/* ============================================================================
 * resume-core — pure helpers for resumable saves (docs/resumable-downloads-plan.md)
 *
 * Framework-free: a plain <script>/importScripts global (`ResumeCore`) in the
 * browser and worker, `require`-able by `node --test`.
 * ========================================================================== */
(function (root) {
  'use strict';

  const CHUNK = 8 * 1024 * 1024;

  /** Next byte range to fetch, or null when the file is complete. */
  function nextRange(pos, size, chunk = CHUNK) {
    if (!(size > 0) || pos >= size) return null;
    return { start: pos, end: Math.min(pos + chunk, size) - 1 };
  }

  /** "bytes 0-99/1234" → { start, end, total }; null when malformed. */
  function parseContentRange(h) {
    const m = /^bytes (\d+)-(\d+)\/(\d+|\*)$/.exec(String(h || '').trim());
    if (!m) return null;
    return { start: Number(m[1]), end: Number(m[2]), total: m[3] === '*' ? null : Number(m[3]) };
  }

  /** Wait before retry `attempt` (0-based): 2 s, 4 s, 8 s, 16 s, 30 s cap. */
  function backoffMs(attempt) {
    return Math.min(30000, 2000 * Math.pow(2, Math.max(0, attempt)));
  }

  /**
   * Where a stored partial may resume from. The partial is only reused when
   * it was written for the very same server copy (ETag) and is not longer
   * than that copy; otherwise start over.
   */
  function resumeFrom(sidecar, partSize, target) {
    if (!sidecar || !target || sidecar.etag !== target.etag || sidecar.size !== target.size) return 0;
    if (!(partSize > 0)) return 0;
    return Math.min(partSize, target.size);
  }

  const ResumeCore = { CHUNK, nextRange, parseContentRange, backoffMs, resumeFrom };
  if (typeof module !== 'undefined' && module.exports) module.exports = ResumeCore;
  else root.ResumeCore = ResumeCore;
})(typeof self !== 'undefined' ? self : this);