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HTTP Range Header Parser (JavaScript)

Parse a single-range HTTP Range header, resolve open ends against a resource size and return a satisfiable byte range.

JavascriptData Integrity

Code

snippet
export function parseRange(header, size) {
  if (!header || typeof header !== 'string') return null;
  const match = /^bytes=(\d*)-(\d*)$/.exec(header.trim());
  if (!match) return { unsatisfiable: true };
  const [, startRaw, endRaw] = match;
  if (startRaw === '' && endRaw === '') return { unsatisfiable: true };
  let start;
  let end;
  if (startRaw === '') {
    const suffix = Number(endRaw);
    if (suffix === 0) return { unsatisfiable: true };
    start = Math.max(size - suffix, 0);
    end = size - 1;
  } else {
    start = Number(startRaw);
    end = endRaw === '' ? size - 1 : Number(endRaw);
  }
  if (start > end || start >= size) return { unsatisfiable: true };
  return { start, end: Math.min(end, size - 1), length: Math.min(end, size - 1) - start + 1 };
}

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