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raczlib.go
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raczlib.go
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// Copyright 2019 The Wuffs Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// ----------------
// Package raczlib provides access to RAC (Random Access Compression) files
// with the Zlib compression codec.
//
// The RAC specification is at
// https://github.com/google/wuffs/blob/master/doc/spec/rac-spec.md
package raczlib
import (
"bytes"
"compress/zlib"
"errors"
"io"
"github.com/google/wuffs/lib/compression"
"github.com/google/wuffs/lib/internal/racdict"
"github.com/google/wuffs/lib/rac"
"github.com/google/wuffs/lib/zlibcut"
)
var (
errInvalidCodec = errors.New("raczlib: invalid codec")
errInvalidDictionary = errors.New("raczlib: invalid dictionary")
)
func u32BE(b []byte) uint32 {
_ = b[3] // Early bounds check to guarantee safety of reads below.
return (uint32(b[0]) << 24) | (uint32(b[1]) << 16) | (uint32(b[2]) << 8) | (uint32(b[3]))
}
// refine returns the last 32 KiB of b, or if b is shorter than that, it
// returns just b.
//
// The Zlib format only supports up to 32 KiB of history or shared dictionary.
func refine(b []byte) []byte {
const n = 32 * 1024
if len(b) > n {
return b[len(b)-n:]
}
return b
}
// CodecReader specializes a rac.Reader to decode Zlib-compressed chunks.
type CodecReader struct {
// cachedReader lets us re-use the memory allocated for a zlib reader, when
// decompressing multiple chunks.
cachedReader compression.Reader
// lim provides a limited view of a RAC file.
lim io.LimitedReader
// dictLoader loads shared dictionaries.
dictLoader racdict.Loader
}
// Close implements rac.CodecReader.
func (r *CodecReader) Close() error {
return nil
}
// Accepts implements rac.CodecReader.
func (r *CodecReader) Accepts(c rac.Codec) bool {
return c == rac.CodecZlib
}
// Clone implements rac.CodecReader.
func (r *CodecReader) Clone() rac.CodecReader {
return &CodecReader{}
}
// MakeDecompressor implements rac.CodecReader.
func (r *CodecReader) MakeDecompressor(racFile io.ReadSeeker, chunk rac.Chunk) (io.Reader, error) {
dict, err := r.dictLoader.Load(racFile, chunk)
if err != nil {
return nil, err
}
if _, err := racFile.Seek(chunk.CPrimary[0], io.SeekStart); err != nil {
return nil, err
}
r.lim.R = racFile
r.lim.N = chunk.CPrimary.Size()
return r.makeDecompressor(&r.lim, dict)
}
// CodecWriter specializes a rac.Writer to encode Zlib-compressed chunks.
type CodecWriter struct {
// These fields help reduce memory allocation by re-using previous byte
// buffers or the zlib.Writer.
compressed bytes.Buffer
cachedWriter *zlib.Writer
// dictSaver saves shared dictionaries.
dictSaver racdict.Saver
}
// Close implements rac.CodecWriter.
func (w *CodecWriter) Close() error {
return nil
}
// Clone implements rac.CodecWriter.
func (w *CodecWriter) Clone() rac.CodecWriter {
return &CodecWriter{}
}
// Compress implements rac.CodecWriter.
func (w *CodecWriter) Compress(p []byte, q []byte, resourcesData [][]byte) (
codec rac.Codec, compressed []byte, secondaryResource int, tertiaryResource int, retErr error) {
return w.dictSaver.Compress(
p, q, resourcesData,
rac.CodecZlib, w.compress, refine,
)
}
func (w *CodecWriter) compress(p []byte, q []byte, dict []byte) ([]byte, error) {
w.compressed.Reset()
zw, err := (*zlib.Writer)(nil), error(nil)
if len(dict) != 0 {
// A zlib.Writer doesn't have a Reset(etc, dict) method, so we have to
// allocate a new zlib.Writer.
zw, err = zlib.NewWriterLevelDict(&w.compressed, zlib.BestCompression, dict)
if err != nil {
return nil, err
}
} else if w.cachedWriter == nil {
zw, err = zlib.NewWriterLevelDict(&w.compressed, zlib.BestCompression, nil)
if err != nil {
return nil, err
}
w.cachedWriter = zw
} else {
zw = w.cachedWriter
zw.Reset(&w.compressed)
}
if len(p) > 0 {
if _, err := zw.Write(p); err != nil {
zw.Close()
return nil, err
}
}
if len(q) > 0 {
if _, err := zw.Write(q); err != nil {
zw.Close()
return nil, err
}
}
if err := zw.Close(); err != nil {
return nil, err
}
return w.compressed.Bytes(), nil
}
// CanCut implements rac.CodecWriter.
func (w *CodecWriter) CanCut() bool {
return true
}
// Cut implements rac.CodecWriter.
func (w *CodecWriter) Cut(codec rac.Codec, encoded []byte, maxEncodedLen int) (encodedLen int, decodedLen int, retErr error) {
if codec != rac.CodecZlib {
return 0, 0, errInvalidCodec
}
return zlibcut.Cut(nil, encoded, maxEncodedLen)
}
// WrapResource implements rac.CodecWriter.
func (w *CodecWriter) WrapResource(raw []byte) ([]byte, error) {
return w.dictSaver.WrapResource(raw, refine)
}