// The single chokepoint for SysMLModel mutations.
//
// Pure: (model, ops) → { model, idMapping, errors }. No DB / network here.
// M5 in-memory: tempIds are used as canonical ids when free, otherwise
// rewritten with a `_n` suffix. The DB layer (later) will replace this with
// server-issued ids.
//
// applyOps is atomic per-batch: if any op fails validation, the entire
// batch is rejected and the original model is returned unchanged. This is
// the contract docs/sync.md §4 specifies.

import type { SysMLModel, Block, Association, Constraint, Requirement } from "../sysml/model";
import type { ModelOp } from "./ops";

export interface ApplyResult {
  model: SysMLModel;
  /** Map of any tempId → assigned canonical id. */
  idMapping: Record<string, string>;
  errors: ApplyError[];
  /** True iff the batch landed without errors. */
  applied: boolean;
}

export interface ApplyError {
  opIndex: number;
  code: "DUPLICATE_ID" | "MISSING_ELEMENT" | "INVALID_PATCH";
  message: string;
}

export function applyOps(model: SysMLModel, ops: ModelOp[]): ApplyResult {
  const idMapping: Record<string, string> = {};
  const errors: ApplyError[] = [];

  // Fail-fast working copy. We mutate this; if errors accumulate we throw it
  // away and return the original.
  let next = cloneModel(model);

  for (let i = 0; i < ops.length; i++) {
    const op = ops[i]!;
    try {
      next = applyOne(next, op, idMapping);
    } catch (err) {
      errors.push({ opIndex: i, code: classifyError(err), message: (err as Error).message });
    }
  }

  if (errors.length > 0) {
    return { model, idMapping: {}, errors, applied: false };
  }
  return { model: next, idMapping, errors: [], applied: true };
}

// ─── Per-op application ──────────────────────────────────────────────────

function applyOne(model: SysMLModel, op: ModelOp, idMapping: Record<string, string>): SysMLModel {
  switch (op.kind) {
    case "add-block": {
      const id = resolveAddId(op.block.id, op.tempId, idMapping, model.blocks.map(b => b.id));
      const block: Block = { ...op.block, id };
      return { ...model, blocks: [...model.blocks, block] };
    }
    case "update-block": {
      const realId = idMapping[op.blockId] ?? op.blockId;
      assertExists(model.blocks, realId, "block");
      return {
        ...model,
        blocks: model.blocks.map(b => (b.id === realId ? { ...b, ...op.patch } : b)),
      };
    }
    case "remove-block": {
      const realId = idMapping[op.blockId] ?? op.blockId;
      assertExists(model.blocks, realId, "block");
      // Cascade: drop incident associations + requirement satisfiers + constraint applies-to
      return {
        ...model,
        blocks: model.blocks.filter(b => b.id !== realId),
        associations: model.associations.filter(a => a.fromBlockId !== realId && a.toBlockId !== realId),
        constraints: model.constraints.map(c => ({ ...c, appliesTo: c.appliesTo.filter(id => id !== realId) })),
        requirements: model.requirements.map(r => ({
          ...r,
          relations: r.relations.filter(rel => rel.kind !== "satisfy" || rel.blockId !== realId),
        })),
      };
    }

    case "add-property": {
      const blockId = idMapping[op.blockId] ?? op.blockId;
      const block = model.blocks.find(b => b.id === blockId);
      if (!block) throw missing("block", blockId);
      const propId = resolveAddId(op.property.id, op.tempId, idMapping, block.properties.map(p => p.id));
      return {
        ...model,
        blocks: model.blocks.map(b =>
          b.id === blockId ? { ...b, properties: [...b.properties, { ...op.property, id: propId }] } : b
        ),
      };
    }
    case "update-property": {
      const blockId = idMapping[op.blockId] ?? op.blockId;
      return {
        ...model,
        blocks: model.blocks.map(b =>
          b.id === blockId
            ? { ...b, properties: b.properties.map(p => (p.id === op.propertyId ? { ...p, ...op.patch } : p)) }
            : b
        ),
      };
    }
    case "remove-property": {
      const blockId = idMapping[op.blockId] ?? op.blockId;
      return {
        ...model,
        blocks: model.blocks.map(b =>
          b.id === blockId ? { ...b, properties: b.properties.filter(p => p.id !== op.propertyId) } : b
        ),
      };
    }

    case "add-association": {
      const id = resolveAddId(op.association.id, op.tempId, idMapping, model.associations.map(a => a.id));
      const association: Association = {
        ...op.association,
        id,
        fromBlockId: idMapping[op.association.fromBlockId] ?? op.association.fromBlockId,
        toBlockId: idMapping[op.association.toBlockId] ?? op.association.toBlockId,
      };
      return { ...model, associations: [...model.associations, association] };
    }
    case "update-association": {
      const realId = idMapping[op.associationId] ?? op.associationId;
      assertExists(model.associations, realId, "association");
      return {
        ...model,
        associations: model.associations.map(a => (a.id === realId ? { ...a, ...op.patch } : a)),
      };
    }
    case "remove-association": {
      const realId = idMapping[op.associationId] ?? op.associationId;
      assertExists(model.associations, realId, "association");
      return { ...model, associations: model.associations.filter(a => a.id !== realId) };
    }

    case "add-constraint": {
      const id = resolveAddId(op.constraint.id, op.tempId, idMapping, model.constraints.map(c => c.id));
      const constraint: Constraint = {
        ...op.constraint,
        id,
        appliesTo: op.constraint.appliesTo.map(b => idMapping[b] ?? b),
      };
      return { ...model, constraints: [...model.constraints, constraint] };
    }
    case "update-constraint": {
      const realId = idMapping[op.constraintId] ?? op.constraintId;
      assertExists(model.constraints, realId, "constraint");
      return {
        ...model,
        constraints: model.constraints.map(c => (c.id === realId ? { ...c, ...op.patch } : c)),
      };
    }
    case "remove-constraint": {
      const realId = idMapping[op.constraintId] ?? op.constraintId;
      assertExists(model.constraints, realId, "constraint");
      return { ...model, constraints: model.constraints.filter(c => c.id !== realId) };
    }

    case "add-requirement": {
      const id = resolveAddId(op.requirement.id, op.tempId, idMapping, model.requirements.map(r => r.id));
      const requirement: Requirement = { ...op.requirement, id };
      return { ...model, requirements: [...model.requirements, requirement] };
    }
    case "update-requirement": {
      const realId = idMapping[op.requirementId] ?? op.requirementId;
      assertExists(model.requirements, realId, "requirement");
      return {
        ...model,
        requirements: model.requirements.map(r => (r.id === realId ? { ...r, ...op.patch } : r)),
      };
    }
    case "remove-requirement": {
      const realId = idMapping[op.requirementId] ?? op.requirementId;
      assertExists(model.requirements, realId, "requirement");
      return { ...model, requirements: model.requirements.filter(r => r.id !== realId) };
    }
    case "add-relation": {
      const realId = idMapping[op.requirementId] ?? op.requirementId;
      assertExists(model.requirements, realId, "requirement");
      return {
        ...model,
        requirements: model.requirements.map(r =>
          r.id === realId ? { ...r, relations: [...r.relations, op.relation] } : r
        ),
      };
    }
    case "remove-relation": {
      const realId = idMapping[op.requirementId] ?? op.requirementId;
      assertExists(model.requirements, realId, "requirement");
      return {
        ...model,
        requirements: model.requirements.map(r =>
          r.id === realId
            ? { ...r, relations: r.relations.filter((_, idx) => idx !== op.relationIndex) }
            : r
        ),
      };
    }
    case "decide-element": {
      const realId = idMapping[op.element.id] ?? op.element.id;
      const flip = <T extends { id: string; reviewStatus?: import("../sysml/model").ReviewStatus; reviewPinned?: boolean }>(arr: T[]): T[] =>
        arr.map(e =>
          e.id === realId
            ? {
                ...e,
                reviewStatus: "accepted" as const,
                reviewPinned: e.reviewStatus === "deprecated" ? true : !!e.reviewPinned,
              }
            : e
        );
      switch (op.element.kind) {
        case "block":
          assertExists(model.blocks, realId, "block");
          return { ...model, blocks: flip(model.blocks) };
        case "association":
          assertExists(model.associations, realId, "association");
          return { ...model, associations: flip(model.associations) };
        case "constraint":
          assertExists(model.constraints, realId, "constraint");
          return { ...model, constraints: flip(model.constraints) };
        case "requirement":
          assertExists(model.requirements, realId, "requirement");
          return { ...model, requirements: flip(model.requirements) };
      }
    }
  }
}

// ─── Helpers ─────────────────────────────────────────────────────────────

function cloneModel(model: SysMLModel): SysMLModel {
  // Shallow-clone collections; nested objects are immutable per our op contract.
  return {
    systemOfInterestId: model.systemOfInterestId,
    blocks: model.blocks.map(b => ({ ...b, properties: [...b.properties], stereotypes: [...b.stereotypes] })),
    associations: model.associations.map(a => ({ ...a })),
    constraints: model.constraints.map(c => ({ ...c, appliesTo: [...c.appliesTo] })),
    requirements: model.requirements.map(r => ({ ...r, relations: [...r.relations] })),
  };
}

/**
 * Decide what id to use for a newly-added element.
 * - If the requested id is unused, it sticks. (Most common path.)
 * - Otherwise, append `_2`, `_3`, … until free, and record the mapping from
 *   the requested id to the assigned id so subsequent ops in the same batch
 *   can reference the new element by its tempId.
 */
function resolveAddId(
  desiredId: string,
  tempId: string | undefined,
  idMapping: Record<string, string>,
  existing: string[]
): string {
  const taken = new Set(existing);
  if (!taken.has(desiredId)) {
    if (tempId) idMapping[tempId] = desiredId;
    return desiredId;
  }
  let suffix = 2;
  while (taken.has(`${desiredId}_${suffix}`)) suffix++;
  const final = `${desiredId}_${suffix}`;
  if (tempId) idMapping[tempId] = final;
  idMapping[desiredId] = final;
  return final;
}

function assertExists<T extends { id: string }>(items: T[], id: string, what: string): void {
  if (!items.find(i => i.id === id)) throw missing(what, id);
}

function missing(what: string, id: string): Error {
  return new Error(`${what} "${id}" not found`);
}

function classifyError(err: unknown): ApplyError["code"] {
  const msg = (err as Error).message ?? "";
  if (msg.includes("not found")) return "MISSING_ELEMENT";
  if (msg.includes("duplicate")) return "DUPLICATE_ID";
  return "INVALID_PATCH";
}