103 lines
2.3 KiB
Go
103 lines
2.3 KiB
Go
// Copyright 2019 The Go Authors. 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 file.
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package ccitt
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import (
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"encoding/binary"
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"io"
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)
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type bitWriter struct {
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w io.Writer
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// order is whether to process w's bytes LSB first or MSB first.
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order Order
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// The high nBits bits of the bits field hold encoded bits to be written to w.
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bits uint64
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nBits uint32
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// bytes[:bw] holds encoded bytes not yet written to w.
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// Overflow protection is ensured by using a multiple of 8 as bytes length.
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bw uint32
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bytes [1024]uint8
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}
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// flushBits copies 64 bits from b.bits to b.bytes. If b.bytes is then full, it
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// is written to b.w.
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func (b *bitWriter) flushBits() error {
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binary.BigEndian.PutUint64(b.bytes[b.bw:], b.bits)
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b.bits = 0
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b.nBits = 0
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b.bw += 8
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if b.bw < uint32(len(b.bytes)) {
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return nil
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}
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b.bw = 0
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if b.order != MSB {
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reverseBitsWithinBytes(b.bytes[:])
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}
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_, err := b.w.Write(b.bytes[:])
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return err
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}
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// close finalizes a bitcode stream by writing any
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// pending bits to bitWriter's underlying io.Writer.
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func (b *bitWriter) close() error {
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// Write any encoded bits to bytes.
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if b.nBits > 0 {
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binary.BigEndian.PutUint64(b.bytes[b.bw:], b.bits)
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b.bw += (b.nBits + 7) >> 3
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}
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if b.order != MSB {
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reverseBitsWithinBytes(b.bytes[:b.bw])
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}
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// Write b.bw bytes to b.w.
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_, err := b.w.Write(b.bytes[:b.bw])
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return err
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}
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// alignToByteBoundary rounds b.nBits up to a multiple of 8.
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// If all 64 bits are used, flush them to bitWriter's bytes.
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func (b *bitWriter) alignToByteBoundary() error {
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if b.nBits = (b.nBits + 7) &^ 7; b.nBits == 64 {
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return b.flushBits()
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}
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return nil
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}
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// writeCode writes a variable length bitcode to b's underlying io.Writer.
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func (b *bitWriter) writeCode(bs bitString) error {
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bits := bs.bits
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nBits := bs.nBits
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if 64-b.nBits >= nBits {
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// b.bits has sufficient room for storing nBits bits.
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b.bits |= uint64(bits) << (64 - nBits - b.nBits)
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b.nBits += nBits
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if b.nBits == 64 {
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return b.flushBits()
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}
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return nil
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}
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// Number of leading bits that fill b.bits.
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i := 64 - b.nBits
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// Fill b.bits then flush and write remaining bits.
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b.bits |= uint64(bits) >> (nBits - i)
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b.nBits = 64
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if err := b.flushBits(); err != nil {
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return err
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}
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nBits -= i
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b.bits = uint64(bits) << (64 - nBits)
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b.nBits = nBits
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return nil
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}
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