// A seekable, streaming byte source backed by HTTP Range requests.
//
// The streaming ONNX parser uses this to:
//   1. Read protobuf wire bytes sequentially.
//   2. Skip over uninteresting regions (notably `raw_data` of initializers)
//      without buffering them in RAM and, when the skip is large enough,
//      without even pulling them from the network — we cancel the current
//      stream and issue a fresh Range request that jumps over the region.
//
// Design goals:
//   - Always know the current absolute offset in the file.
//   - Pull more data from the network only when needed.
//   - When a skip is small (< SKIP_RANGE_THRESHOLD), discard from the
//     in-flight stream; when large, cancel and re-issue Range from the
//     post-skip offset (saves bandwidth).
//   - Report total bytes actually transferred so the UI can show savings.
//   - Be cancellable from the outside (AbortController).

export interface ByteSourceOpts {
  /** Range threshold (bytes) above which we cancel + re-issue instead of stream-skipping. */
  skipRangeThreshold?: number;
  /** Optional AbortSignal to cancel the entire source. */
  signal?: AbortSignal;
  /** Called every time we receive more data. Useful for UI progress. */
  onTransfer?: (deltaBytes: number, totalTransferred: number) => void;
}

// Skip-vs-stream break-even. Below this, streaming-skip is cheaper than
// paying a fresh Range request RTT (~30–200 ms on Hub CDN, throughput
// typically 10–30 MB/s).
//
// We pick the threshold adaptively from the total file size — see
// `pickAdaptiveThreshold()` below — because the right answer is very
// different between a 50 MB quantized GPT-2 (lots of ~350 KB int8 tensors,
// we *want* to Range-jump even small blobs) and a 5 GB fp16 Llama (we want
// to amortize fewer, bigger jumps).
//
// 1 MB is the safe ceiling: above that, paying a fresh RTT is always
// worthwhile because we'd otherwise download a megabyte of dead bytes.
const SKIP_RANGE_CEILING = 1024 * 1024; // 1 MB
const SKIP_RANGE_FLOOR = 32 * 1024; // 32 KB

/**
 * Pick a per-file skip threshold from the total file size.
 *
 * Rules of thumb the formula encodes:
 *  - Tiny files (< 50 MB, typical quantized BERT-class): aggressive 32 KB
 *    floor. We accept dozens of RTTs because the total file is small enough
 *    that each saved KB matters and TLS reuse keeps each RTT cheap.
 *  - Medium (50 MB – 500 MB, quantized GPT-2 / DistilBERT / ViT class):
 *    interpolate up to ~128 KB. We still want to skip most quantized
 *    tensors which sit in the 200–800 KB range.
 *  - Large (≥ 500 MB, fp16/fp32 LLaMA-class): full 1 MB ceiling. Tensors
 *    are big enough that RTT cost is dwarfed by the bandwidth save.
 */
export function pickAdaptiveThreshold(totalSize: number): number {
  if (totalSize <= 0) return SKIP_RANGE_CEILING;
  if (totalSize < 50 * 1024 * 1024) return SKIP_RANGE_FLOOR;
  if (totalSize >= 500 * 1024 * 1024) return SKIP_RANGE_CEILING;
  // Linear ramp 32 KB → 1 MB over the 50 MB → 500 MB band.
  const t = (totalSize - 50 * 1024 * 1024) / (450 * 1024 * 1024);
  const v = SKIP_RANGE_FLOOR + t * (SKIP_RANGE_CEILING - SKIP_RANGE_FLOOR);
  return Math.round(v);
}

export class ByteSource {
  readonly url: string;
  readonly totalSize: number;
  private opts: Required<Omit<ByteSourceOpts, "signal" | "onTransfer">> & {
    signal?: AbortSignal;
    onTransfer?: ByteSourceOpts["onTransfer"];
  };

  private reader: ReadableStreamDefaultReader<Uint8Array> | null = null;
  /** Absolute offset where the current Range request is supposed to end (exclusive). */
  private streamEnd = 0;

  /** A small ring of buffered chunks pulled from the reader but not yet consumed. */
  private buffer: Uint8Array[] = [];
  private bufferStart = 0; // absolute offset of buffer[0][0]
  /** Offset within buffer[0]. */
  private bufferHead = 0;
  /** Total bytes currently held across all buffer chunks. */
  private bufferedBytes = 0;

  /** Absolute offset of the next byte to be returned to the caller. */
  pos = 0;

  /** Total bytes received over the network so far. */
  bytesTransferred = 0;

  constructor(url: string, totalSize: number, opts: ByteSourceOpts = {}) {
    this.url = url;
    this.totalSize = totalSize;
    this.opts = {
      skipRangeThreshold:
        opts.skipRangeThreshold ?? pickAdaptiveThreshold(totalSize),
      signal: opts.signal,
      onTransfer: opts.onTransfer,
    };
  }

  /** Exposed for the UI: what threshold are we actually using? */
  get skipRangeThreshold(): number {
    return this.opts.skipRangeThreshold;
  }

  /** Bytes available to read without pulling more from the network. */
  private get availableInBuffer(): number {
    return this.bufferedBytes - this.bufferHead;
  }

  /** Open a Range stream starting at `from` (absolute offset), ending at end (exclusive, defaults to totalSize). */
  private async openRangeStream(from: number, end?: number): Promise<void> {
    if (this.reader) await this.cancelStream();
    const stop = end ?? this.totalSize;
    if (from >= stop) {
      this.streamEnd = stop;
      return;
    }
    const rangeHeader = `bytes=${from}-${stop - 1}`;
    const res = await fetch(this.url, {
      headers: { Range: rangeHeader },
      signal: this.opts.signal,
    });
    if (!(res.status === 206 || (res.status === 200 && from === 0))) {
      throw new Error(
        `Range request failed (status ${res.status}). The CDN may not support Range; falling back is not implemented.`,
      );
    }
    if (!res.body) throw new Error("Response has no body.");
    this.reader = res.body.getReader();
    this.streamEnd = stop;
  }

  private async cancelStream(): Promise<void> {
    if (!this.reader) return;
    try {
      await this.reader.cancel();
    } catch {
      // ignored: cancel failures are non-fatal.
    }
    this.reader = null;
  }

  /** Make sure at least `n` bytes are available in the buffer, pulling from the stream as needed. */
  private async ensureAvailable(n: number): Promise<void> {
    if (this.pos + n > this.totalSize) {
      throw new Error(
        `Read past end of file: pos=${this.pos}, want=${n}, total=${this.totalSize}`,
      );
    }
    if (!this.reader || this.pos < this.bufferStart + this.bufferHead) {
      // Stream is closed or seek happened — (re)open at current pos.
      await this.openRangeStream(this.pos);
      this.resetBuffer(this.pos);
    }
    while (this.availableInBuffer < n) {
      if (!this.reader) {
        // Stream ended but we expected more.
        if (this.pos + this.availableInBuffer >= this.streamEnd) {
          if (this.streamEnd < this.totalSize) {
            await this.openRangeStream(this.pos + this.availableInBuffer);
            continue;
          }
        }
        throw new Error("Stream ended unexpectedly before requested bytes arrived.");
      }
      const { done, value } = await this.reader.read();
      if (done) {
        this.reader = null;
        if (this.pos + this.availableInBuffer >= this.streamEnd && this.streamEnd < this.totalSize) {
          await this.openRangeStream(this.pos + this.availableInBuffer);
          continue;
        }
        if (this.availableInBuffer < n) {
          throw new Error("Stream EOF before requested bytes arrived.");
        }
        break;
      }
      if (!value || value.byteLength === 0) continue;
      this.buffer.push(value);
      this.bufferedBytes += value.byteLength;
      this.bytesTransferred += value.byteLength;
      this.opts.onTransfer?.(value.byteLength, this.bytesTransferred);
    }
  }

  private resetBuffer(startPos: number): void {
    this.buffer = [];
    this.bufferedBytes = 0;
    this.bufferHead = 0;
    this.bufferStart = startPos;
  }

  /** Read exactly `n` bytes into a fresh Uint8Array (concatenating internal chunks if needed). */
  async readBytes(n: number): Promise<Uint8Array> {
    if (n <= 0) return new Uint8Array(0);
    await this.ensureAvailable(n);
    const out = new Uint8Array(n);
    let written = 0;
    while (written < n) {
      const head = this.buffer[0];
      const headRemaining = head.byteLength - this.bufferHead;
      const take = Math.min(headRemaining, n - written);
      out.set(head.subarray(this.bufferHead, this.bufferHead + take), written);
      written += take;
      this.bufferHead += take;
      if (this.bufferHead >= head.byteLength) {
        this.buffer.shift();
        this.bufferedBytes -= head.byteLength;
        this.bufferStart += head.byteLength;
        this.bufferHead = 0;
      }
    }
    this.pos += n;
    return out;
  }

  /** Read a single byte (faster path for varint decoding). */
  async readByte(): Promise<number> {
    if (this.availableInBuffer === 0) await this.ensureAvailable(1);
    const head = this.buffer[0];
    const b = head[this.bufferHead++];
    if (this.bufferHead >= head.byteLength) {
      this.buffer.shift();
      this.bufferedBytes -= head.byteLength;
      this.bufferStart += head.byteLength;
      this.bufferHead = 0;
    }
    this.pos += 1;
    return b;
  }

  /**
   * Advance the read position by `n` bytes without exposing them to the caller.
   * - If n <= skipRangeThreshold, we still pull/discard those bytes from the
   *   current stream (cheap, no RTT).
   * - If n > skipRangeThreshold, we cancel the current stream and jump
   *   forward via a new Range request. This is how we save real bandwidth
   *   on big initializers.
   */
  async skipBytes(n: number): Promise<void> {
    if (n <= 0) return;
    if (this.pos + n > this.totalSize) {
      throw new Error(
        `Skip past end of file: pos=${this.pos}, skip=${n}, total=${this.totalSize}`,
      );
    }

    // First, eat what we already have buffered.
    if (this.availableInBuffer > 0) {
      const eat = Math.min(this.availableInBuffer, n);
      let remaining = eat;
      while (remaining > 0) {
        const head = this.buffer[0];
        const headRemaining = head.byteLength - this.bufferHead;
        const take = Math.min(headRemaining, remaining);
        this.bufferHead += take;
        remaining -= take;
        if (this.bufferHead >= head.byteLength) {
          this.buffer.shift();
          this.bufferedBytes -= head.byteLength;
          this.bufferStart += head.byteLength;
          this.bufferHead = 0;
        }
      }
      this.pos += eat;
      n -= eat;
      if (n === 0) return;
    }

    if (n > this.opts.skipRangeThreshold) {
      // Big skip: cancel current stream, jump forward via new Range.
      await this.cancelStream();
      this.resetBuffer(this.pos + n);
      this.pos += n;
      return;
    }

    // Small skip: stream-discard.
    while (n > 0) {
      if (!this.reader) {
        await this.openRangeStream(this.pos);
        this.resetBuffer(this.pos);
        continue;
      }
      const { done, value } = await this.reader.read();
      if (done) {
        this.reader = null;
        if (this.streamEnd < this.totalSize) {
          await this.openRangeStream(this.pos);
          continue;
        }
        throw new Error("Stream EOF during skip.");
      }
      if (!value || value.byteLength === 0) continue;
      this.bytesTransferred += value.byteLength;
      this.opts.onTransfer?.(value.byteLength, this.bytesTransferred);
      if (value.byteLength <= n) {
        // Entire chunk discarded.
        this.pos += value.byteLength;
        n -= value.byteLength;
      } else {
        // Partial chunk: keep the unused tail in the buffer.
        const tail = value.subarray(n);
        this.pos += n;
        this.bufferStart = this.pos;
        this.bufferHead = 0;
        this.buffer = [tail];
        this.bufferedBytes = tail.byteLength;
        n = 0;
      }
    }
  }

  /** Cancel any pending network and free buffers. */
  async close(): Promise<void> {
    await this.cancelStream();
    this.buffer = [];
    this.bufferedBytes = 0;
    this.bufferHead = 0;
  }

  /** Pre-fetch the total length via HEAD if not known at construction. */
  static async create(url: string, opts: ByteSourceOpts = {}): Promise<ByteSource> {
    const head = await fetch(url, { method: "HEAD", signal: opts.signal });
    if (!head.ok) throw new Error(`HEAD ${url} failed: ${head.status}`);
    const len = Number(head.headers.get("content-length") ?? 0);
    if (!Number.isFinite(len) || len <= 0) {
      throw new Error(`Server did not return a usable Content-Length for ${url}.`);
    }
    const acceptRanges = head.headers.get("accept-ranges");
    if (acceptRanges && acceptRanges.toLowerCase() === "none") {
      throw new Error(`Server says it does not support Range requests for ${url}.`);
    }
    return new ByteSource(url, len, opts);
  }
}