import type { BlobRange } from './types'

/**
 * Resolve a requested {@link BlobRange} against an object's real size, the way
 * every byte-storing blob store has to before it slices.
 *
 * Clamps `length` to the end of the object and treats an absent `length` as
 * "to the end", so the result is always the slice actually served — which is
 * what `BlobObject.range` reports and what a `206` response's `Content-Range`
 * is built from.
 *
 * Throws on an `offset` outside the object: that is a caller error, not a
 * store error. A serve route knows the size (it is on the artifact record) and
 * answers `416` from it, so a store only ever sees a satisfiable range unless
 * something upstream is wrong — and silently returning an empty body there
 * would serve a `206` that claims bytes it does not carry.
 *
 * @example
 * ```ts
 * const { offset, length } = resolveBlobRange(bytes.byteLength, range)
 * const slice = bytes.subarray(offset, offset + length)
 * ```
 */
export function resolveBlobRange(
  size: number,
  range: BlobRange,
): { offset: number; length: number } {
  const { offset } = range
  if (!Number.isInteger(offset) || offset < 0 || offset >= size) {
    throw new RangeError(
      `Blob range offset ${offset} is outside the object (size ${size}).`,
    )
  }
  const remaining = size - offset
  if (range.length === undefined) return { offset, length: remaining }
  if (!Number.isInteger(range.length) || range.length < 0) {
    throw new RangeError(`Blob range length ${range.length} is not valid.`)
  }
  return { offset, length: Math.min(range.length, remaining) }
}

/**
 * Resolve an HTTP `Range` header against a known object size, for a route that
 * serves artifact bytes.
 *
 * Returns the {@link BlobRange} to pass to `retrieveBlob` / `BlobStore.get`,
 * `'unsatisfiable'` when the request names bytes the object does not have — answer
 * `416`, whose `content-range` is the literal `bytes` `*` then a slash then the
 * size — or `undefined` when there is no range to honour and the whole object
 * should be served: an absent header, an invalid byte-range-spec (`bytes=100-50`,
 * which RFC 9110 says to ignore rather than reject), and the forms this does not
 * implement (multiple ranges, units other than `bytes`), which a server is
 * always free to answer in full.
 *
 * @example
 * ```ts
 * const range = parseRangeHeader(request.headers.get('range'), record.size)
 * if (range === 'unsatisfiable') return new Response(null, { status: 416 })
 * const blob = await retrieveBlob(
 *   persistence,
 *   record,
 *   range ? { range } : undefined,
 * )
 * ```
 */
export function parseRangeHeader(
  header: string | null | undefined,
  size: number,
): BlobRange | 'unsatisfiable' | undefined {
  const match = /^bytes=(\d*)-(\d*)$/.exec(header?.trim() ?? '')
  if (!match) return undefined
  const [, rawStart, rawEnd] = match
  // `bytes=-` names nothing at all.
  if (rawStart === '' && rawEnd === '') return undefined

  // `bytes=-500` is the LAST 500 bytes, not "from 0 to 500" — the one form
  // that is easy to read backwards, and reading it backwards serves the wrong
  // bytes with a 206 that claims they are the right ones.
  if (rawStart === '') {
    const suffix = Number(rawEnd)
    // A zero-length suffix names no bytes, and NO range is satisfiable against
    // a zero-byte object — without the size check, `bytes=-1` on an empty
    // artifact resolves to `{ offset: 0 }` and throws out of the store instead
    // of answering 416.
    if (suffix === 0 || size === 0) return 'unsatisfiable'
    return { offset: Math.max(0, size - suffix) }
  }

  const start = Number(rawStart)
  const end = rawEnd === '' ? undefined : Number(rawEnd)
  // `bytes=100-50` is an INVALID byte-range-spec, not an unsatisfiable one
  // (RFC 9110 §14.1.1). An invalid spec is ignored and the whole
  // representation is served — answering 416 would fail a request that is
  // supposed to succeed. Checked before satisfiability so the size cannot turn
  // an ignorable spec into a 416.
  if (end !== undefined && end < start) return undefined
  if (start >= size) return 'unsatisfiable'
  if (end === undefined) return { offset: start }
  // `end` is inclusive, and past the end of the object it simply clamps.
  return { offset: start, length: Math.min(end, size - 1) - start + 1 }
}