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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 []byte
        next    []uint8
        prev    []uint8
        head    uint8
}

// 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 := &moveToFrontDecoder{
                symbols: symbols,
                next:    make([]uint8, len(symbols)),
                prev:    make([]uint8, 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 := &moveToFrontDecoder{
                symbols: make([]uint8, n),
                next:    make([]uint8, n),
                prev:    make([]uint8, n),
        }

        for i := 0; i < n; i++ {
                m.symbols[i] = byte(i)
        }

        m.threadLinkedList()
        return m
}

// threadLinkedList creates the initial linked-list pointers.
func (m *moveToFrontDecoder) threadLinkedList() {
        if len(m.symbols) == 0 {
                return
        }

        m.prev[0] = uint8(len(m.symbols) - 1)

        for i := 0; i < len(m.symbols)-1; i++ {
                m.next[i] = uint8(i + 1)
                m.prev[i+1] = uint8(i)
        }

        m.next[len(m.symbols)-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]
}