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108 lines
2.8 KiB
108 lines
2.8 KiB
// Copyright 2015, Joe Tsai. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE.md file.
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// Package internal is a collection of common compression algorithms.
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//
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// For performance reasons, these packages lack strong error checking and
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// require that the caller to ensure that strict invariants are kept.
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package internal
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var (
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// IdentityLUT returns the input key itself.
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IdentityLUT = func() (lut [256]byte) {
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for i := range lut {
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lut[i] = uint8(i)
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}
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return lut
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}()
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// ReverseLUT returns the input key with its bits reversed.
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ReverseLUT = func() (lut [256]byte) {
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for i := range lut {
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b := uint8(i)
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b = (b&0xaa)>>1 | (b&0x55)<<1
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b = (b&0xcc)>>2 | (b&0x33)<<2
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b = (b&0xf0)>>4 | (b&0x0f)<<4
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lut[i] = b
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}
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return lut
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}()
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)
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// ReverseUint32 reverses all bits of v.
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func ReverseUint32(v uint32) (x uint32) {
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x |= uint32(ReverseLUT[byte(v>>0)]) << 24
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x |= uint32(ReverseLUT[byte(v>>8)]) << 16
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x |= uint32(ReverseLUT[byte(v>>16)]) << 8
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x |= uint32(ReverseLUT[byte(v>>24)]) << 0
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return x
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}
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// ReverseUint32N reverses the lower n bits of v.
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func ReverseUint32N(v uint32, n uint) (x uint32) {
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return ReverseUint32(v << (32 - n))
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}
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// ReverseUint64 reverses all bits of v.
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func ReverseUint64(v uint64) (x uint64) {
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x |= uint64(ReverseLUT[byte(v>>0)]) << 56
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x |= uint64(ReverseLUT[byte(v>>8)]) << 48
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x |= uint64(ReverseLUT[byte(v>>16)]) << 40
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x |= uint64(ReverseLUT[byte(v>>24)]) << 32
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x |= uint64(ReverseLUT[byte(v>>32)]) << 24
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x |= uint64(ReverseLUT[byte(v>>40)]) << 16
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x |= uint64(ReverseLUT[byte(v>>48)]) << 8
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x |= uint64(ReverseLUT[byte(v>>56)]) << 0
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return x
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}
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// ReverseUint64N reverses the lower n bits of v.
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func ReverseUint64N(v uint64, n uint) (x uint64) {
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return ReverseUint64(v << (64 - n))
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}
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// MoveToFront is a data structure that allows for more efficient move-to-front
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// transformations. This specific implementation assumes that the alphabet is
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// densely packed within 0..255.
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type MoveToFront struct {
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dict [256]uint8 // Mapping from indexes to values
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tail int // Number of tail bytes that are already ordered
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}
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func (m *MoveToFront) Encode(vals []uint8) {
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copy(m.dict[:], IdentityLUT[:256-m.tail]) // Reset dict to be identity
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var max int
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for i, val := range vals {
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var idx uint8 // Reverse lookup idx in dict
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for di, dv := range m.dict {
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if dv == val {
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idx = uint8(di)
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break
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}
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}
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vals[i] = idx
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max |= int(idx)
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copy(m.dict[1:], m.dict[:idx])
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m.dict[0] = val
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}
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m.tail = 256 - max - 1
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}
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func (m *MoveToFront) Decode(idxs []uint8) {
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copy(m.dict[:], IdentityLUT[:256-m.tail]) // Reset dict to be identity
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var max int
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for i, idx := range idxs {
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val := m.dict[idx] // Forward lookup val in dict
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idxs[i] = val
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max |= int(idx)
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copy(m.dict[1:], m.dict[:idx])
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m.dict[0] = val
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}
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m.tail = 256 - max - 1
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}
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