import { markRaw, reactive, shallowReactive, shallowRef } from "vue";
import type {
  OriginAction,
  OriginHistoryMode,
  OriginView,
} from "../navigation";
import { clamp01, type OriginRect } from "../shared";
import type {
  MutableOriginOperation,
  OriginContext,
  OriginFinishOptions,
  OriginSceneNodeState,
} from "./types";

/** Mutable storage for one mounted scene node. */
export interface SceneNodeRecord {
  key: string;
  sequence: number;
  view: OriginView;
  /** Retained-history link: the entry a committed back returns to. */
  previousNodeKey?: string;
  state: OriginSceneNodeState;
  /** Visual link: the operation edge positioning this node as its target. */
  incomingOperationId?: number;
}

/**
 * Why a scene operation was rejected.
 *
 * - `missing-node`: the node no longer exists.
 * - `missing-origin`: the origin node does not exist.
 * - `inert-origin`: the origin is parked or exposed and cannot originate.
 * - `origin-busy`: the origin already has a live outgoing operation.
 * - `invalid-action`: the action cannot be executed as written.
 * - `no-previous-entry`: back was requested without a retained entry.
 * - `undecided-forward`: back was requested through an unreleased forward.
 * - `target-busy`: the retained back target is transitioning, or still
 *   positions the origin.
 * - `outside-scene`: `useOrigin()` was called outside an `<OriginScene>` view.
 */
export type OriginSceneErrorCode =
  | "missing-node"
  | "missing-origin"
  | "inert-origin"
  | "origin-busy"
  | "invalid-action"
  | "no-previous-entry"
  | "undecided-forward"
  | "target-busy"
  | "outside-scene";

/** Error thrown or rejected by scene operations, with a stable `code`. */
export class OriginSceneError extends Error {
  override readonly name = "OriginSceneError";

  constructor(
    readonly code: OriginSceneErrorCode,
    message: string,
  ) {
    super(message);
  }
}

/**
 * Progress at which a release commits when the choreography does not supply
 * its own `commitThreshold`.
 */
export const DEFAULT_COMMIT_THRESHOLD = 0.36;

/**
 * Release velocity (progress units per second) that commits a flick at any
 * progress when the choreography does not supply its own `commitVelocity`.
 */
export const DEFAULT_COMMIT_VELOCITY = 0.3;

/** Decide a release outcome from explicit options or progress/velocity. */
export const shouldCommit = (
  operation: MutableOriginOperation,
  options: OriginFinishOptions,
): boolean => {
  if (options.commit !== undefined) return options.commit;
  const { commitThreshold, commitVelocity } = operation.choreography;
  return (
    operation.progress >= (commitThreshold ?? DEFAULT_COMMIT_THRESHOLD) ||
    operation.velocity >= (commitVelocity ?? DEFAULT_COMMIT_VELOCITY)
  );
};

/**
 * The scene model: nodes, operation edges, and the rules that rewrite them.
 *
 * Two independent structures share these records:
 *
 * - **History** links each node to its retained previous entry through
 *   `previousNodeKey`. It changes only when operations commit or cancel.
 * - **Visual graph** links each target to the operation that positions it
 *   through `incomingOperationId`; each operation links back to its origin.
 *   Nodes without an incoming edge are listed in `roots`.
 *
 * Nodes are stored flat so Vue never re-parents a component when edges are
 * added, spliced, or removed. Every method is synchronous; timing (mounting,
 * measuring, settling) belongs to `Scene`.
 */
export class SceneGraph {
  readonly nodes = shallowReactive(new Map<string, SceneNodeRecord>());
  readonly operations = shallowReactive(
    new Map<number, MutableOriginOperation>(),
  );
  readonly roots = shallowRef<string[]>([]);
  /**
   * Connected resting edges temporarily removed by a back operation, keyed by
   * that back operation's id. Cancelling the back operation restores the edge.
   */
  private readonly suspendedEdges = new Map<number, MutableOriginOperation>();
  /**
   * Operations indexed by origin key, in insertion order. Every addition and
   * removal goes through {@link attach} and {@link detach} to keep it in sync
   * with `operations`.
   */
  private readonly outgoing = shallowReactive(
    new Map<string, readonly MutableOriginOperation[]>(),
  );
  private nodeSequence = 0;
  private operationSequence = 0;

  constructor(initial: readonly OriginView[]) {
    this.roots.value = initial.map((view) => this.createNode(view).key);
  }

  // Queries -----------------------------------------------------------------

  node(key: string): SceneNodeRecord | undefined {
    return this.nodes.get(key);
  }

  operation(id: number): MutableOriginOperation | undefined {
    return this.operations.get(id);
  }

  /** The edge positioning `node` as its target, including resting edges. */
  incomingOf(node: SceneNodeRecord): MutableOriginOperation | undefined {
    return node.incomingOperationId
      ? this.operations.get(node.incomingOperationId)
      : undefined;
  }

  /** Incoming edges positioning `node`, from the node outward to its root. */
  incomingChain(node: SceneNodeRecord): MutableOriginOperation[] {
    const chain: MutableOriginOperation[] = [];
    const visited = new Set<string>();
    let current: SceneNodeRecord | undefined = node;
    while (current && !visited.has(current.key)) {
      visited.add(current.key);
      const incoming = this.incomingOf(current);
      if (!incoming) break;
      chain.push(incoming);
      current = this.nodes.get(incoming.originKey);
    }
    return chain;
  }

  /** The unfinished operation originated by a node, if any. */
  liveOutgoingOf(nodeKey: string): MutableOriginOperation | undefined {
    return this.outgoing
      .get(nodeKey)
      ?.find((operation) => operation.phase !== "finished");
  }

  /**
   * The operation whose source effects apply to a node. Unlike
   * {@link liveOutgoingOf}, this includes a finished, connected resting edge.
   */
  presentedOutgoingOf(nodeKey: string): MutableOriginOperation | undefined {
    return this.outgoing.get(nodeKey)?.[0];
  }

  /** Retained entries preceding `node`, oldest first. */
  historyOf(node: SceneNodeRecord): SceneNodeRecord[] {
    const history: SceneNodeRecord[] = [];
    const visited = new Set<string>([node.key]);
    let previousKey = node.previousNodeKey;
    while (previousKey && !visited.has(previousKey)) {
      visited.add(previousKey);
      const previous = this.nodes.get(previousKey);
      if (!previous) break;
      history.push(previous);
      previousKey = previous.previousNodeKey;
    }
    return history.reverse();
  }

  contextFor(nodeKey: string): OriginContext {
    const node = this.nodes.get(nodeKey);
    if (!node)
      throw new OriginSceneError(
        "missing-node",
        `Origin scene node "${nodeKey}" no longer exists.`,
      );
    const history = this.historyOf(node).map((entry) => entry.view);
    return {
      nodeKey,
      view: node.view,
      canGoBack: Boolean(node.previousNodeKey),
      previous: history.at(-1),
      history,
    };
  }

  /**
   * Whether a node should currently receive input.
   *
   * Parked and exposed entries are inert. A target being cancelled is already
   * scheduled to disappear, and a source settling toward commit has ceded new
   * interactions to its target. This is node-local fate, not a global "active
   * view".
   */
  isInteractive(nodeKey: string): boolean {
    const node = this.nodes.get(nodeKey);
    if (!node || node.state === "parked" || node.state === "exposed")
      return false;
    if (this.incomingOf(node)?.intent === "cancel") return false;
    return this.liveOutgoingOf(nodeKey)?.intent !== "commit";
  }

  // Transitions -------------------------------------------------------------

  /**
   * Validate an action and add its operation edge (and, for push/replace, its
   * target node). The edge starts in the `preparing` phase.
   *
   * @throws If the origin cannot originate, or the action cannot be resolved.
   */
  begin(
    originKey: string,
    action: OriginAction,
    originRect: OriginRect | undefined,
  ): MutableOriginOperation {
    const origin = this.assertCanOriginate(originKey);
    const history = action.history ?? "push";
    if (action.choreography.persistAtRest && history !== "push")
      throw new OriginSceneError(
        "invalid-action",
        "persistAtRest choreographies require retained-history push navigation.",
      );

    const { target, suspendedEdge } =
      history === "back"
        ? this.resolveBackTarget(origin)
        : { target: this.createForwardTarget(origin, action, history) };

    const operation = this.createOperation({
      originKey,
      targetKey: target.key,
      entryTargetKey: target.key,
      choreography: action.choreography,
      placement: action.placement ?? "above",
      history,
      originRect,
    });
    target.incomingOperationId = operation.id;
    target.state = "transitioning";
    origin.state = "transitioning";
    if (suspendedEdge) this.suspendedEdges.set(operation.id, suspendedEdge);
    return operation;
  }

  /** Apply interactive progress to an unfinished operation. */
  update(id: number, progress: number, velocity: number): void {
    const operation = this.operations.get(id);
    if (!operation || operation.phase === "finished") return;
    operation.progress = clamp01(progress);
    operation.velocity = velocity;
    if (operation.phase !== "preparing") operation.phase = "interactive";
  }

  /**
   * Finalize a committed operation.
   *
   * A normal push parks its origin and collapses the temporary visual edge, so
   * the target takes the origin's place in the visual graph. A connected
   * (`persistAtRest`) push keeps the edge at progress 1 and leaves the origin
   * exposed but inert. Replace and back remove the origin.
   *
   * @returns Whether the operation's nodes still existed to be committed.
   */
  commit(id: number): boolean {
    const operation = this.operations.get(id);
    if (!operation) return false;
    const origin = this.nodes.get(operation.originKey);
    const target = this.nodes.get(operation.targetKey);
    if (!origin || !target) return false;

    if (operation.history === "push" && operation.choreography.persistAtRest) {
      operation.progress = 1;
      operation.velocity = 0;
      operation.intent = "commit";
      operation.phase = "finished";
      origin.state = "exposed";
      this.activate(target);
      return true;
    }

    this.transferPosition(origin, target);
    operation.phase = "finished";
    this.detach(operation);
    this.suspendedEdges.delete(operation.id);

    if (operation.history === "push") {
      origin.incomingOperationId = undefined;
      origin.state = "parked";
    } else {
      // Back pops the current entry; replace discards it in favor of the new
      // target. Both preserve every earlier retained instance, and any entry
      // still retained above the origin now sits on the target instead.
      for (const node of this.nodes.values())
        if (node.previousNodeKey === origin.key && node !== target)
          node.previousNodeKey = target.key;
      this.removeNode(origin.key);
    }

    this.activate(target);
    return true;
  }

  /**
   * Finalize a cancelled operation, restoring the origin.
   *
   * Everything the edge created or now positions is undone. Entries from the
   * origin's retained history (a back target, or a replacement of one) return
   * to history; anything newer is discarded. A cancelled back that suspended
   * a connected resting edge reconnects it instead.
   */
  cancel(id: number): void {
    const operation = this.operations.get(id);
    if (!operation) return;
    const origin = this.nodes.get(operation.originKey);
    const entry = this.nodes.get(operation.entryTargetKey);
    const current = this.nodes.get(operation.targetKey);
    const suspendedEdge = this.suspendedEdges.get(id);
    const retained = origin ? this.retainedKeys(origin) : new Set<string>();
    operation.phase = "finished";
    this.detach(operation);
    this.suspendedEdges.delete(id);

    if (suspendedEdge && origin && entry && current === entry) {
      // Put the retained page back into its former position, then reconnect
      // the resting page→drawer edge. Both nodes keep their VNodes.
      this.discardOutgoing(entry.key, new Set(retained));
      this.transferPosition(origin, entry);
      this.attach(suspendedEdge);
      origin.incomingOperationId = suspendedEdge.id;
      entry.state = "exposed";
    } else {
      for (const node of new Set([entry, current])) {
        if (!node || node === origin) continue;
        if (retained.has(node.key)) this.returnToHistory(node, retained);
        else this.removeBranch(node.key, new Set(retained));
      }
    }

    if (origin) this.activate(origin);
  }

  // Transition steps --------------------------------------------------------

  private assertCanOriginate(originKey: string): SceneNodeRecord {
    const origin = this.nodes.get(originKey);
    if (!origin)
      throw new OriginSceneError(
        "missing-origin",
        `Cannot animate from missing origin "${originKey}".`,
      );
    if (origin.state === "parked" || origin.state === "exposed")
      throw new OriginSceneError(
        "inert-origin",
        `Cannot animate from ${origin.state} origin "${originKey}".`,
      );
    if (this.liveOutgoingOf(originKey))
      throw new OriginSceneError(
        "origin-busy",
        `Origin "${originKey}" already has an outgoing operation. ` +
          "A descendant target may start its own operation instead.",
      );
    return origin;
  }

  /**
   * Resolve the retained previous entry for a back operation.
   *
   * If the origin's incoming edge comes from that previous entry (a forward
   * push still settling, or a connected resting edge), the edge is collapsed
   * first. Creating a new edge back to its origin would otherwise form X→Y→X.
   * Reciprocal choreographies begin at the same visual endpoints, so this
   * handoff is continuous without remounting either node.
   */
  private resolveBackTarget(origin: SceneNodeRecord): {
    target: SceneNodeRecord;
    suspendedEdge?: MutableOriginOperation;
  } {
    const previousKey = origin.previousNodeKey;
    if (!previousKey)
      throw new OriginSceneError(
        "no-previous-entry",
        `Cannot go back from origin "${origin.key}" without a previous entry.`,
      );
    const target = this.nodes.get(previousKey);
    if (!target)
      throw new OriginSceneError(
        "no-previous-entry",
        `Cannot go back to missing retained entry "${previousKey}".`,
      );

    let suspendedEdge: MutableOriginOperation | undefined;
    const incoming = this.incomingOf(origin);
    if (incoming?.originKey === target.key) {
      if (incoming.intent !== "commit")
        throw new OriginSceneError(
          "undecided-forward",
          "Cannot go back through an undecided forward operation.",
        );
      if (incoming.phase !== "finished") this.commit(incoming.id);

      const resting = this.operations.get(incoming.id);
      if (resting?.phase === "finished" && resting.choreography.persistAtRest) {
        // Re-root A→page→drawer as A→drawer before adding drawer→page.
        this.detach(resting);
        this.transferPosition(target, origin);
        target.incomingOperationId = undefined;
        suspendedEdge = resting;
      }
    }

    // A target still positioning the origin (through edges other than the
    // one collapsed above) would close a visual cycle.
    if (
      this.incomingChain(origin).some(
        (operation) => operation.originKey === target.key,
      )
    )
      throw new OriginSceneError(
        "target-busy",
        `Retained target "${target.key}" still positions origin "${origin.key}".`,
      );
    if (this.liveOutgoingOf(target.key))
      throw new OriginSceneError(
        "target-busy",
        `Retained target "${target.key}" already has an outgoing operation.`,
      );
    if (target.incomingOperationId)
      throw new OriginSceneError(
        "target-busy",
        `Retained target "${target.key}" is already transitioning.`,
      );
    return { target, suspendedEdge };
  }

  /** Create the target node for a push or replace operation. */
  private createForwardTarget(
    origin: SceneNodeRecord,
    action: OriginAction,
    history: Exclude<OriginHistoryMode, "back">,
  ): SceneNodeRecord {
    if (!action.target)
      throw new OriginSceneError(
        "invalid-action",
        `A "${history}" operation requires a target view recipe.`,
      );
    // A replacement skips the origin in the retained chain. The origin stays
    // mounted until commit so an interactive replacement can still cancel.
    const previousKey =
      history === "push" ? origin.key : origin.previousNodeKey;
    return this.createNode(action.target, previousKey);
  }

  /**
   * Make a node presentable again. A node still holding a connected resting
   * edge (an open drawer, say) stays exposed rather than becoming active.
   */
  private activate(node: SceneNodeRecord): void {
    const resting = this.presentedOutgoingOf(node.key);
    if (resting?.phase === "finished") {
      node.state = "exposed";
      return;
    }
    node.state = "active";
    this.refreshPresentedState(node);
  }

  /**
   * Recompute whether a presented node is participating in a live operation.
   * Parked and exposed nodes keep their retained state.
   */
  private refreshPresentedState(node: SceneNodeRecord): void {
    if (node.state === "parked" || node.state === "exposed") return;
    const incoming = this.incomingOf(node);
    node.state =
      (incoming && incoming.phase !== "finished") ||
      this.liveOutgoingOf(node.key)
        ? "transitioning"
        : "active";
  }

  // Structural primitives ---------------------------------------------------

  private createNode(
    view: OriginView,
    previousNodeKey?: string,
  ): SceneNodeRecord {
    const sequence = ++this.nodeSequence;
    const node: SceneNodeRecord = shallowReactive({
      key: `${view.key ?? view.name ?? "view"}::${sequence}`,
      sequence,
      view: markRaw(view),
      previousNodeKey,
      state: "active",
      incomingOperationId: undefined,
    });
    this.nodes.set(node.key, node);
    return node;
  }

  private createOperation(
    init: Pick<
      MutableOriginOperation,
      | "originKey"
      | "targetKey"
      | "entryTargetKey"
      | "choreography"
      | "placement"
      | "history"
      | "originRect"
    >,
  ): MutableOriginOperation {
    const operation = reactive<MutableOriginOperation>({
      id: ++this.operationSequence,
      originKey: init.originKey,
      targetKey: init.targetKey,
      entryTargetKey: init.entryTargetKey,
      choreography: markRaw(init.choreography),
      placement: init.placement,
      history: init.history,
      progress: 0,
      velocity: 0,
      phase: "preparing",
      intent: "undecided",
      originRect: init.originRect,
    });
    this.attach(operation);
    return operation;
  }

  private attach(operation: MutableOriginOperation): void {
    this.operations.set(operation.id, operation);
    const siblings = this.outgoing.get(operation.originKey) ?? [];
    this.outgoing.set(operation.originKey, [...siblings, operation]);
  }

  private detach(operation: MutableOriginOperation): void {
    this.operations.delete(operation.id);
    const siblings = (this.outgoing.get(operation.originKey) ?? []).filter(
      (sibling) => sibling.id !== operation.id,
    );
    if (siblings.length) this.outgoing.set(operation.originKey, siblings);
    else this.outgoing.delete(operation.originKey);
  }

  private removeNode(nodeKey: string): void {
    this.nodes.delete(nodeKey);
    this.roots.value = this.roots.value.filter((key) => key !== nodeKey);
  }

  /**
   * Resolve the operations a node started, because the node is leaving
   * the visible graph: nodes they created (push/replace targets, including
   * replacements whose history skips this node) are removed, and retained
   * entries they revealed (back targets) are restored as if that back
   * operation were cancelled.
   */
  /** Park a retained entry again, discarding what it started meanwhile. */
  private returnToHistory(node: SceneNodeRecord, retained: Set<string>): void {
    this.discardOutgoing(node.key, new Set(retained));
    node.incomingOperationId = undefined;
    node.state = "parked";
  }

  /** A node's key plus its retained history, which a cancel must keep. */
  private retainedKeys(node: SceneNodeRecord): Set<string> {
    return new Set([node.key, ...this.historyOf(node).map(({ key }) => key)]);
  }

  private discardOutgoing(nodeKey: string, visited = new Set<string>()): void {
    for (const operation of [...(this.outgoing.get(nodeKey) ?? [])]) {
      // Includes finished resting edges: an open drawer closes with its page.
      if (operation.history === "back") {
        this.cancel(operation.id);
      } else {
        this.detach(operation);
        this.removeBranch(operation.entryTargetKey, visited);
        this.removeBranch(operation.targetKey, visited);
      }
    }
  }

  /**
   * Remove a node, every node whose retained history passes through it, and
   * every operation touching any of them. Earlier entries are untouched.
   * Operations the node itself started are discarded first.
   */
  private removeBranch(nodeKey: string, visited = new Set<string>()): void {
    if (visited.has(nodeKey)) return;
    visited.add(nodeKey);

    for (const node of [...this.nodes.values()])
      if (node.previousNodeKey === nodeKey)
        this.removeBranch(node.key, visited);
    this.discardOutgoing(nodeKey, visited);
    for (const operation of [...this.operations.values()])
      if (
        operation.originKey === nodeKey ||
        operation.targetKey === nodeKey ||
        operation.entryTargetKey === nodeKey
      )
        this.detach(operation);
    this.removeNode(nodeKey);
  }

  /**
   * Give `target` the visual-graph position currently held by `from`: either
   * as the target of `from`'s incoming edge, or as a root. Neither Vue VNode
   * moves; only edge bookkeeping changes.
   */
  private transferPosition(from: SceneNodeRecord, target: SceneNodeRecord) {
    const parent = this.incomingOf(from);
    if (parent) {
      parent.targetKey = target.key;
      target.incomingOperationId = parent.id;
    } else {
      this.roots.value = this.roots.value.map((key) =>
        key === from.key ? target.key : key,
      );
      target.incomingOperationId = undefined;
    }
  }
}