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