// Async protobuf wire-format reader on top of a `ByteSource`.
//
// We implement just what we need to parse ONNX:
//   - tag    : varint of (field_number << 3) | wire_type
//   - varint : LEB128, up to 10 bytes
//   - I32/I64: fixed little-endian
//   - LEN    : length-delimited, varint length followed by N bytes
//
// We expose helpers to read primitives and a `skipField(wireType)` helper
// that can stream-skip large LEN fields via the ByteSource's optimized
// skipBytes. This is the foundation for the ONNX raw_data optimization.

import { ByteSource } from "./byteSource";

export const WIRE_VARINT = 0;
export const WIRE_I64 = 1;
export const WIRE_LEN = 2;
export const WIRE_SGROUP = 3; // deprecated
export const WIRE_EGROUP = 4; // deprecated
export const WIRE_I32 = 5;

export interface ProtoTag {
  field: number;
  wire: number;
}

/** Decoded varint up to 64 bits. Values that fit in a JS number are returned as number, otherwise bigint. */
export type Varint = number | bigint;

export class ProtoReader {
  private src: ByteSource;
  /** Optional absolute end position. Reads past this throw EOM. */
  private endPos: number;

  constructor(src: ByteSource, endPos?: number) {
    this.src = src;
    this.endPos = endPos ?? src.totalSize;
  }

  get pos(): number {
    return this.src.pos;
  }

  get end(): number {
    return this.endPos;
  }

  get hasMore(): boolean {
    return this.src.pos < this.endPos;
  }

  /** Create a sub-reader that ends at `start + length`. */
  subreader(length: number): ProtoReader {
    return new ProtoReader(this.src, this.src.pos + length);
  }

  /** Read an unsigned varint. Returns number if it fits, bigint otherwise. */
  async readVarint(): Promise<Varint> {
    // Fast path: stay in JS number-land for varints whose data fits in
    // ~28 bits. We read up to 4 bytes here (4 × 7 = 28 bits) which is
    // safe because `<< 21` still produces a positive 32-bit signed int.
    // The fifth byte at shift=28 risks truncating the upper data bits
    // (JS bitwise ops are 32-bit), so we switch to BigInt from byte 5 on.
    let lo = 0;
    for (let i = 0; i < 4; i++) {
      const b = await this.src.readByte();
      lo |= (b & 0x7f) << (i * 7);
      lo = lo >>> 0;
      if ((b & 0x80) === 0) return lo;
    }
    // Slow path: switch to BigInt for byte 5 onwards. Bit positions
    // 28..62 are exactly representable, and `int64 = -1` (10-byte
    // varint with all bits set) is handled by `readVarintNumber` which
    // re-interprets values above MAX_SAFE_INTEGER as signed int64.
    let big = BigInt(lo);
    let shift = 28n;
    for (let i = 0; i < 6; i++) {
      const b = await this.src.readByte();
      big |= BigInt(b & 0x7f) << shift;
      shift += 7n;
      if ((b & 0x80) === 0) {
        if (big <= BigInt(Number.MAX_SAFE_INTEGER)) return Number(big);
        return big;
      }
    }
    throw new Error("Varint exceeds 10 bytes.");
  }

  /**
   * Read a varint into a number.
   *
   * Lengths and small unsigned counters fit easily in JS numbers. For int64
   * fields, protobuf encodes signed values via sign-extended unsigned varints:
   * for example, `int64 = -1` is the 10-byte varint `FF FF FF FF FF FF FF FF FF 01`
   * which decodes as `2^64 - 1`. We handle that by reinterpreting any varint
   * larger than Number.MAX_SAFE_INTEGER as a signed int64 (subtract 2^64) and
   * returning it if the result fits in a JS number.
   *
   * This is correct for every ONNX field we read (dim_value, dims, ir_version,
   * model_version, opset versions, lengths) because none of them ever store an
   * unsigned magnitude above 2^53.
   */
  async readVarintNumber(): Promise<number> {
    const v = await this.readVarint();
    if (typeof v === "number") return v;
    const TWO_64 = BigInt("18446744073709551616");
    const MAX_SAFE = BigInt(Number.MAX_SAFE_INTEGER);
    const MIN_SAFE = BigInt(Number.MIN_SAFE_INTEGER);
    let signed = v;
    if (v > MAX_SAFE) signed = v - TWO_64;
    if (signed >= MIN_SAFE && signed <= MAX_SAFE) return Number(signed);
    throw new Error(`Varint out of safe integer range: ${v}`);
  }

  async readTag(): Promise<ProtoTag> {
    const v = await this.readVarintNumber();
    return { field: v >>> 3, wire: v & 0x07 };
  }

  async readLengthDelimited(): Promise<Uint8Array> {
    const len = await this.readVarintNumber();
    return this.src.readBytes(len);
  }

  async readString(): Promise<string> {
    const bytes = await this.readLengthDelimited();
    return new TextDecoder("utf-8").decode(bytes);
  }

  async readBytes(n: number): Promise<Uint8Array> {
    return this.src.readBytes(n);
  }

  async readByte(): Promise<number> {
    return this.src.readByte();
  }

  async readI32LE(): Promise<number> {
    const b = await this.src.readBytes(4);
    return new DataView(b.buffer, b.byteOffset, 4).getUint32(0, true);
  }

  async readI64LE(): Promise<bigint> {
    const b = await this.src.readBytes(8);
    return new DataView(b.buffer, b.byteOffset, 8).getBigUint64(0, true);
  }

  async readFloat32(): Promise<number> {
    const b = await this.src.readBytes(4);
    return new DataView(b.buffer, b.byteOffset, 4).getFloat32(0, true);
  }

  /** Skip a single field of the given wire type. */
  async skipField(wire: number): Promise<void> {
    switch (wire) {
      case WIRE_VARINT: {
        // Read until MSB cleared, max 10 bytes.
        for (let i = 0; i < 10; i++) {
          const b = await this.src.readByte();
          if ((b & 0x80) === 0) return;
        }
        throw new Error("Varint runaway during skip.");
      }
      case WIRE_I64:
        await this.src.skipBytes(8);
        return;
      case WIRE_I32:
        await this.src.skipBytes(4);
        return;
      case WIRE_LEN: {
        const len = await this.readVarintNumber();
        await this.src.skipBytes(len);
        return;
      }
      case WIRE_SGROUP:
      case WIRE_EGROUP:
        throw new Error("Group wire types are not supported (deprecated).");
      default:
        throw new Error(`Unknown wire type ${wire}.`);
    }
  }

  /** Convenience: skip everything until `endPos`. */
  async skipToEnd(): Promise<void> {
    if (this.src.pos < this.endPos) {
      await this.src.skipBytes(this.endPos - this.src.pos);
    }
  }
}