighborIndex < neighbors.length) {
        const neighbor = neighbors[neighborIndex];
        stack[stack.length - 1] = [node, neighbors, neighborIndex + 1, false];
        if (recursionStack.has(neighbor)) {
          // Back edge found - cycle detected
          graph.isAcyclic = Option.some(false);
          return false;
        }
        if (!visited.has(neighbor)) {
          stack.push([neighbor, [], 0, true]);
        }
      } else {
        // Done with this node - backtrack
        recursionStack.delete(node);
        stack.pop();
      }
    }
  }
  // Cache the result
  graph.isAcyclic = Option.some(true);
  return true;
};
/**
 * Checks if an undirected graph is bipartite.
 *
 * A bipartite graph is one whose vertices can be divided into two disjoint sets
 * such that no two vertices within the same set are adjacent. Uses BFS coloring
 * to determine bipartiteness.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * // Bipartite graph (alternating coloring possible)
 * const bipartite = Graph.undirected<string, string>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   const c = Graph.addNode(mutable, "C")
 *   const d = Graph.addNode(mutable, "D")
 *   Graph.addEdge(mutable, a, b, "edge") // Set 1: {A, C}, Set 2: {B, D}
 *   Graph.addEdge(mutable, b, c, "edge")
 *   Graph.addEdge(mutable, c, d, "edge")
 * })
 * console.log(Graph.isBipartite(bipartite)) // true
 *
 * // Non-bipartite graph (odd cycle)
 * const triangle = Graph.undirected<string, string>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   const c = Graph.addNode(mutable, "C")
 *   Graph.addEdge(mutable, a, b, "edge")
 *   Graph.addEdge(mutable, b, c, "edge")
 *   Graph.addEdge(mutable, c, a, "edge") // Triangle (3-cycle)
 * })
 * console.log(Graph.isBipartite(triangle)) // false
 * ```
 *
 * @since 3.18.0
 * @category algorithms
 */
export const isBipartite = graph => {
  const coloring = new Map();
  const discovered = new Set();
  let isBipartiteGraph = true;
  // Get all nodes to handle disconnected components
  for (const startNode of graph.nodes.keys()) {
    if (!discovered.has(startNode)) {
      // Start BFS coloring from this component
      const queue = [startNode];
      coloring.set(startNode, 0); // Color start node with 0
      discovered.add(startNode);
      while (queue.length > 0 && isBipartiteGraph) {
        const current = queue.shift();
        const currentColor = coloring.get(current);
        const neighborColor = currentColor === 0 ? 1 : 0;
        // Get all neighbors for undirected graph
        const nodeNeighbors = getUndirectedNeighbors(graph, current);
        for (const neighbor of nodeNeighbors) {
          if (!discovered.has(neighbor)) {
            // Color unvisited neighbor with opposite color
            coloring.set(neighbor, neighborColor);
            discovered.add(neighbor);
            queue.push(neighbor);
          } else {
            // Check if neighbor has the same color (conflict)
            if (coloring.get(neighbor) === currentColor) {
              isBipartiteGraph = false;
              break;
            }
          }
        }
      }
      // Early exit if not bipartite
      if (!isBipartiteGraph) {
        break;
      }
    }
  }
  return isBipartiteGraph;
};
/**
 * Get neighbors for undirected graphs by checking both adjacency and reverse adjacency.
 * For undirected graphs, we need to find the other endpoint of each edge incident to the node.
 */
const getUndirectedNeighbors = (graph, nodeIndex) => {
  const neighbors = new Set();
  // Check edges where this node is the source
  const adjacencyList = getMapSafe(graph.adjacency, nodeIndex);
  if (Option.isSome(adjacencyList)) {
    for (const edgeIndex of adjacencyList.value) {
      const edge = getMapSafe(graph.edges, edgeIndex);
      if (Option.isSome(edge)) {
        // For undirected graphs, the neighbor is the other endpoint
        const otherNode = edge.value.source === nodeIndex ? edge.value.target : edge.value.source;
        neighbors.add(otherNode);
      }
    }
  }
  return Array.from(neighbors);
};
/**
 * Find connected components in an undirected graph.
 * Each component is represented as an array of node indices.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.undirected<string, string>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   const c = Graph.addNode(mutable, "C")
 *   const d = Graph.addNode(mutable, "D")
 *   Graph.addEdge(mutable, a, b, "edge") // Component 1: A-B
 *   Graph.addEdge(mutable, c, d, "edge") // Component 2: C-D
 * })
 *
 * const components = Graph.connectedComponents(graph)
 * console.log(components) // [[0, 1], [2, 3]]
 * ```
 *
 * @since 3.18.0
 * @category algorithms
 */
export const connectedComponents = graph => {
  const visited = new Set();
  const components = [];
  for (const startNode of graph.nodes.keys()) {
    if (!visited.has(startNode)) {
      // DFS to find all nodes in this component
      const component = [];
      const stack = [startNode];
      while (stack.length > 0) {
        const current = stack.pop();
        if (!visited.has(current)) {
          visited.add(current);
          component.push(current);
          // Add all unvisited neighbors to stack
          const nodeNeighbors = getUndirectedNeighbors(graph, current);
          for (const neighbor of nodeNeighbors) {
            if (!visited.has(neighbor)) {
              stack.push(neighbor);
            }
          }
        }
      }
      components.push(component);
    }
  }
  return components;
};
/**
 * Find strongly connected components in a directed graph using Kosaraju's algorithm.
 * Each SCC is represented as an array of node indices.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, string>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   const c = Graph.addNode(mutable, "C")
 *   Graph.addEdge(mutable, a, b, "A->B")
 *   Graph.addEdge(mutable, b, c, "B->C")
 *   Graph.addEdge(mutable, c, a, "C->A") // Creates SCC: A-B-C
 * })
 *
 * const sccs = Graph.stronglyConnectedComponents(graph)
 * console.log(sccs) // [[0, 1, 2]]
 * ```
 *
 * @since 3.18.0
 * @category algorithms
 */
export const stronglyConnectedComponents = graph => {
  const visited = new Set();
  const finishOrder = [];
  for (const startNode of graph.nodes.keys()) {
    if (visited.has(startNode)) {
      continue;
    }
    const stack = [[startNode, [], 0, true]];
    while (stack.length > 0) {
      const [node, nodeNeighbors, neighborIndex, isFirstVisit] = stack[stack.length - 1];
      if (isFirstVisit) {
        if (visited.has(node)) {
          stack.pop();
          continue;
        }
        visited.add(node);
        const nodeNeighborsList = neighbors(graph, node);
        stack[stack.length - 1] = [node, nodeNeighborsList, 0, false];
        continue;
      }
      // Process next neighbor
      if (neighborIndex < nodeNeighbors.length) {
        const neighbor = nodeNeighbors[neighborIndex];
        stack[stack.length - 1] = [node, nodeNeighbors, neighborIndex + 1, false];
        if (!visited.has(neighbor)) {
          stack.push([neighbor, [], 0, true]);
        }
      } else {
        // Done with this node - add to finish order (post-order)
        finishOrder.push(node);
        stack.pop();
      }
    }
  }
  // Step 2: Stack-safe DFS on transpose graph in reverse finish order
  visited.clear();
  const sccs = [];
  for (let i = finishOrder.length - 1; i >= 0; i--) {
    const startNode = finishOrder[i];
    if (visited.has(startNode)) {
      continue;
    }
    const scc = [];
    const stack = [startNode];
    while (stack.length > 0) {
      const node = stack.pop();
      if (visited.has(node)) {
        continue;
      }
      visited.add(node);
      scc.push(node);
      // Use reverse adjacency (transpose graph)
      const reverseAdjacency = getMapSafe(graph.reverseAdjacency, node);
      if (Option.isSome(reverseAdjacency)) {
        for (const edgeIndex of reverseAdjacency.value) {
          const edge = getMapSafe(graph.edges, edgeIndex);
          if (Option.isSome(edge)) {
            const predecessor = edge.value.source;
            if (!visited.has(predecessor)) {
              stack.push(predecessor);
            }
          }
        }
      }
    }
    sccs.push(scc);
  }
  return sccs;
};
/**
 * Find the shortest path between two nodes using Dijkstra's algorithm.
 *
 * Dijkstra's algorithm works with non-negative edge weights and finds the shortest
 * path from a source node to a target node in O((V + E) log V) time complexity.
 *
 * @example
 * ```ts
 * import { Graph, Option } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   const c = Graph.addNode(mutable, "C")
 *   Graph.addEdge(mutable, a, b, 5)
 *   Graph.addEdge(mutable, a, c, 10)
 *   Graph.addEdge(mutable, b, c, 2)
 * })
 *
 * const result = Graph.dijkstra(graph, 0, 2, (edgeData) => edgeData)
 * if (Option.isSome(result)) {
 *   console.log(result.value.path) // [0, 1, 2] - shortest path A->B->C
 *   console.log(result.value.distance) // 7 - total distance
 * }
 * ```
 *
 * @since 3.18.0
 * @category algorithms
 */
export const dijkstra = (graph, source, target, edgeWeight) => {
  // Validate that source and target nodes exist
  if (!graph.nodes.has(source)) {
    throw new Error(`Source node ${source} does not exist`);
  }
  if (!graph.nodes.has(target)) {
    throw new Error(`Target node ${target} does not exist`);
  }
  // Early return if source equals target
  if (source === target) {
    return Option.some({
      path: [source],
      distance: 0,
      edgeWeights: []
    });
  }
  // Distance tracking and priority queue simulation
  const distances = new Map();
  const previous = new Map();
  const visited = new Set();
  // Initialize distances
  // Iterate directly over node keys
  for (const node of graph.nodes.keys()) {
    distances.set(node, node === source ? 0 : Infinity);
    previous.set(node, null);
  }
  // Simple priority queue using array (can be optimized with proper heap)
  const priorityQueue = [{
    node: source,
    distance: 0
  }];
  while (priorityQueue.length > 0) {
    // Find minimum distance node (priority queue extract-min)
    let minIndex = 0;
    for (let i = 1; i < priorityQueue.length; i++) {
      if (priorityQueue[i].distance < priorityQueue[minIndex].distance) {
        minIndex = i;
      }
    }
    const current = priorityQueue.splice(minIndex, 1)[0];
    const currentNode = current.node;
    // Skip if already visited (can happen with duplicate entries)
    if (visited.has(currentNode)) {
      continue;
    }
    visited.add(currentNode);
    // Early termination if we reached the target
    if (currentNode === target) {
      break;
    }
    // Get current distance
    const currentDistance = distances.get(currentNode);
    // Examine all outgoing edges
    const adjacencyList = getMapSafe(graph.adjacency, currentNode);
    if (Option.isSome(adjacencyList)) {
      for (const edgeIndex of adjacencyList.value) {
        const edge = getMapSafe(graph.edges, edgeIndex);
        if (Option.isSome(edge)) {
          const neighbor = edge.value.target;
          const weight = edgeWeight(edge.value.data);
          // Validate non-negative weights
          if (weight < 0) {
            throw new Error(`Dijkstra's algorithm requires non-negative edge weights, found ${weight}`);
          }
          const newDistance = currentDistance + weight;
          const neighborDistance = distances.get(neighbor);
          // Relaxation step
          if (newDistance < neighborDistance) {
            distances.set(neighbor, newDistance);
            previous.set(neighbor, {
              node: currentNode,
              edgeData: edge.value.data
            });
            // Add to priority queue if not visited
            if (!visited.has(neighbor)) {
              priorityQueue.push({
                node: neighbor,
                distance: newDistance
              });
            }
          }
        }
      }
    }
  }
  // Check if target is reachable
  const targetDistance = distances.get(target);
  if (targetDistance === Infinity) {
    return Option.none(); // No path exists
  }
  // Reconstruct path
  const path = [];
  const edgeWeights = [];
  let currentNode = target;
  while (currentNode !== null) {
    path.unshift(currentNode);
    const prev = previous.get(currentNode);
    if (prev !== null) {
      edgeWeights.unshift(prev.edgeData);
      currentNode = prev.node;
    } else {
      currentNode = null;
    }
  }
  return Option.some({
    path,
    distance: targetDistance,
    edgeWeights
  });
};
/**
 * Find shortest paths between all pairs of nodes using Floyd-Warshall algorithm.
 *
 * Floyd-Warshall algorithm computes shortest paths between all pairs of nodes in O(V³) time.
 * It can handle negative edge weights and detect negative cycles.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   const c = Graph.addNode(mutable, "C")
 *   Graph.addEdge(mutable, a, b, 3)
 *   Graph.addEdge(mutable, b, c, 2)
 *   Graph.addEdge(mutable, a, c, 7)
 * })
 *
 * const result = Graph.floydWarshall(graph, (edgeData) => edgeData)
 * const distanceAToC = result.distances.get(0)?.get(2) // 5 (A->B->C)
 * const pathAToC = result.paths.get(0)?.get(2) // [0, 1, 2]
 * ```
 *
 * @since 3.18.0
 * @category algorithms
 */
export const floydWarshall = (graph, edgeWeight) => {
  // Get all nodes for Floyd-Warshall algorithm (needs array for nested iteration)
  const allNodes = Array.from(graph.nodes.keys());
  // Initialize distance matrix
  const dist = new Map();
  const next = new Map();
  const edgeMatrix = new Map();
  // Initialize with infinity for all pairs
  for (const i of allNodes) {
    dist.set(i, new Map());
    next.set(i, new Map());
    edgeMatrix.set(i, new Map());
    for (const j of allNodes) {
      dist.get(i).set(j, i === j ? 0 : Infinity);
      next.get(i).set(j, null);
      edgeMatrix.get(i).set(j, null);
    }
  }
  // Set edge weights
  for (const [, edgeData] of graph.edges) {
    const weight = edgeWeight(edgeData.data);
    const i = edgeData.source;
    const j = edgeData.target;
    // Use minimum weight if multiple edges exist
    const currentWeight = dist.get(i).get(j);
    if (weight < currentWeight) {
      dist.get(i).set(j, weight);
      next.get(i).set(j, j);
      edgeMatrix.get(i).set(j, edgeData.data);
    }
  }
  // Floyd-Warshall main loop
  for (const k of allNodes) {
    for (const i of allNodes) {
      for (const j of allNodes) {
        const distIK = dist.get(i).get(k);
        const distKJ = dist.get(k).get(j);
        const distIJ = dist.get(i).get(j);
        if (distIK !== Infinity && distKJ !== Infinity && distIK + distKJ < distIJ) {
          dist.get(i).set(j, distIK + distKJ);
          next.get(i).set(j, next.get(i).get(k));
        }
      }
    }
  }
  // Check for negative cycles
  for (const i of allNodes) {
    if (dist.get(i).get(i) < 0) {
      throw new Error(`Negative cycle detected involving node ${i}`);
    }
  }
  // Build result paths and edge weights
  const paths = new Map();
  const resultEdgeWeights = new Map();
  for (const i of allNodes) {
    paths.set(i, new Map());
    resultEdgeWeights.set(i, new Map());
    for (const j of allNodes) {
      if (i === j) {
        paths.get(i).set(j, [i]);
        resultEdgeWeights.get(i).set(j, []);
      } else if (dist.get(i).get(j) === Infinity) {
        paths.get(i).set(j, null);
        resultEdgeWeights.get(i).set(j, []);
      } else {
        // Reconstruct path iteratively
        const path = [];
        const weights = [];
        let current = i;
        path.push(current);
        while (current !== j) {
          const nextNode = next.get(current).get(j);
          if (nextNode === null) break;
          const edgeData = edgeMatrix.get(current).get(nextNode);
          if (edgeData !== null) {
            weights.push(edgeData);
          }
          current = nextNode;
          path.push(current);
        }
        paths.get(i).set(j, path);
        resultEdgeWeights.get(i).set(j, weights);
      }
    }
  }
  return {
    distances: dist,
    paths,
    edgeWeights: resultEdgeWeights
  };
};
/**
 * Find the shortest path between two nodes using A* pathfinding algorithm.
 *
 * A* is an extension of Dijkstra's algorithm that uses a heuristic function to guide
 * the search towards the target, potentially finding paths faster than Dijkstra's.
 * The heuristic must be admissible (never overestimate the actual cost).
 *
 * @example
 * ```ts
 * import { Graph, Option } from "effect"
 *
 * const graph = Graph.directed<{x: number, y: number}, number>((mutable) => {
 *   const a = Graph.addNode(mutable, {x: 0, y: 0})
 *   const b = Graph.addNode(mutable, {x: 1, y: 0})
 *   const c = Graph.addNode(mutable, {x: 2, y: 0})
 *   Graph.addEdge(mutable, a, b, 1)
 *   Graph.addEdge(mutable, b, c, 1)
 * })
 *
 * // Manhattan distance heuristic
 * const heuristic = (nodeData: {x: number, y: number}, targetData: {x: number, y: number}) =>
 *   Math.abs(nodeData.x - targetData.x) + Math.abs(nodeData.y - targetData.y)
 *
 * const result = Graph.astar(graph, 0, 2, (edgeData) => edgeData, heuristic)
 * if (Option.isSome(result)) {
 *   console.log(result.value.path) // [0, 1, 2] - shortest path
 *   console.log(result.value.distance) // 2 - total distance
 * }
 * ```
 *
 * @since 3.18.0
 * @category algorithms
 */
export const astar = (graph, source, target, edgeWeight, heuristic) => {
  // Validate that source and target nodes exist
  if (!graph.nodes.has(source)) {
    throw new Error(`Source node ${source} does not exist`);
  }
  if (!graph.nodes.has(target)) {
    throw new Error(`Target node ${target} does not exist`);
  }
  // Early return if source equals target
  if (source === target) {
    return Option.some({
      path: [source],
      distance: 0,
      edgeWeights: []
    });
  }
  // Get target node data for heuristic calculations
  const targetNodeData = getMapSafe(graph.nodes, target);
  if (Option.isNone(targetNodeData)) {
    throw new Error(`Target node ${target} data not found`);
  }
  // Distance tracking (g-score) and f-score (g + h)
  const gScore = new Map();
  const fScore = new Map();
  const previous = new Map();
  const visited = new Set();
  // Initialize scores
  // Iterate directly over node keys
  for (const node of graph.nodes.keys()) {
    gScore.set(node, node === source ? 0 : Infinity);
    fScore.set(node, Infinity);
    previous.set(node, null);
  }
  // Calculate initial f-score for source
  const sourceNodeData = getMapSafe(graph.nodes, source);
  if (Option.isSome(sourceNodeData)) {
    const h = heuristic(sourceNodeData.value, targetNodeData.value);
    fScore.set(source, h);
  }
  // Priority queue using f-score (total estimated cost)
  const openSet = [{
    node: source,
    fScore: fScore.get(source)
  }];
  while (openSet.length > 0) {
    // Find node with lowest f-score
    let minIndex = 0;
    for (let i = 1; i < openSet.length; i++) {
      if (openSet[i].fScore < openSet[minIndex].fScore) {
        minIndex = i;
      }
    }
    const current = openSet.splice(minIndex, 1)[0];
    const currentNode = current.node;
    // Skip if already visited
    if (visited.has(currentNode)) {
      continue;
    }
    visited.add(currentNode);
    // Early termination if we reached the target
    if (currentNode === target) {
      break;
    }
    // Get current g-score
    const currentGScore = gScore.get(currentNode);
    // Examine all outgoing edges
    const adjacencyList = getMapSafe(graph.adjacency, currentNode);
    if (Option.isSome(adjacencyList)) {
      for (const edgeIndex of adjacencyList.value) {
        const edge = getMapSafe(graph.edges, edgeIndex);
        if (Option.isSome(edge)) {
          const neighbor = edge.value.target;
          const weight = edgeWeight(edge.value.data);
          // Validate non-negative weights
          if (weight < 0) {
            throw new Error(`A* algorithm requires non-negative edge weights, found ${weight}`);
          }
          const tentativeGScore = currentGScore + weight;
          const neighborGScore = gScore.get(neighbor);
          // If this path to neighbor is better than any previous one
          if (tentativeGScore < neighborGScore) {
            // Update g-score and previous
            gScore.set(neighbor, tentativeGScore);
            previous.set(neighbor, {
              node: currentNode,
              edgeData: edge.value.data
            });
            // Calculate f-score using heuristic
            const neighborNodeData = getMapSafe(graph.nodes, neighbor);
            if (Option.isSome(neighborNodeData)) {
              const h = heuristic(neighborNodeData.value, targetNodeData.value);
              const f = tentativeGScore + h;
              fScore.set(neighbor, f);
              // Add to open set if not visited
              if (!visited.has(neighbor)) {
                openSet.push({
                  node: neighbor,
                  fScore: f
                });
              }
            }
          }
        }
      }
    }
  }
  // Check if target is reachable
  const targetGScore = gScore.get(target);
  if (targetGScore === Infinity) {
    return Option.none(); // No path exists
  }
  // Reconstruct path
  const path = [];
  const edgeWeights = [];
  let currentNode = target;
  while (currentNode !== null) {
    path.unshift(currentNode);
    const prev = previous.get(currentNode);
    if (prev !== null) {
      edgeWeights.unshift(prev.edgeData);
      currentNode = prev.node;
    } else {
      currentNode = null;
    }
  }
  return Option.some({
    path,
    distance: targetGScore,
    edgeWeights
  });
};
/**
 * Find the shortest path between two nodes using Bellman-Ford algorithm.
 *
 * Bellman-Ford algorithm can handle negative edge weights and detects negative cycles.
 * It has O(VE) time complexity, slower than Dijkstra's but more versatile.
 * Returns Option.none() if a negative cycle is detected that affects the path.
 *
 * @example
 * ```ts
 * import { Graph, Option } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   const c = Graph.addNode(mutable, "C")
 *   Graph.addEdge(mutable, a, b, -1)  // Negative weight allowed
 *   Graph.addEdge(mutable, b, c, 3)
 *   Graph.addEdge(mutable, a, c, 5)
 * })
 *
 * const result = Graph.bellmanFord(graph, 0, 2, (edgeData) => edgeData)
 * if (Option.isSome(result)) {
 *   console.log(result.value.path) // [0, 1, 2] - shortest path A->B->C
 *   console.log(result.value.distance) // 2 - total distance
 * }
 * ```
 *
 * @since 3.18.0
 * @category algorithms
 */
export const bellmanFord = (graph, source, target, edgeWeight) => {
  // Validate that source and target nodes exist
  if (!graph.nodes.has(source)) {
    throw new Error(`Source node ${source} does not exist`);
  }
  if (!graph.nodes.has(target)) {
    throw new Error(`Target node ${target} does not exist`);
  }
  // Early return if source equals target
  if (source === target) {
    return Option.some({
      path: [source],
      distance: 0,
      edgeWeights: []
    });
  }
  // Initialize distances and predecessors
  const distances = new Map();
  const previous = new Map();
  // Iterate directly over node keys
  for (const node of graph.nodes.keys()) {
    distances.set(node, node === source ? 0 : Infinity);
    previous.set(node, null);
  }
  // Collect all edges for relaxation
  const edges = [];
  for (const [, edgeData] of graph.edges) {
    const weight = edgeWeight(edgeData.data);
    edges.push({
      source: edgeData.source,
      target: edgeData.target,
      weight,
      edgeData: edgeData.data
    });
  }
  // Relax edges up to V-1 times
  const nodeCount = graph.nodes.size;
  for (let i = 0; i < nodeCount - 1; i++) {
    let hasUpdate = false;
    for (const edge of edges) {
      const sourceDistance = distances.get(edge.source);
      const targetDistance = distances.get(edge.target);
      // Relaxation step
      if (sourceDistance !== Infinity && sourceDistance + edge.weight < targetDistance) {
        distances.set(edge.target, sourceDistance + edge.weight);
        previous.set(edge.target, {
          node: edge.source,
          edgeData: edge.edgeData
        });
        hasUpdate = true;
      }
    }
    // Early termination if no updates
    if (!hasUpdate) {
      break;
    }
  }
  // Check for negative cycles
  for (const edge of edges) {
    const sourceDistance = distances.get(edge.source);
    const targetDistance = distances.get(edge.target);
    if (sourceDistance !== Infinity && sourceDistance + edge.weight < targetDistance) {
      // Negative cycle detected - check if it affects the path to target
      const affectedNodes = new Set();
      const queue = [edge.target];
      while (queue.length > 0) {
        const node = queue.shift();
        if (affectedNodes.has(node)) continue;
        affectedNodes.add(node);
        // Add all nodes reachable from this node
        const adjacencyList = getMapSafe(graph.adjacency, node);
        if (Option.isSome(adjacencyList)) {
          for (const edgeIndex of adjacencyList.value) {
            const edge = getMapSafe(graph.edges, edgeIndex);
            if (Option.isSome(edge)) {
              queue.push(edge.value.target);
            }
          }
        }
      }
      // If target is affected by negative cycle, return null
      if (affectedNodes.has(target)) {
        return Option.none();
      }
    }
  }
  // Check if target is reachable
  const targetDistance = distances.get(target);
  if (targetDistance === Infinity) {
    return Option.none(); // No path exists
  }
  // Reconstruct path
  const path = [];
  const edgeWeights = [];
  let currentNode = target;
  while (currentNode !== null) {
    path.unshift(currentNode);
    const prev = previous.get(currentNode);
    if (prev !== null) {
      edgeWeights.unshift(prev.edgeData);
      currentNode = prev.node;
    } else {
      currentNode = null;
    }
  }
  return Option.some({
    path,
    distance: targetDistance,
    edgeWeights
  });
};
/**
 * Concrete class for iterables that produce [NodeIndex, NodeData] tuples.
 *
 * This class provides a common abstraction for all iterables that return node data,
 * including traversal iterators (DFS, BFS, etc.) and element iterators (nodes, externals).
 * It uses a mapEntry function pattern for flexible iteration and transformation.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   Graph.addEdge(mutable, a, b, 1)
 * })
 *
 * // Both traversal and element iterators return NodeWalker
 * const dfsNodes: Graph.NodeWalker<string> = Graph.dfs(graph, { startNodes: [0] })
 * const allNodes: Graph.NodeWalker<string> = Graph.nodes(graph)
 *
 * // Common interface for working with node iterables
 * function processNodes<N>(nodeIterable: Graph.NodeWalker<N>): Array<number> {
 *   return Array.from(Graph.indices(nodeIterable))
 * }
 *
 * // Access node data using values() or entries()
 * const nodeData = Array.from(Graph.values(dfsNodes)) // ["A", "B"]
 * const nodeEntries = Array.from(Graph.entries(allNodes)) // [[0, "A"], [1, "B"]]
 * ```
 *
 * @since 3.18.0
 * @category models
 */
export class Walker {
  // @ts-ignore
  [Symbol.iterator];
  /**
   * Visits each element and maps it to a value using the provided function.
   *
   * Takes a function that receives the index and data,
   * and returns an iterable of the mapped values. Skips elements that
   * no longer exist in the graph.
   *
   * @example
   * ```ts
   * import { Graph } from "effect"
   *
   * const graph = Graph.directed<string, number>((mutable) => {
   *   const a = Graph.addNode(mutable, "A")
   *   const b = Graph.addNode(mutable, "B")
   *   Graph.addEdge(mutable, a, b, 1)
   * })
   *
   * const dfs = Graph.dfs(graph, { startNodes: [0] })
   *
   * // Map to just the node data
   * const values = Array.from(dfs.visit((index, data) => data))
   * console.log(values) // ["A", "B"]
   *
   * // Map to custom objects
   * const custom = Array.from(dfs.visit((index, data) => ({ id: index, name: data })))
   * console.log(custom) // [{ id: 0, name: "A" }, { id: 1, name: "B" }]
   * ```
   *
   * @since 3.18.0
   * @category iterators
   */
  visit;
  constructor(
  /**
   * Visits each element and maps it to a value using the provided function.
   *
   * Takes a function that receives the index and data,
   * and returns an iterable of the mapped values. Skips elements that
   * no longer exist in the graph.
   *
   * @example
   * ```ts
   * import { Graph } from "effect"
   *
   * const graph = Graph.directed<string, number>((mutable) => {
   *   const a = Graph.addNode(mutable, "A")
   *   const b = Graph.addNode(mutable, "B")
   *   Graph.addEdge(mutable, a, b, 1)
   * })
   *
   * const dfs = Graph.dfs(graph, { startNodes: [0] })
   *
   * // Map to just the node data
   * const values = Array.from(dfs.visit((index, data) => data))
   * console.log(values) // ["A", "B"]
   *
   * // Map to custom objects
   * const custom = Array.from(dfs.visit((index, data) => ({ id: index, name: data })))
   * console.log(custom) // [{ id: 0, name: "A" }, { id: 1, name: "B" }]
   * ```
   *
   * @since 3.18.0
   * @category iterators
   */
  visit) {
    this.visit = visit;
    this[Symbol.iterator] = visit((index, data) => [index, data])[Symbol.iterator];
  }
}
/**
 * Returns an iterator over the indices in the walker.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   Graph.addEdge(mutable, a, b, 1)
 * })
 *
 * const dfs = Graph.dfs(graph, { startNodes: [0] })
 * const indices = Array.from(Graph.indices(dfs))
 * console.log(indices) // [0, 1]
 * ```
 *
 * @since 3.18.0
 * @category utilities
 */
export const indices = walker => walker.visit((index, _) => index);
/**
 * Returns an iterator over the values (data) in the walker.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   Graph.addEdge(mutable, a, b, 1)
 * })
 *
 * const dfs = Graph.dfs(graph, { startNodes: [0] })
 * const values = Array.from(Graph.values(dfs))
 * console.log(values) // ["A", "B"]
 * ```
 *
 * @since 3.18.0
 * @category utilities
 */
export const values = walker => walker.visit((_, data) => data);
/**
 * Returns an iterator over [index, data] entries in the walker.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   Graph.addEdge(mutable, a, b, 1)
 * })
 *
 * const dfs = Graph.dfs(graph, { startNodes: [0] })
 * const entries = Array.from(Graph.entries(dfs))
 * console.log(entries) // [[0, "A"], [1, "B"]]
 * ```
 *
 * @since 3.18.0
 * @category utilities
 */
export const entries = walker => walker.visit((index, data) => [index, data]);
/**
 * Creates a new DFS iterator with optional configuration.
 *
 * The iterator maintains a stack of nodes to visit and tracks discovered nodes.
 * It provides lazy evaluation of the depth-first search.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   const c = Graph.addNode(mutable, "C")
 *   Graph.addEdge(mutable, a, b, 1)
 *   Graph.addEdge(mutable, b, c, 1)
 * })
 *
 * // Start from a specific node
 * const dfs1 = Graph.dfs(graph, { startNodes: [0] })
 * for (const nodeIndex of Graph.indices(dfs1)) {
 *   console.log(nodeIndex) // Traverses in DFS order: 0, 1, 2
 * }
 *
 * // Empty iterator (no starting nodes)
 * const dfs2 = Graph.dfs(graph)
 * // Can be used programmatically
 * ```
 *
 * @since 3.18.0
 * @category iterators
 */
export const dfs = (graph, config = {}) => {
  const startNodes = config.startNodes ?? [];
  const direction = config.direction ?? "outgoing";
  // Validate that all start nodes exist
  for (const nodeIndex of startNodes) {
    if (!hasNode(graph, nodeIndex)) {
      throw new Error(`Start node ${nodeIndex} does not exist`);
    }
  }
  return new Walker(f => ({
    [Symbol.iterator]: () => {
      const stack = [...startNodes];
      const discovered = new Set();
      const nextMapped = () => {
        while (stack.length > 0) {
          const current = stack.pop();
          if (discovered.has(current)) {
            continue;
          }
          discovered.add(current);
          const nodeDataOption = getMapSafe(graph.nodes, current);
          if (Option.isNone(nodeDataOption)) {
            continue;
          }
          const neighbors = neighborsDirected(graph, current, direction);
          for (let i = neighbors.length - 1; i >= 0; i--) {
            const neighbor = neighbors[i];
            if (!discovered.has(neighbor)) {
              stack.push(neighbor);
            }
          }
          return {
            done: false,
            value: f(current, nodeDataOption.value)
          };
        }
        return {
          done: true,
          value: undefined
        };
      };
      return {
        next: nextMapped
      };
    }
  }));
};
/**
 * Creates a new BFS iterator with optional configuration.
 *
 * The iterator maintains a queue of nodes to visit and tracks discovered nodes.
 * It provides lazy evaluation of the breadth-first search.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   const c = Graph.addNode(mutable, "C")
 *   Graph.addEdge(mutable, a, b, 1)
 *   Graph.addEdge(mutable, b, c, 1)
 * })
 *
 * // Start from a specific node
 * const bfs1 = Graph.bfs(graph, { startNodes: [0] })
 * for (const nodeIndex of Graph.indices(bfs1)) {
 *   console.log(nodeIndex) // Traverses in BFS order: 0, 1, 2
 * }
 *
 * // Empty iterator (no starting nodes)
 * const bfs2 = Graph.bfs(graph)
 * // Can be used programmatically
 * ```
 *
 * @since 3.18.0
 * @category iterators
 */
export const bfs = (graph, config = {}) => {
  const startNodes = config.startNodes ?? [];
  const direction = config.direction ?? "outgoing";
  // Validate that all start nodes exist
  for (const nodeIndex of startNodes) {
    if (!hasNode(graph, nodeIndex)) {
      throw new Error(`Start node ${nodeIndex} does not exist`);
    }
  }
  return new Walker(f => ({
    [Symbol.iterator]: () => {
      const queue = [...startNodes];
      const discovered = new Set();
      const nextMapped = () => {
        while (queue.length > 0) {
          const current = queue.shift();
          if (!discovered.has(current)) {
            discovered.add(current);
            const neighbors = neighborsDirected(graph, current, direction);
            for (const neighbor of neighbors) {
              if (!discovered.has(neighbor)) {
                queue.push(neighbor);
              }
            }
            const nodeData = getNode(graph, current);
            if (Option.isSome(nodeData)) {
              return {
                done: false,
                value: f(current, nodeData.value)
              };
            }
            return nextMapped();
          }
        }
        return {
          done: true,
          value: undefined
        };
      };
      return {
        next: nextMapped
      };
    }
  }));
};
/**
 * Creates a new topological sort iterator with optional configuration.
 *
 * The iterator uses Kahn's algorithm to lazily produce nodes in topological order.
 * Throws an error if the graph contains cycles.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   const c = Graph.addNode(mutable, "C")
 *   Graph.addEdge(mutable, a, b, 1)
 *   Graph.addEdge(mutable, b, c, 1)
 * })
 *
 * // Standard topological sort
 * const topo1 = Graph.topo(graph)
 * for (const nodeIndex of Graph.indices(topo1)) {
 *   console.log(nodeIndex) // 0, 1, 2 (topological order)
 * }
 *
 * // With initial nodes
 * const topo2 = Graph.topo(graph, { initials: [0] })
 *
 * // Throws error for cyclic graph
 * const cyclicGraph = Graph.directed<string, number>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   Graph.addEdge(mutable, a, b, 1)
 *   Graph.addEdge(mutable, b, a, 2) // Creates cycle
 * })
 *
 * try {
 *   Graph.topo(cyclicGraph) // Throws: "Cannot perform topological sort on cyclic graph"
 * } catch (error) {
 *   console.log((error as Error).message)
 * }
 * ```
 *
 * @since 3.18.0
 * @category iterators
 */
export const topo = (graph, config = {}) => {
  // Check if graph is acyclic first
  if (!isAcyclic(graph)) {
    throw new Error("Cannot perform topological sort on cyclic graph");
  }
  const initials = config.initials ?? [];
  // Validate that all initial nodes exist
  for (const nodeIndex of initials) {
    if (!hasNode(graph, nodeIndex)) {
      throw new Error(`Initial node ${nodeIndex} does not exist`);
    }
  }
  return new Walker(f => ({
    [Symbol.iterator]: () => {
      const inDegree = new Map();
      const remaining = new Set();
      const queue = [...initials];
      // Initialize in-degree counts
      for (const [nodeIndex] of graph.nodes) {
        inDegree.set(nodeIndex, 0);
        remaining.add(nodeIndex);
      }
      // Calculate in-degrees
      for (const [, edgeData] of graph.edges) {
        const currentInDegree = inDegree.get(edgeData.target) || 0;
        inDegree.set(edgeData.target, currentInDegree + 1);
      }
      // Add nodes with zero in-degree to queue if no initials provided
      if (initials.length === 0) {
        for (const [nodeIndex, degree] of inDegree) {
          if (degree === 0) {
            queue.push(nodeIndex);
          }
        }
      }
      const nextMapped = () => {
        while (queue.length > 0) {
          const current = queue.shift();
          if (remaining.has(current)) {
            remaining.delete(current);
            // Process outgoing edges, reducing in-degree of targets
            const neighbors = neighborsDirected(graph, current, "outgoing");
            for (const neighbor of neighbors) {
              if (remaining.has(neighbor)) {
                const currentInDegree = inDegree.get(neighbor) || 0;
                const newInDegree = currentInDegree - 1;
                inDegree.set(neighbor, newInDegree);
                // If in-degree becomes 0, add to queue
                if (newInDegree === 0) {
                  queue.push(neighbor);
                }
              }
            }
            const nodeData = getNode(graph, current);
            if (Option.isSome(nodeData)) {
              return {
                done: false,
                value: f(current, nodeData.value)
              };
            }
            return nextMapped();
          }
        }
        return {
          done: true,
          value: undefined
        };
      };
      return {
        next: nextMapped
      };
    }
  }));
};
/**
 * Creates a new DFS postorder iterator with optional configuration.
 *
 * The iterator maintains a stack with visit state tracking and emits nodes
 * in postorder (after all descendants have been processed). Essential for
 * dependency resolution and tree destruction algorithms.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   const root = Graph.addNode(mutable, "root")
 *   const child1 = Graph.addNode(mutable, "child1")
 *   const child2 = Graph.addNode(mutable, "child2")
 *   Graph.addEdge(mutable, root, child1, 1)
 *   Graph.addEdge(mutable, root, child2, 1)
 * })
 *
 * // Postorder: children before parents
 * const postOrder = Graph.dfsPostOrder(graph, { startNodes: [0] })
 * for (const node of postOrder) {
 *   console.log(node) // 1, 2, 0
 * }
 * ```
 *
 * @since 3.18.0
 * @category iterators
 */
export const dfsPostOrder = (graph, config = {}) => {
  const startNodes = config.startNodes ?? [];
  const direction = config.direction ?? "outgoing";
  // Validate that all start nodes exist
  for (const nodeIndex of startNodes) {
    if (!hasNode(graph, nodeIndex)) {
      throw new Error(`Start node ${nodeIndex} does not exist`);
    }
  }
  return new Walker(f => ({
    [Symbol.iterator]: () => {
      const stack = [];
      const discovered = new Set();
      const finished = new Set();
      // Initialize stack with start nodes
      for (let i = startNodes.length - 1; i >= 0; i--) {
        stack.push({
          node: startNodes[i],
          visitedChildren: false
        });
      }
      const nextMapped = () => {
        while (stack.length > 0) {
          const current = stack[stack.length - 1];
          if (!discovered.has(current.node)) {
            discovered.add(current.node);
            current.visitedChildren = false;
          }
          if (!current.visitedChildren) {
            current.visitedChildren = true;
            const neighbors = neighborsDirected(graph, current.node, direction);
            for (let i = neighbors.length - 1; i >= 0; i--) {
              const neighbor = neighbors[i];
              if (!discovered.has(neighbor) && !finished.has(neighbor)) {
                stack.push({
                  node: neighbor,
                  visitedChildren: false
                });
              }
            }
          } else {
            const nodeToEmit = stack.pop().node;
            if (!finished.has(nodeToEmit)) {
              finished.add(nodeToEmit);
              const nodeData = getNode(graph, nodeToEmit);
              if (Option.isSome(nodeData)) {
                return {
                  done: false,
                  value: f(nodeToEmit, nodeData.value)
                };
              }
              return nextMapped();
            }
          }
        }
        return {
          done: true,
          value: undefined
        };
      };
      return {
        next: nextMapped
      };
    }
  }));
};
/**
 * Creates an iterator over all node indices in the graph.
 *
 * The iterator produces node indices in the order they were added to the graph.
 * This provides access to all nodes regardless of connectivity.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   const c = Graph.addNode(mutable, "C")
 *   Graph.addEdge(mutable, a, b, 1)
 * })
 *
 * const indices = Array.from(Graph.indices(Graph.nodes(graph)))
 * console.log(indices) // [0, 1, 2]
 * ```
 *
 * @since 3.18.0
 * @category iterators
 */
export const nodes = graph => new Walker(f => ({
  [Symbol.iterator]() {
    const nodeMap = graph.nodes;
    const iterator = nodeMap.entries();
    return {
      next() {
        const result = iterator.next();
        if (result.done) {
          return {
            done: true,
            value: undefined
          };
        }
        const [nodeIndex, nodeData] = result.value;
        return {
          done: false,
          value: f(nodeIndex, nodeData)
        };
      }
    };
  }
}));
/**
 * Creates an iterator over all edge indices in the graph.
 *
 * The iterator produces edge indices in the order they were added to the graph.
 * This provides access to all edges regardless of connectivity.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   const a = Graph.addNode(mutable, "A")
 *   const b = Graph.addNode(mutable, "B")
 *   const c = Graph.addNode(mutable, "C")
 *   Graph.addEdge(mutable, a, b, 1)
 *   Graph.addEdge(mutable, b, c, 2)
 * })
 *
 * const indices = Array.from(Graph.indices(Graph.edges(graph)))
 * console.log(indices) // [0, 1]
 * ```
 *
 * @since 3.18.0
 * @category iterators
 */
export const edges = graph => new Walker(f => ({
  [Symbol.iterator]() {
    const edgeMap = graph.edges;
    const iterator = edgeMap.entries();
    return {
      next() {
        const result = iterator.next();
        if (result.done) {
          return {
            done: true,
            value: undefined
          };
        }
        const [edgeIndex, edgeData] = result.value;
        return {
          done: false,
          value: f(edgeIndex, edgeData)
        };
      }
    };
  }
}));
/**
 * Creates an iterator over external nodes (nodes without edges in specified direction).
 *
 * External nodes are nodes that have no outgoing edges (direction="outgoing") or
 * no incoming edges (direction="incoming"). These are useful for finding
 * sources, sinks, or isolated nodes.
 *
 * @example
 * ```ts
 * import { Graph } from "effect"
 *
 * const graph = Graph.directed<string, number>((mutable) => {
 *   const source = Graph.addNode(mutable, "source")     // 0 - no incoming
 *   const middle = Graph.addNode(mutable, "middle")     // 1 - has both
 *   const sink = Graph.addNode(mutable, "sink")         // 2 - no outgoing
 *   const isolated = Graph.addNode(mutable, "isolated") // 3 - no edges
 *
 *   Graph.addEdge(mutable, source, middle, 1)
 *   Graph.addEdge(mutable, middle, sink, 2)
 * })
 *
 * // Nodes with no outgoing edges (sinks + isolated)
 * const sinks = Array.from(Graph.indices(Graph.externals(graph, { direction: "outgoing" })))
 * console.log(sinks) // [2, 3]
 *
 * // Nodes with no incoming edges (sources + isolated)
 * const sources = Array.from(Graph.indices(Graph.externals(graph, { direction: "incoming" })))
 * console.log(sources) // [0, 3]
 * ```
 *
 * @since 3.18.0
 * @category iterators
 */
export const externals = (graph, config = {}) => {
  const direction = config.direction ?? "outgoing";
  return new Walker(f => ({
    [Symbol.iterator]: () => {
      const nodeMap = graph.nodes;
      const adjacencyMap = direction === "incoming" ? graph.reverseAdjacency : graph.adjacency;
      const nodeIterator = nodeMap.entries();
      const nextMapped = () => {
        let current = nodeIterator.next();
        while (!current.done) {
          const [nodeIndex, nodeData] = current.value;
          const adjacencyList = getMapSafe(adjacencyMap, nodeIndex);
          // Node is external if it has no edges in the specified direction
          if (Option.isNone(adjacencyList) || adjacencyList.value.length === 0) {
            return {
              done: false,
              value: f(nodeIndex, nodeData)
            };
          }
          current = nodeIterator.next();
        }
        return {
          done: true,
          value: undefined
        };
      };
      return {
        next: nextMapped
      };
    }
  }));
};
//# sourceMappingURL=Graph.js.map