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// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package bzip2
// moveToFrontDecoder implements a move-to-front list. Such a list is an
// efficient way to transform a string with repeating elements into one with
// many small valued numbers, which is suitable for entropy encoding. It works
// by starting with an initial list of symbols and references symbols by their
// index into that list. When a symbol is referenced, it's moved to the front
// of the list. Thus, a repeated symbol ends up being encoded with many zeros,
// as the symbol will be at the front of the list after the first access.
type moveToFrontDecoder []byte
// newMTFDecoder creates a move-to-front decoder with an explicit initial list
// of symbols.
func newMTFDecoder(symbols []byte) moveToFrontDecoder {
if len(symbols) > 256 {
panic("too many symbols")
return moveToFrontDecoder(symbols)
// newMTFDecoderWithRange creates a move-to-front decoder with an initial
// symbol list of 0...n-1.
func newMTFDecoderWithRange(n int) moveToFrontDecoder {
if n > 256 {
panic("newMTFDecoderWithRange: cannot have > 256 symbols")
m := make([]byte, n)
for i := 0; i < n; i++ {
m[i] = byte(i)
return moveToFrontDecoder(m)
func (m moveToFrontDecoder) Decode(n int) (b byte) {
// Implement move-to-front with a simple copy. This approach
// beats more sophisticated approaches in benchmarking, probably
// because it has high locality of reference inside of a
// single cache line (most move-to-front operations have n < 64).
b = m[n]
copy(m[1:], m[:n])
m[0] = b
// First returns the symbol at the front of the list.
func (m moveToFrontDecoder) First() byte {
return m[0]