smartarchive/ts/bzip2/bititerator.ts

import type { IBitReader } from '../interfaces.js';

const BITMASK = [0, 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f, 0xff] as const;

/**
 * Creates a bit reader function for BZIP2 decompression.
 * Takes a buffer iterator as input and returns a function that reads bits.
 */
export function bitIterator(nextBuffer: () => Buffer): IBitReader {
  let bit = 0;
  let byte = 0;
  let bytes = nextBuffer();
  let _bytesRead = 0;

  const reader = function (n: number | null): number | void {
    if (n === null && bit !== 0) {
      // align to byte boundary
      bit = 0;
      byte++;
      return;
    }

    let result = 0;
    let remaining = n as number;

    while (remaining > 0) {
      if (byte >= bytes.length) {
        byte = 0;
        bytes = nextBuffer();
      }

      const left = 8 - bit;

      if (bit === 0 && remaining > 0) {
        _bytesRead++;
      }

      if (remaining >= left) {
        result <<= left;
        result |= BITMASK[left] & bytes[byte++];
        bit = 0;
        remaining -= left;
      } else {
        result <<= remaining;
        result |= (bytes[byte] & (BITMASK[remaining] << (8 - remaining - bit))) >> (8 - remaining - bit);
        bit += remaining;
        remaining = 0;
      }
    }

    return result;
  } as IBitReader;

  Object.defineProperty(reader, 'bytesRead', {
    get: () => _bytesRead,
    enumerable: true,
  });

  return reader;
}