// An in-memory stand-in for the CMS: just enough of /api/edit, /api/nodes and
// /api/schemas to develop and test the editor with no server and no login.
// Semantics follow docs/onpage-editor.md (atomic batches, inverse ops, 409 on a
// stale version, patches = re-rendered subtrees). It is deliberately small - when
// real behaviour differs, fix the mock, then the editor.
export interface MNode { id: string; dataType: string; data: Record<string, any>; version: number; tags: string[] }
export interface MEdge { id: string; from: string; to: string; type: string; handle?: string; sort: number }

export class HttpError extends Error {
  constructor(readonly status: number, message: string) {
    super(message)
  }
}

type Op = Record<string, any>

export class Graph {
  nodes = new Map<string, MNode>()
  edges = new Map<string, MEdge>()
  private seq = 100

  constructor(seed?: { nodes: Omit<MNode, 'version' | 'tags'>[]; edges: Omit<MEdge, 'type'>[] }) {
    if (seed) {
      for (const n of seed.nodes) this.nodes.set(n.id, { version: 1, tags: [], ...n })
      for (const e of seed.edges) this.edges.set(e.id, { type: 'child', ...e })
    }
  }

  private id(p: string) {
    return `${p}${++this.seq}`
  }
  node(id: string): MNode {
    const n = this.nodes.get(id)
    if (!n) throw new HttpError(404, `node ${id} not found`)
    return n
  }
  edge(id: string): MEdge {
    const e = this.edges.get(id)
    if (!e) throw new HttpError(404, `edge ${id} not found`)
    return e
  }
  children(id: string): MEdge[] {
    return [...this.edges.values()].filter((e) => e.from === id && e.type === 'child').sort((a, b) => a.sort - b.sort)
  }
  parents(id: string): MEdge[] {
    return [...this.edges.values()].filter((e) => e.to === id)
  }
  edgesOf(id: string) {
    return { from: this.children(id), into: this.parents(id) }
  }

  private renumber(parent: string) {
    this.children(parent).forEach((e, i) => (e.sort = i))
  }
  /** Place edge `e` at `index` among its parent's children. */
  private place(e: MEdge, index?: number) {
    const sibs = this.children(e.from).filter((x) => x.id !== e.id)
    const at = index === undefined || index > sibs.length ? sibs.length : Math.max(0, index)
    sibs.splice(at, 0, e)
    sibs.forEach((x, i) => (x.sort = i))
  }
  private subtree(edgeId: string): { nodes: MNode[]; edges: MEdge[] } {
    const root = this.edge(edgeId)
    const nodes: MNode[] = []
    const edges: MEdge[] = [root]
    const walk = (id: string) => {
      nodes.push(this.node(id))
      for (const c of this.children(id)) {
        edges.push(c)
        walk(c.to)
      }
    }
    walk(root.to)
    return { nodes, edges }
  }

  /** Apply a batch atomically. Returns results, inverse (already reversed) and touched ids. */
  edit(ops: Op[]) {
    if (!Array.isArray(ops) || ops.length < 1 || ops.length > 200) throw new HttpError(400, 'ops must hold 1-200 operations')
    const snapshot = structuredClone({ nodes: [...this.nodes], edges: [...this.edges], seq: this.seq })
    const results: Array<{ node?: string; edge?: string }> = []
    const inverse: Op[] = []
    const touched = new Set<string>()
    try {
      ops.forEach((op, i) => {
        try {
          const { result, inv, touch } = this.applyOne(op)
          results.push(result)
          inverse.unshift(...inv)
          touch.forEach((t) => touched.add(t))
        } catch (e) {
          if (e instanceof HttpError) throw new HttpError(e.status, `op ${i} (${op.op}): ${e.message}`)
          throw e
        }
      })
    } catch (e) {
      this.nodes = new Map(snapshot.nodes)
      this.edges = new Map(snapshot.edges)
      this.seq = snapshot.seq
      throw e
    }
    return { results, inverse, touched: [...touched] }
  }

  private applyOne(op: Op): { result: { node?: string; edge?: string }; inv: Op[]; touch: string[] } {
    switch (op.op) {
      case 'insert': {
        this.node(op.parent)
        if (!op.type) throw new HttpError(400, 'type required')
        const node: MNode = { id: op.id ?? this.id('n'), dataType: op.type, data: { ...(op.data ?? {}) }, version: 1, tags: [] }
        this.nodes.set(node.id, node)
        const edge: MEdge = { id: this.id('e'), from: op.parent, to: node.id, type: 'child', handle: op.handle, sort: 0 }
        this.edges.set(edge.id, edge)
        this.place(edge, op.index)
        return { result: { node: node.id, edge: edge.id }, inv: [{ op: 'delete', edge: edge.id }], touch: [op.parent] }
      }
      case 'set': {
        const n = this.node(op.id)
        if (op.version !== undefined && op.version !== n.version) throw new HttpError(409, `stale version (have ${n.version}, got ${op.version})`)
        const undo: Record<string, unknown> = {}
        for (const [k, v] of Object.entries<any>(op.data ?? {})) {
          if (['lastModified', 'status'].includes(k)) throw new HttpError(400, `lifecycle key ${k} cannot be set`)
          undo[k] = k in n.data ? n.data[k] : null
          if (v === null) delete n.data[k]
          else n.data[k] = v
        }
        n.version++
        return { result: { node: n.id }, inv: [{ op: 'set', id: n.id, data: undo }], touch: [n.id] }
      }
      case 'move': {
        const e = this.edge(op.edge)
        const before = { parent: e.from, handle: e.handle, index: this.children(e.from).findIndex((x) => x.id === e.id) }
        const to = op.parent ?? e.from
        if (this.isDescendant(to, e.to)) throw new HttpError(400, 'move would create a cycle')
        const oldParent = e.from
        e.from = to
        if ('handle' in op) e.handle = op.handle
        this.renumber(oldParent)
        this.place(e, op.index)
        return {
          result: { edge: e.id },
          inv: [{ op: 'move', edge: e.id, parent: before.parent, handle: before.handle, index: before.index }],
          touch: [...new Set([oldParent, to])],
        }
      }
      case 'delete': {
        const e = this.edge(op.edge)
        const snap = this.subtree(e.id)
        for (const x of snap.edges) this.edges.delete(x.id)
        // Subtree nodes survive only if still composed elsewhere (shared blocks).
        for (const n of snap.nodes) if (this.parents(n.id).length === 0) this.nodes.delete(n.id)
        this.renumber(e.from)
        return { result: {}, inv: [{ op: 'restore', edge: e.id, nodes: structuredClone(snap.nodes), edges: structuredClone(snap.edges) }], touch: [e.from] }
      }
      case 'restore': {
        for (const n of op.nodes ?? []) this.nodes.set(n.id, structuredClone(n))
        for (const e of op.edges ?? []) this.edges.set(e.id, structuredClone(e))
        const root = this.edge(op.edge)
        this.place(root, root.sort)
        return { result: { edge: root.id, node: root.to }, inv: [{ op: 'delete', edge: root.id }], touch: [root.from] }
      }
      case 'graft': {
        // Link an existing node (tree.link) or create a subtree. The editor uses `tree: { link }` to attach
        // searched nodes into slots.
        if (op.tree?.link) {
          this.node(op.parent)
          const target = this.node(op.tree.link)
          const edge: MEdge = { id: this.id('e'), from: op.parent, to: target.id, type: 'child', handle: op.handle ?? op.tree.handle, sort: 0 }
          this.edges.set(edge.id, edge)
          this.place(edge, op.index)
          return { result: { node: target.id, edge: edge.id }, inv: [{ op: 'delete', edge: edge.id }], touch: [op.parent] }
        }
        throw new HttpError(400, 'graft without tree.link is not implemented by the mock')
      }
      case 'duplicate': {
        const e = this.edge(op.edge)
        const map = new Map<string, string>()
        const clone = (id: string): string => {
          const src = this.node(id)
          const copy: MNode = { ...structuredClone(src), id: this.id('n'), version: 1 }
          this.nodes.set(copy.id, copy)
          map.set(id, copy.id)
          for (const c of this.children(id)) {
            const ce: MEdge = { ...c, id: this.id('e'), from: copy.id, to: clone(c.to) }
            this.edges.set(ce.id, ce)
          }
          return copy.id
        }
        const nid = clone(e.to)
        const ne: MEdge = { ...e, id: this.id('e'), to: nid }
        this.edges.set(ne.id, ne)
        this.place(ne, e.sort + 1)
        return { result: { node: nid, edge: ne.id }, inv: [{ op: 'delete', edge: ne.id }], touch: [e.from] }
      }
      default:
        throw new HttpError(400, `op "${op.op}" is not implemented by the mock`)
    }
  }

  private isDescendant(id: string, ancestor: string): boolean {
    if (id === ancestor) return true
    return this.children(ancestor).some((c) => this.isDescendant(id, c.to))
  }

  /** Root = a node nobody composes. */
  roots(): MNode[] {
    return [...this.nodes.values()].filter((n) => this.parents(n.id).length === 0)
  }
}