// HTTP for clawhdf5-wasm's openUrl (see src/lib.rs and src/lazy.rs).
//
// The Rust side decides which byte ranges a read needs; this file fetches
// them with `fetch` and `Range` headers and checks every answer, so a server
// that ignores the range, answers with other bytes, or serves a file that
// changed since it was opened is an error, never data. wasm-bindgen copies
// it into the package (pkg/snippets/...).

const DEFAULT_MAX_DOWNLOAD = 512 * 1024 * 1024;
const DEFAULT_PARALLEL = 6;

function fetcher(opts) {
  const f = opts?.fetch ?? globalThis.fetch;
  if (typeof f !== "function") {
    throw new Error("openUrl: no fetch() in this environment (pass opts.fetch)");
  }
  return f;
}

// The caller's headers (`opts.headers`: a Headers, [name, value] pairs or a
// plain object, as fetch takes them; names come out in lower case) with
// `extra` over them. A Range of the caller's is dropped: this file asks for
// the ranges.
function init(opts, extra, method = "GET", signal = undefined) {
  const headers = {};
  if (opts?.headers != null) {
    for (const [name, value] of new Headers(opts.headers)) {
      if (name !== "range") headers[name] = value;
    }
  }
  Object.assign(headers, extra);
  return { method, headers, credentials: opts?.credentials, signal };
}

// `opts.parallel`: range requests in flight at once.
function parallelism(opts) {
  const p = opts?.parallel ?? DEFAULT_PARALLEL;
  if (!Number.isSafeInteger(p) || p < 1) {
    throw new Error(`openUrl: parallel must be a positive integer, got ${String(p)}`);
  }
  return p;
}

// "bytes a-b/total" -> { start, end (exclusive), total | null }; null when
// the page cannot see the header (cross-origin, not exposed).
function contentRange(resp, url) {
  const v = resp.headers.get("Content-Range");
  if (v === null) return null;
  const m = /^bytes (\d+)-(\d+)\/(\d+|\*)$/.exec(v.trim());
  if (!m) throw new Error(`${url}: the server sent an unusable Content-Range: ${v}`);
  return { start: Number(m[1]), end: Number(m[2]) + 1, total: m[3] === "*" ? null : Number(m[3]) };
}

// What pins the file: its ETag, else its Last-Modified (null if neither is
// visible to this page).
function validatorOf(resp) {
  return resp.headers.get("ETag") ?? resp.headers.get("Last-Modified");
}

async function discard(resp) {
  try {
    await resp.body?.cancel();
  } catch {
    // Nothing to release.
  }
}

// The body, refusing more than `limit` bytes as they arrive: it is piped
// through a TransformStream that errors the moment the count passes the
// limit, which cancels the body and so aborts the request. Whatever the
// server declares or sends, the page never holds more than `limit` bytes of
// it. (`tooBig(n)` makes the error; n is the count so far.) A reader loop
// would do the same, but it stalls on small bodies in headless Chromium
// under --virtual-time-budget, which the page test uses; a pipe does not.
async function readCapped(resp, limit, tooBig) {
  const declared = resp.headers.get("Content-Length");
  if (declared !== null && Number(declared) > limit) {
    await discard(resp);
    throw tooBig(declared);
  }
  if (!resp.body) {
    // No stream to read from (some fetch implementations): all at once.
    const all = new Uint8Array(await resp.arrayBuffer());
    if (all.length > limit) throw tooBig(all.length);
    return all;
  }
  let n = 0;
  let over = null;
  const capped = resp.body.pipeThrough(new TransformStream({
    transform(chunk, ctl) {
      n += chunk.length;
      if (n > limit) {
        over = tooBig(`over ${limit}`);
        ctl.error(over);
        return;
      }
      ctl.enqueue(chunk);
    },
  }));
  try {
    return new Uint8Array(await new Response(capped).arrayBuffer());
  } catch (e) {
    throw over ?? e;
  }
}

// The whole body of a 200 answer (a server without range support), at
// most `limit` (maxDownload) bytes.
function readAll(resp, limit, url) {
  return readCapped(resp, limit, (n) =>
    new Error(`${url} is ${n} bytes, more than maxDownload (${limit}); ` +
      "the server does not support range requests, so the whole file would have to be downloaded"));
}

// The body of a 206 answer, which may not be longer than the `limit` bytes
// asked for at `start` (the caller checks the exact length).
function readLimited(resp, limit, url, start) {
  return readCapped(resp, limit, () =>
    new Error(`${url}: asked for ${limit} bytes at offset ${start}, the server sent more`));
}

/**
 * Ask for the file's first `firstLen` bytes. A server that honours the
 * range (206) gives `{ length, first, validator, requests }`; one that
 * answers 200 sends the whole file, which is kept (`{ whole, requests }`)
 * when `opts.fallback` is "download" (the default) and the file is at most
 * `opts.maxDownload` bytes, and is an error otherwise.
 */
export async function probe(url, firstLen, opts) {
  const f = fetcher(opts);
  parallelism(opts);
  const resp = await f(url, init(opts, { Range: `bytes=0-${firstLen - 1}` }));
  if (resp.status === 206) {
    const cr = contentRange(resp, url);
    if (cr && cr.start !== 0) {
      await discard(resp);
      throw new Error(`${url}: asked for bytes from 0, the server sent bytes from ${cr.start}`);
    }
    const first = await readLimited(resp, firstLen, url, 0);
    let length = cr?.total ?? null;
    let requests = 1;
    if (length === null) {
      // Content-Range is not readable here: a cross-origin server that does
      // not list it in Access-Control-Expose-Headers. Content-Length of a
      // HEAD request is always readable.
      const head = await f(url, init(opts, {}, "HEAD"));
      requests++;
      const cl = head.headers.get("Content-Length");
      if (!head.ok || cl === null) {
        throw new Error(`${url}: cannot learn the file's size (a cross-origin server must send ` +
          "Access-Control-Expose-Headers: Content-Range, or answer HEAD with Content-Length)");
      }
      length = Number(cl);
    }
    if (!Number.isSafeInteger(length) || length < 0) {
      throw new Error(`${url}: the server gave a file size of ${length} bytes; openUrl reads files ` +
        "of up to 2^53 - 1 bytes (the largest offset a JavaScript number holds exactly)");
    }
    if (first.length !== Math.min(firstLen, length)) {
      throw new Error(`${url}: asked for the first ${firstLen} bytes of ${length}, got ${first.length}`);
    }
    return { length, first, validator: validatorOf(resp), requests };
  }
  if (resp.status === 200) {
    if ((opts?.fallback ?? "download") !== "download") {
      await discard(resp);
      throw new Error(`${url}: the server does not support HTTP range requests (it answered 200 ` +
        "to a Range request); open it with { fallback: \"download\" } to download the whole file");
    }
    const whole = await readAll(resp, opts?.maxDownload ?? DEFAULT_MAX_DOWNLOAD, url);
    return { whole, requests: 1 };
  }
  await discard(resp);
  throw new Error(`${url}: HTTP ${resp.status} ${resp.statusText ?? ""}`.trim());
}

/**
 * Fetch `ranges` ([start0, end0, start1, end1, ...], ends exclusive) of a
 * file opened by `probe`, at most `opts.parallel` (default 6) at a time.
 * Every answer must be a 206 with exactly the bytes asked for, from the same
 * file (validator and length). When one request fails, the others in
 * flight are aborted and no more are made; that failure is the error.
 */
export async function fetchRanges(url, ranges, opts, validator, length) {
  const f = fetcher(opts);
  const parallel = parallelism(opts);
  const n = ranges.length / 2;
  const out = new Array(n);
  const abort = new AbortController();
  let next = 0;
  async function one(i) {
    const start = ranges[2 * i];
    const end = ranges[2 * i + 1];
    const resp = await f(url, init(opts, { Range: `bytes=${start}-${end - 1}` }, "GET", abort.signal));
    if (resp.status !== 206) {
      await discard(resp);
      throw new Error(resp.status === 200
        ? `${url}: the server stopped honouring range requests`
        : `${url}: HTTP ${resp.status} ${resp.statusText ?? ""}`.trim());
    }
    const cr = contentRange(resp, url);
    const v = validatorOf(resp);
    if ((validator != null && v !== null && v !== validator) ||
        (cr?.total != null && cr.total !== length)) {
      await discard(resp);
      throw new Error(`${url} changed on the server since it was opened`);
    }
    if (cr && (cr.start !== start || cr.end !== end)) {
      await discard(resp);
      throw new Error(`${url}: asked for bytes ${start}-${end - 1}, the server sent ${cr.start}-${cr.end - 1}`);
    }
    const body = await readLimited(resp, end - start, url, start);
    if (body.length !== end - start) {
      throw new Error(`${url}: asked for ${end - start} bytes at offset ${start}, got ${body.length}`);
    }
    out[i] = body;
  }
  async function worker() {
    while (next < n && !abort.signal.aborted) {
      try {
        await one(next++);
      } catch (e) {
        // The first failure stops the rest: requests in flight are aborted
        // (their AbortErrors are not reported) and no new ones start.
        if (!abort.signal.aborted) {
          abort.abort();
          throw e;
        }
        return;
      }
    }
  }
  await Promise.all(Array.from({ length: Math.min(parallel, n) }, worker));
  return out;
}