Lesson 16 · Reading multi-bit values - Build a PNG Encoder and Decoder

Reading multi-bit values

Most DEFLATE fields are several bits wide - lengths, distances, header counts. Today you read n bits at once as an integer, assembled in the same low-bit-first order, so higher lessons can ask for whole values.

The goal

Read an n-bit unsigned integer from the bit reader, with the first bit read as the least significant.

Start here - the target

TO DO

Scenario: Reading a value of several bits

Givena bit reader positioned at the start of the byte 0xB4

When3 bits are read as a value, then 5 more bits are read as a value

Thenthe first value is 4 and the second value is 22

Andeach value places its first-read bit in position 0, so the low bits of 0xB4 give 4 and the high bits give 22

Background

A single-bit reader is not enough; DEFLATE constantly reads small fixed-width numbers - a 2-bit block type, a 5-bit count, a handful of extra bits on a length. ReadBits(n) calls ReadBit n times and, keeping the low-bit-first rule, puts the first bit read into position 0, the second into position 1, and so on. So reading 3 bits from 0xB4 takes its low three bits (100 in stream order) and assembles them as 4, then reading 5 bits takes the remaining 10110 and assembles them as 22.

This is where the ordering rule pays off concretely: the same bits, read low-first and placed low-first, reconstruct exactly the values the encoder wrote. Note that this scheme is used for the numeric fields; Huffman codes have their own bit order, which is the next thing you will meet. For now, ReadBits gives every fixed-width field in the format.

Make it work

func (r *BitReader) ReadBits(n uint) uint32 {
  var v uint32
  for i := uint(0); i < n; i++ {
    v |= r.ReadBit() << i   // first bit read -> bit 0
  }
  return v
}

CheckpointDONE

You can read multi-bit DEFLATE values in the correct order. Commit and stop here.