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// 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 (
"errors"
"fmt"
"io"
"math/bits"
"github.com/klauspost/compress/internal/le"
)
// 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
value uint64 // Maybe use [16]byte, but shifting is awkward.
cursor int // offset where next read should end
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
// 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.cursor = len(in)
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 int(b.get32BitsFast(n))
}
// get32BitsFast requires that at least one bit is requested every time.
// There are no checks if the buffer is filled.
func (b *bitReader) get32BitsFast(n uint8) uint32 {
const regMask = 64 - 1
v := uint32((b.value << (b.bitsRead & regMask)) >> ((regMask + 1 - n) & regMask))
b.bitsRead += n
return 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
}
b.cursor -= 4
b.value = (b.value << 32) | uint64(le.Load32(b.in, b.cursor))
b.bitsRead -= 32
}
// fillFastStart() assumes the bitreader is empty and there is at least 8 bytes to read.
func (b *bitReader) fillFastStart() {
b.cursor -= 8
b.value = le.Load64(b.in, b.cursor)
b.bitsRead = 0
}
// fill() will make sure at least 32 bits are available.
func (b *bitReader) fill() {
if b.bitsRead < 32 {
return
}
if b.cursor >= 4 {
b.cursor -= 4
b.value = (b.value << 32) | uint64(le.Load32(b.in, b.cursor))
b.bitsRead -= 32
return
}
b.bitsRead -= uint8(8 * b.cursor)
for b.cursor > 0 {
b.cursor -= 1
b.value = (b.value << 8) | uint64(b.in[b.cursor])
}
}
// finished returns true if all bits have been read from the bit stream.
func (b *bitReader) finished() bool {
return b.cursor == 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 8*uint(b.cursor) + 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
b.cursor = 0
if !b.finished() {
return fmt.Errorf("%d extra bits on block, should be 0", b.remain())
}
if b.bitsRead > 64 {
return io.ErrUnexpectedEOF
}
return nil
}
func highBits(val uint32) (n uint32) {
return uint32(bits.Len32(val) - 1)
}
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