137 lines
3.3 KiB
Go
137 lines
3.3 KiB
Go
// Copyright 2019+ Klaus Post. All rights reserved.
|
|
// License information can be found in the LICENSE file.
|
|
// Based on work by Yann Collet, released under BSD License.
|
|
|
|
package zstd
|
|
|
|
import (
|
|
"encoding/binary"
|
|
"errors"
|
|
"io"
|
|
"math/bits"
|
|
)
|
|
|
|
// bitReader reads a bitstream in reverse.
|
|
// The last set bit indicates the start of the stream and is used
|
|
// for aligning the input.
|
|
type bitReader struct {
|
|
in []byte
|
|
off uint // next byte to read is at in[off - 1]
|
|
value uint64 // Maybe use [16]byte, but shifting is awkward.
|
|
bitsRead uint8
|
|
}
|
|
|
|
// init initializes and resets the bit reader.
|
|
func (b *bitReader) init(in []byte) error {
|
|
if len(in) < 1 {
|
|
return errors.New("corrupt stream: too short")
|
|
}
|
|
b.in = in
|
|
b.off = uint(len(in))
|
|
// The highest bit of the last byte indicates where to start
|
|
v := in[len(in)-1]
|
|
if v == 0 {
|
|
return errors.New("corrupt stream, did not find end of stream")
|
|
}
|
|
b.bitsRead = 64
|
|
b.value = 0
|
|
if len(in) >= 8 {
|
|
b.fillFastStart()
|
|
} else {
|
|
b.fill()
|
|
b.fill()
|
|
}
|
|
b.bitsRead += 8 - uint8(highBits(uint32(v)))
|
|
return nil
|
|
}
|
|
|
|
// getBits will return n bits. n can be 0.
|
|
func (b *bitReader) getBits(n uint8) int {
|
|
if n == 0 /*|| b.bitsRead >= 64 */ {
|
|
return 0
|
|
}
|
|
return b.getBitsFast(n)
|
|
}
|
|
|
|
// getBitsFast requires that at least one bit is requested every time.
|
|
// There are no checks if the buffer is filled.
|
|
func (b *bitReader) getBitsFast(n uint8) int {
|
|
const regMask = 64 - 1
|
|
v := uint32((b.value << (b.bitsRead & regMask)) >> ((regMask + 1 - n) & regMask))
|
|
b.bitsRead += n
|
|
return int(v)
|
|
}
|
|
|
|
// fillFast() will make sure at least 32 bits are available.
|
|
// There must be at least 4 bytes available.
|
|
func (b *bitReader) fillFast() {
|
|
if b.bitsRead < 32 {
|
|
return
|
|
}
|
|
// 2 bounds checks.
|
|
v := b.in[b.off-4:]
|
|
v = v[:4]
|
|
low := (uint32(v[0])) | (uint32(v[1]) << 8) | (uint32(v[2]) << 16) | (uint32(v[3]) << 24)
|
|
b.value = (b.value << 32) | uint64(low)
|
|
b.bitsRead -= 32
|
|
b.off -= 4
|
|
}
|
|
|
|
// fillFastStart() assumes the bitreader is empty and there is at least 8 bytes to read.
|
|
func (b *bitReader) fillFastStart() {
|
|
// Do single re-slice to avoid bounds checks.
|
|
b.value = binary.LittleEndian.Uint64(b.in[b.off-8:])
|
|
b.bitsRead = 0
|
|
b.off -= 8
|
|
}
|
|
|
|
// fill() will make sure at least 32 bits are available.
|
|
func (b *bitReader) fill() {
|
|
if b.bitsRead < 32 {
|
|
return
|
|
}
|
|
if b.off >= 4 {
|
|
v := b.in[b.off-4:]
|
|
v = v[:4]
|
|
low := (uint32(v[0])) | (uint32(v[1]) << 8) | (uint32(v[2]) << 16) | (uint32(v[3]) << 24)
|
|
b.value = (b.value << 32) | uint64(low)
|
|
b.bitsRead -= 32
|
|
b.off -= 4
|
|
return
|
|
}
|
|
for b.off > 0 {
|
|
b.value = (b.value << 8) | uint64(b.in[b.off-1])
|
|
b.bitsRead -= 8
|
|
b.off--
|
|
}
|
|
}
|
|
|
|
// finished returns true if all bits have been read from the bit stream.
|
|
func (b *bitReader) finished() bool {
|
|
return b.off == 0 && b.bitsRead >= 64
|
|
}
|
|
|
|
// overread returns true if more bits have been requested than is on the stream.
|
|
func (b *bitReader) overread() bool {
|
|
return b.bitsRead > 64
|
|
}
|
|
|
|
// remain returns the number of bits remaining.
|
|
func (b *bitReader) remain() uint {
|
|
return b.off*8 + 64 - uint(b.bitsRead)
|
|
}
|
|
|
|
// close the bitstream and returns an error if out-of-buffer reads occurred.
|
|
func (b *bitReader) close() error {
|
|
// Release reference.
|
|
b.in = nil
|
|
if b.bitsRead > 64 {
|
|
return io.ErrUnexpectedEOF
|
|
}
|
|
return nil
|
|
}
|
|
|
|
func highBits(val uint32) (n uint32) {
|
|
return uint32(bits.Len32(val) - 1)
|
|
}
|