import { StudioGatewayError } from "../../application/ports/studio-gateway-error.ts";
import type { ProblemDetails } from "../../contracts/studio/contract.ts";
import { stableStringify } from "../stable-stringify.ts";

export type CursorPayload = { binding: string; lastValue: string; lastId: string };
const UUID = /^[0-9a-f]{8}-[0-9a-f]{4}-[1-5][0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/i;

function invalid(detail: string) {
  const problem: ProblemDetails = {
    type: "https://techlog.local/problems/request-validation-failed", title: "Request validation failed",
    status: 422, detail, code: "REQUEST_VALIDATION_FAILED", retryable: false,
    // wire와 같은 자리: `details`가 `ValidationErrorDetails` 모양이다 (Task 3
    // fix round 1 — 예전엔 `fieldErrors`가 최상위 필드였다).
    details: { fieldErrors: [{ path: "/cursor", message: detail }] },
  };
  return new StudioGatewayError(problem);
}

export function cursorBinding(value: unknown) { return stableStringify(value); }

export function encodeCursor(payload: CursorPayload): string {
  const bytes = new TextEncoder().encode(stableStringify(payload));
  let binary = "";
  for (const byte of bytes) binary += String.fromCharCode(byte);
  return btoa(binary).replaceAll("+", "-").replaceAll("/", "_").replace(/=+$/, "");
}

export function decodeCursor(cursor: string, binding: string): CursorPayload {
  try {
    const base64 = cursor.replaceAll("-", "+").replaceAll("_", "/").padEnd(Math.ceil(cursor.length / 4) * 4, "=");
    const bytes = Uint8Array.from(atob(base64), (character) => character.charCodeAt(0));
    const parsed = JSON.parse(new TextDecoder().decode(bytes)) as Partial<CursorPayload>;
    if (parsed.binding !== binding || typeof parsed.lastValue !== "string" || typeof parsed.lastId !== "string" || !UUID.test(parsed.lastId)) {
      throw invalid("Cursor does not match the normalized filters and sort.");
    }
    return parsed as CursorPayload;
  } catch (error) {
    if (error instanceof StudioGatewayError) throw error;
    throw invalid("Cursor is malformed.");
  }
}