Static Arithmetic Coding

hard · compression, arithmetic-coding, entropy

Static Arithmetic (Range) Coding

Implement a static arithmetic coder using fixed frequencies.

Functions

func BuildFrequencies(data []byte) []int
func ArithmeticEncode(data []byte, freqs []int) []byte
func ArithmeticDecode(encoded []byte, freqs []int) []byte

Behavior

freqs has length 257 (symbols 0..255 plus EOF at index 256).
BuildFrequencies counts bytes in data and sets freqs[256] = 1.
ArithmeticEncode encodes data followed by an explicit EOF symbol.
ArithmeticDecode decodes until EOF is seen.

Constraints

Total frequency sum must fit within 20 bits (sum <= 1<<20).
Input length <= 4096 (to keep arithmetic ranges safe).

Notes

Use a fixed-precision arithmetic coder with renormalization and underflow handling.
Bitstream is MSB-first; trailing zero padding is allowed.

package solution

// BitWriter writes bits MSB-first.
type BitWriter struct {
	buf   []byte
	cur   byte
	nbits uint8
}

func NewBitWriter() *BitWriter { return &BitWriter{} }

func (bw *BitWriter) WriteBit(bit uint8) {
	if bit&1 == 1 {
		bw.cur |= 1 << (7 - bw.nbits)
	}
	bw.nbits++
	if bw.nbits == 8 {
		bw.buf = append(bw.buf, bw.cur)
		bw.cur = 0
		bw.nbits = 0
	}
}

func (bw *BitWriter) Bytes() []byte {
	out := make([]byte, len(bw.buf))
	copy(out, bw.buf)
	if bw.nbits > 0 {
		out = append(out, bw.cur)
	}
	return out
}

// BitReader reads bits MSB-first.
type BitReader struct {
	data  []byte
	idx   int
	nbits uint8
}

func NewBitReader(data []byte) *BitReader { return &BitReader{data: data} }

func (br *BitReader) ReadBit() (uint8, bool) {
	if br.idx >= len(br.data) {
		return 0, false
	}
	b := br.data[br.idx]
	bit := (b >> (7 - br.nbits)) & 1
	br.nbits++
	if br.nbits == 8 {
		br.idx++
		br.nbits = 0
	}
	return bit, true
}

// BuildFrequencies counts bytes in data and appends EOF=1.
func BuildFrequencies(data []byte) []int {
	// TODO: implement
	return nil
}

// ArithmeticEncode encodes data + EOF using freqs.
func ArithmeticEncode(data []byte, freqs []int) []byte {
	// TODO: implement
	return nil
}

// ArithmeticDecode decodes until EOF using freqs.
func ArithmeticDecode(encoded []byte, freqs []int) []byte {
	// TODO: implement
	return nil
}