import type { JsonPatchError, Operation } from "fast-json-patch";

import jsonpatch from "fast-json-patch";
import z from "zod";

import { type ResumeData, resumeDataSchema } from "@/schema/resume/data";

/**
 * A Zod schema that models JSON Patch (RFC 6902) operations as a discriminated union on `op`.
 * This ensures required fields (`value` for add/replace/test, `from` for move/copy) are
 * validated at the request boundary rather than failing later at the `fast-json-patch` layer.
 */
export const jsonPatchOperationSchema = z.discriminatedUnion("op", [
  z.object({ op: z.literal("add"), path: z.string(), value: z.any() }),
  z.object({ op: z.literal("remove"), path: z.string() }),
  z.object({ op: z.literal("replace"), path: z.string(), value: z.any() }),
  z.object({ op: z.literal("move"), path: z.string(), from: z.string() }),
  z.object({ op: z.literal("copy"), path: z.string(), from: z.string() }),
  z.object({ op: z.literal("test"), path: z.string(), value: z.any() }),
]);

/**
 * A structured error thrown when a JSON Patch operation fails.
 * Contains only the relevant details -- never the full document tree.
 */
export class ResumePatchError extends Error {
  /** The error code from `fast-json-patch`, e.g. `TEST_OPERATION_FAILED`. */
  code: string;
  /** The zero-based index of the failing operation in the operations array. */
  index: number;
  /** The operation object that caused the failure. */
  operation: Operation;

  constructor(code: string, message: string, index: number, operation: Operation) {
    super(message);
    this.name = "ResumePatchError";
    this.code = code;
    this.index = index;
    this.operation = operation;
  }
}

/**
 * Human-readable messages for each `fast-json-patch` error code.
 * These are returned to the API consumer instead of the raw library output.
 */
const patchErrorMessages: Record<string, string> = {
  SEQUENCE_NOT_AN_ARRAY: "Patch sequence must be an array.",
  OPERATION_NOT_AN_OBJECT: "Operation is not an object.",
  OPERATION_OP_INVALID: "Operation `op` property is not one of the operations defined in RFC 6902.",
  OPERATION_PATH_INVALID: "Operation `path` property is not a valid JSON Pointer string.",
  OPERATION_FROM_REQUIRED: "Operation `from` property is required for `move` and `copy` operations.",
  OPERATION_VALUE_REQUIRED: "Operation `value` property is required for `add`, `replace`, and `test` operations.",
  OPERATION_VALUE_CANNOT_CONTAIN_UNDEFINED:
    "Operation `value` contains an `undefined` value, which is not valid in JSON.",
  OPERATION_PATH_CANNOT_ADD: "Cannot perform an `add` operation at the desired path.",
  OPERATION_PATH_UNRESOLVABLE: "Cannot perform the operation at a path that does not exist.",
  OPERATION_FROM_UNRESOLVABLE: "Cannot perform the operation from a path that does not exist.",
  OPERATION_PATH_ILLEGAL_ARRAY_INDEX: "Array index in path must be an unsigned base-10 integer.",
  OPERATION_VALUE_OUT_OF_BOUNDS: "The specified array index is greater than the number of elements in the array.",
  TEST_OPERATION_FAILED: "Test operation failed -- the value at the given path did not match the expected value.",
};

/**
 * Checks whether an error is a `JsonPatchError` from `fast-json-patch`.
 * The library doesn't export the class directly, so we duck-type it.
 */
function isJsonPatchError(error: unknown): error is JsonPatchError {
  return error instanceof Error && "index" in error && "operation" in error;
}

/**
 * Converts a `JsonPatchError` into a clean `ResumePatchError` that omits the document tree.
 */
function toResumePatchError(error: JsonPatchError): ResumePatchError {
  const code = error.name;
  const message = patchErrorMessages[code] ?? error.message;
  const index = error.index ?? 0;
  const operation = error.operation as Operation;

  return new ResumePatchError(code, message, index, operation);
}

/**
 * Applies an array of JSON Patch (RFC 6902) operations to a `ResumeData` object.
 *
 * This function validates the operations before applying them, then validates the
 * resulting document against the `resumeDataSchema` to ensure the patched data is
 * still a valid resume.
 *
 * The original `data` object is not mutated; a deep clone is created internally.
 *
 * @see https://docs.rxresu.me/guides/using-the-patch-api — for usage examples and API details.
 * @see https://datatracker.ietf.org/doc/html/rfc6902 — JSON Patch specification.
 *
 * @param data - The current resume data to patch.
 * @param operations - An array of JSON Patch operations to apply.
 * @returns The patched and validated `ResumeData` object.
 * @throws {ResumePatchError} If the operations are structurally invalid or target non-existent paths.
 * @throws {ResumePatchError} If a `test` operation does not match.
 * @throws {Error} If the patched document does not conform to the `ResumeData` schema.
 */
export function applyResumePatches(data: ResumeData, operations: Operation[]): ResumeData {
  // Validate operations structurally before applying.
  const validationError = jsonpatch.validate(operations, data);
  if (validationError) throw toResumePatchError(validationError);

  // Apply operations. applyPatch throws on `test` failures.
  let patched: ResumeData;

  try {
    const result = jsonpatch.applyPatch(data, operations, false, false);
    patched = result.newDocument;
  } catch (error: unknown) {
    if (isJsonPatchError(error)) throw toResumePatchError(error);
    throw error;
  }

  // Validate the result still conforms to ResumeData.
  const parsed = resumeDataSchema.safeParse(patched);
  if (!parsed.success) throw new Error(`Patch produced invalid resume data: ${parsed.error.message}`);

  return parsed.data;
}