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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 struct {
// Rather than actually keep the list in memory, the symbols are stored
// as a circular, double linked list with the symbol indexed by head
// at the front of the list.
symbols [256]byte
next [256]uint8
prev [256]uint8
head uint8
len int
}
// 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")
}
m := new(moveToFrontDecoder)
copy(m.symbols[:], symbols)
m.len = len(symbols)
m.threadLinkedList()
return m
}
// 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 := new(moveToFrontDecoder)
for i := 0; i < n; i++ {
m.symbols[byte(i)] = byte(i)
}
m.len = n
m.threadLinkedList()
return m
}
// threadLinkedList creates the initial linked-list pointers.
func (m *moveToFrontDecoder) threadLinkedList() {
if m.len == 0 {
return
}
m.prev[0] = uint8(m.len - 1)
for i := byte(0); int(i) < m.len-1; i++ {
m.next[i] = uint8(i + 1)
m.prev[i+1] = uint8(i)
}
m.next[m.len-1] = 0
}
func (m *moveToFrontDecoder) Decode(n int) (b byte) {
// Most of the time, n will be zero so it's worth dealing with this
// simple case.
if n == 0 {
return m.symbols[m.head]
}
i := m.head
for j := 0; j < n; j++ {
i = m.next[i]
}
b = m.symbols[i]
m.next[m.prev[i]] = m.next[i]
m.prev[m.next[i]] = m.prev[i]
m.next[i] = m.head
m.prev[i] = m.prev[m.head]
m.next[m.prev[m.head]] = i
m.prev[m.head] = i
m.head = i
return
}
// First returns the symbol at the front of the list.
func (m *moveToFrontDecoder) First() byte {
return m.symbols[m.head]
}
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