import { migrateSave, type SaveDocument, type SaveParse } from "@deadhead/schema";

/**
 * Save export/import (decision 0.5.92): one text code holding the whole save,
 * gzip-compressed and base64url-encoded behind a short prefix, so it survives
 * being pasted into a chat, a note or a file.
 */
export const SAVE_CODE_PREFIX = "DEADHEAD1.";

async function pipe(
  bytes: Uint8Array,
  stream: CompressionStream | DecompressionStream,
): Promise<Uint8Array> {
  const piped = new Response(bytes as BodyInit).body!.pipeThrough(stream);
  return new Uint8Array(await new Response(piped).arrayBuffer());
}

function toBase64Url(bytes: Uint8Array): string {
  let binary = "";
  for (let i = 0; i < bytes.length; i += 0x8000) {
    binary += String.fromCharCode(...bytes.subarray(i, i + 0x8000));
  }
  return btoa(binary).replace(/\+/g, "-").replace(/\//g, "_").replace(/=+$/, "");
}

function fromBase64Url(text: string): Uint8Array {
  const binary = atob(text.replace(/-/g, "+").replace(/_/g, "/"));
  return Uint8Array.from(binary, (c) => c.charCodeAt(0));
}

/** The save as an export code. */
export async function encodeSave(save: SaveDocument): Promise<string> {
  const json = new TextEncoder().encode(JSON.stringify(save));
  return SAVE_CODE_PREFIX + toBase64Url(await pipe(json, new CompressionStream("gzip")));
}

/**
 * Reads an export code: decodes, migrates an older `saveVersion` and validates.
 * Never throws; anything that isn't a good save gives a readable error.
 */
export async function decodeSave(
  code: string,
  migrate: (raw: unknown) => SaveParse = migrateSave,
): Promise<SaveParse> {
  // Pasting often adds spaces or line breaks.
  const text = code.replace(/\s+/g, "");
  if (!text.startsWith(SAVE_CODE_PREFIX)) {
    return { ok: false, error: "That isn't a Deadhead save code." };
  }
  let raw: unknown;
  try {
    const bytes = await pipe(
      fromBase64Url(text.slice(SAVE_CODE_PREFIX.length)),
      new DecompressionStream("gzip"),
    );
    raw = JSON.parse(new TextDecoder().decode(bytes));
  } catch {
    return { ok: false, error: "The save code is incomplete or damaged." };
  }
  return migrate(raw);
}