// Git LFS pointers.
//
// A repository that tracks a large file with Git LFS commits a small text
// file in its place, naming the object by its hash. The file itself is on an
// LFS server, which this one is not. So a reader that finds a pointer holds
// the name of an image, never its pixels, and a page that shows one has to
// say so rather than print 132 bytes of text as a PNG.

/** The first line of every pointer this reader knows. */
const VERSION_LINE = 'version https://git-lfs.github.com/spec/v1';

/** A pointer is a few short lines. Anything longer is the file itself. */
const MAX_POINTER_BYTES = 1024;

const OID_LINE = /^oid sha256:([0-9a-f]{64})$/m;
const SIZE_LINE = /^size (\d{1,19})$/m;

/**
 * @typedef {Object} LfsPointer
 * @property {string} oid - sha256 of the file the pointer stands for
 * @property {number} size - length of that file in bytes
 */

/**
 * Read a blob as a Git LFS pointer.
 * @param {Uint8Array|null|undefined} bytes
 * @returns {LfsPointer|null} null for any blob that is the file itself
 */
export function lfsPointer(bytes) {
  if (!bytes || bytes.length === 0 || bytes.length > MAX_POINTER_BYTES) {
    return null;
  }
  let text;
  try {
    // A pointer is UTF-8 text. Bytes that are not decode as nothing, which
    // is the answer for every file that carries its own content.
    text = new TextDecoder('utf-8', { fatal: true }).decode(bytes);
  } catch {
    return null;
  }
  if (!text.startsWith(VERSION_LINE)) return null;

  const oid = OID_LINE.exec(text);
  const size = SIZE_LINE.exec(text);
  if (!oid || !size) return null;
  return { oid: oid[1], size: Number(size[1]) };
}