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huffman_tree.go
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/
huffman_tree.go
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package main
import (
"container/heap"
"encoding/binary"
"errors"
"fmt"
"io"
"math"
)
var errInvalidCompressedData = errors.New("invalid compressed data")
type node struct {
Frequency uint32 `json:"fr,omitempty"`
Left *node `json:"l,omitempty"`
Right *node `json:"r,omitempty"`
IsLeaf bool `json:"ilf,omitempty"`
Value byte `json:"v,omitempty"`
}
// this needs to use a heap for nodes
func buildHuffmanTree(r io.Reader) (*node, error) {
if r == nil {
return nil, nil
}
byteFrequency := map[byte]uint32{}
for {
buff := make([]byte, buffSize)
n, err := r.Read(buff)
if err != nil && !errors.Is(err, io.EOF) {
return nil, err
}
for i := 0; i < n; i++ {
byteFrequency[buff[i]]++
}
if errors.Is(err, io.EOF) {
break
}
}
// ensure there are more than one character
for _, c := range []byte{'a', 'b'} {
if _, ok := byteFrequency[c]; !ok {
byteFrequency[c] = 0
}
}
nodes := &nodeHeap{}
for b, freq := range byteFrequency {
heap.Push(nodes, node{
Frequency: freq,
IsLeaf: true,
Value: b,
})
}
for {
if nodes.Len() == 1 {
break
}
n1 := heap.Pop(nodes).(node)
n2 := heap.Pop(nodes).(node)
heap.Push(nodes, node{
Left: &n1,
Right: &n2,
Frequency: n1.Frequency + n2.Frequency,
})
}
return &(*nodes)[0], nil
}
func (n *node) buildPrefixCodeTable() map[byte][]bool {
table := map[byte][]bool{}
representation := []bool{}
explore(n, representation, table)
return table
}
// explore explores tree nodes, while assigning binary representation to leaf nodes in the code table
func explore(n *node, representation []bool, table map[byte][]bool) {
if n == nil {
return
}
if n.IsLeaf {
cp := make([]bool, len(representation))
copy(cp, representation)
table[n.Value] = cp
return
}
explore(n.Left, append(representation, false), table)
explore(n.Right, append(representation, true), table)
}
func (n *node) compress(r io.Reader) ([]byte, error) {
prefixCodeTable := n.buildPrefixCodeTable()
bits := []bool{}
for {
data := make([]byte, buffSize)
n, err := r.Read(data)
if err != nil && !errors.Is(err, io.EOF) {
return nil, err
}
for i := 0; i < n; i++ {
code, ok := prefixCodeTable[data[i]]
if !ok {
// should never happen
return nil, errors.New("byte '%c' is not found in prefix code table")
}
bits = append(bits, code...)
}
if errors.Is(err, io.EOF) {
break
}
}
binBytes := make([]byte, int(math.Ceil(float64(len(bits))/8)))
for idx := range bits {
currentByte := &binBytes[idx/8]
if bits[idx] {
*currentByte += 1 << (7 - idx%8)
}
}
result := make([]byte, 4+len(binBytes))
binary.BigEndian.PutUint32(result, uint32(len(bits)))
copy(result[4:], binBytes)
return result, nil
}
func (root *node) decompress(bits []bool, w io.Writer) error {
if len(bits) == 0 {
return nil
}
data := make([]byte, 0, buffSize)
cur := root
for i := 0; i <= len(bits); i++ {
if cur == nil {
return fmt.Errorf("invalid char code: %w", errInvalidCompressedData)
}
if cur.IsLeaf {
data = append(data, cur.Value)
if len(data) == buffSize {
_, err := w.Write(data)
if err != nil {
return fmt.Errorf("write failed: %w", err)
}
data = make([]byte, 0, buffSize)
}
cur = root
if i == len(bits) {
break
}
i--
continue
}
if i == len(bits) {
// last bit must be a leaf
return fmt.Errorf("incorrect last character code: %w", errInvalidCompressedData)
}
if !bits[i] {
cur = cur.Left
continue
}
cur = cur.Right
}
_, err := w.Write(data)
if err != nil {
return fmt.Errorf("write failed: %w", err)
}
return nil
}