Module @4bitlabs/readers

A bit-reader allows for bits level access to a sequence of bytes, allowing bit-level reads that easily cross byte-level boundaries. You can think of a bit-reader like a long sequence of bits that can be shifted off, providing access to later bits. Consider:

const source = Uint8Array.of(0b1111_0011, 0b1100_1111, 0b1010_1010);
Copy

If you wanted the most-significant 4-bits of this byte sequence, you could use a bitmask and a bitwise shifts:

const value = (source[0] & 0b1111_0000) >>> 4; // 15
Copy

This can be useful for simple encoded data, however, can become unweildly when crossing multiple bytes. Let's say you wanted to get the bits

        From           To
         |-------------|
         v             v
0b1111_0011_1100_1111_1010_1010
Copy

With bitwise operators on a Uint8Array, you'd have to:

const value =
  // select and shift the most-significant bits
  ((source[0] & 0b0000_0011) << 10) |
  // select and shift the middle bits
  (source[1] << 2) |
  // select and shift the least-significant bits
  ((source[2] & 0b1100_0000) >>> 6);
Copy

With a BitReader, you can instead say:

const reader = createBitReader(source);
reader.skip(6); // skip the first 6 bits
const value = reader.read(12); // take the next 12 bits
Copy

This can be very useful when parsing densely-packed data-structures, especially when they use variable-length encoding.

See

createBitReader