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