mirror of
https://github.com/go-gitea/gitea.git
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847 lines
20 KiB
Go
847 lines
20 KiB
Go
// Copyright 2009 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 flate implements the DEFLATE compressed data format, described in
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// RFC 1951. The gzip and zlib packages implement access to DEFLATE-based file
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// formats.
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package flate
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import (
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"bufio"
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"io"
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"strconv"
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"sync"
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)
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const (
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maxCodeLen = 16 // max length of Huffman code
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// The next three numbers come from the RFC section 3.2.7, with the
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// additional proviso in section 3.2.5 which implies that distance codes
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// 30 and 31 should never occur in compressed data.
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maxNumLit = 286
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maxNumDist = 30
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numCodes = 19 // number of codes in Huffman meta-code
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)
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// Initialize the fixedHuffmanDecoder only once upon first use.
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var fixedOnce sync.Once
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var fixedHuffmanDecoder huffmanDecoder
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// A CorruptInputError reports the presence of corrupt input at a given offset.
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type CorruptInputError int64
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func (e CorruptInputError) Error() string {
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return "flate: corrupt input before offset " + strconv.FormatInt(int64(e), 10)
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}
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// An InternalError reports an error in the flate code itself.
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type InternalError string
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func (e InternalError) Error() string { return "flate: internal error: " + string(e) }
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// A ReadError reports an error encountered while reading input.
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//
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// Deprecated: No longer returned.
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type ReadError struct {
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Offset int64 // byte offset where error occurred
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Err error // error returned by underlying Read
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}
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func (e *ReadError) Error() string {
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return "flate: read error at offset " + strconv.FormatInt(e.Offset, 10) + ": " + e.Err.Error()
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}
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// A WriteError reports an error encountered while writing output.
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//
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// Deprecated: No longer returned.
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type WriteError struct {
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Offset int64 // byte offset where error occurred
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Err error // error returned by underlying Write
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}
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func (e *WriteError) Error() string {
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return "flate: write error at offset " + strconv.FormatInt(e.Offset, 10) + ": " + e.Err.Error()
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}
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// Resetter resets a ReadCloser returned by NewReader or NewReaderDict to
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// to switch to a new underlying Reader. This permits reusing a ReadCloser
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// instead of allocating a new one.
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type Resetter interface {
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// Reset discards any buffered data and resets the Resetter as if it was
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// newly initialized with the given reader.
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Reset(r io.Reader, dict []byte) error
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}
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// The data structure for decoding Huffman tables is based on that of
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// zlib. There is a lookup table of a fixed bit width (huffmanChunkBits),
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// For codes smaller than the table width, there are multiple entries
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// (each combination of trailing bits has the same value). For codes
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// larger than the table width, the table contains a link to an overflow
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// table. The width of each entry in the link table is the maximum code
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// size minus the chunk width.
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//
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// Note that you can do a lookup in the table even without all bits
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// filled. Since the extra bits are zero, and the DEFLATE Huffman codes
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// have the property that shorter codes come before longer ones, the
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// bit length estimate in the result is a lower bound on the actual
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// number of bits.
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//
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// See the following:
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// http://www.gzip.org/algorithm.txt
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// chunk & 15 is number of bits
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// chunk >> 4 is value, including table link
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const (
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huffmanChunkBits = 9
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huffmanNumChunks = 1 << huffmanChunkBits
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huffmanCountMask = 15
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huffmanValueShift = 4
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)
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type huffmanDecoder struct {
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min int // the minimum code length
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chunks [huffmanNumChunks]uint32 // chunks as described above
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links [][]uint32 // overflow links
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linkMask uint32 // mask the width of the link table
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}
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// Initialize Huffman decoding tables from array of code lengths.
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// Following this function, h is guaranteed to be initialized into a complete
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// tree (i.e., neither over-subscribed nor under-subscribed). The exception is a
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// degenerate case where the tree has only a single symbol with length 1. Empty
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// trees are permitted.
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func (h *huffmanDecoder) init(bits []int) bool {
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// Sanity enables additional runtime tests during Huffman
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// table construction. It's intended to be used during
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// development to supplement the currently ad-hoc unit tests.
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const sanity = false
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if h.min != 0 {
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*h = huffmanDecoder{}
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}
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// Count number of codes of each length,
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// compute min and max length.
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var count [maxCodeLen]int
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var min, max int
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for _, n := range bits {
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if n == 0 {
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continue
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}
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if min == 0 || n < min {
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min = n
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}
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if n > max {
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max = n
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}
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count[n]++
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}
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// Empty tree. The decompressor.huffSym function will fail later if the tree
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// is used. Technically, an empty tree is only valid for the HDIST tree and
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// not the HCLEN and HLIT tree. However, a stream with an empty HCLEN tree
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// is guaranteed to fail since it will attempt to use the tree to decode the
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// codes for the HLIT and HDIST trees. Similarly, an empty HLIT tree is
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// guaranteed to fail later since the compressed data section must be
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// composed of at least one symbol (the end-of-block marker).
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if max == 0 {
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return true
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}
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code := 0
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var nextcode [maxCodeLen]int
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for i := min; i <= max; i++ {
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code <<= 1
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nextcode[i] = code
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code += count[i]
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}
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// Check that the coding is complete (i.e., that we've
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// assigned all 2-to-the-max possible bit sequences).
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// Exception: To be compatible with zlib, we also need to
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// accept degenerate single-code codings. See also
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// TestDegenerateHuffmanCoding.
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if code != 1<<uint(max) && !(code == 1 && max == 1) {
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return false
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}
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h.min = min
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if max > huffmanChunkBits {
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numLinks := 1 << (uint(max) - huffmanChunkBits)
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h.linkMask = uint32(numLinks - 1)
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// create link tables
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link := nextcode[huffmanChunkBits+1] >> 1
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h.links = make([][]uint32, huffmanNumChunks-link)
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for j := uint(link); j < huffmanNumChunks; j++ {
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reverse := int(reverseByte[j>>8]) | int(reverseByte[j&0xff])<<8
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reverse >>= uint(16 - huffmanChunkBits)
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off := j - uint(link)
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if sanity && h.chunks[reverse] != 0 {
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panic("impossible: overwriting existing chunk")
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}
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h.chunks[reverse] = uint32(off<<huffmanValueShift | (huffmanChunkBits + 1))
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h.links[off] = make([]uint32, numLinks)
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}
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}
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for i, n := range bits {
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if n == 0 {
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continue
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}
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code := nextcode[n]
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nextcode[n]++
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chunk := uint32(i<<huffmanValueShift | n)
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reverse := int(reverseByte[code>>8]) | int(reverseByte[code&0xff])<<8
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reverse >>= uint(16 - n)
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if n <= huffmanChunkBits {
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for off := reverse; off < len(h.chunks); off += 1 << uint(n) {
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// We should never need to overwrite
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// an existing chunk. Also, 0 is
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// never a valid chunk, because the
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// lower 4 "count" bits should be
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// between 1 and 15.
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if sanity && h.chunks[off] != 0 {
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panic("impossible: overwriting existing chunk")
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}
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h.chunks[off] = chunk
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}
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} else {
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j := reverse & (huffmanNumChunks - 1)
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if sanity && h.chunks[j]&huffmanCountMask != huffmanChunkBits+1 {
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// Longer codes should have been
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// associated with a link table above.
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panic("impossible: not an indirect chunk")
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}
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value := h.chunks[j] >> huffmanValueShift
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linktab := h.links[value]
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reverse >>= huffmanChunkBits
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for off := reverse; off < len(linktab); off += 1 << uint(n-huffmanChunkBits) {
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if sanity && linktab[off] != 0 {
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panic("impossible: overwriting existing chunk")
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}
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linktab[off] = chunk
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}
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}
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}
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if sanity {
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// Above we've sanity checked that we never overwrote
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// an existing entry. Here we additionally check that
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// we filled the tables completely.
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for i, chunk := range h.chunks {
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if chunk == 0 {
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// As an exception, in the degenerate
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// single-code case, we allow odd
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// chunks to be missing.
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if code == 1 && i%2 == 1 {
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continue
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}
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panic("impossible: missing chunk")
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}
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}
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for _, linktab := range h.links {
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for _, chunk := range linktab {
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if chunk == 0 {
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panic("impossible: missing chunk")
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}
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}
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}
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}
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return true
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}
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// The actual read interface needed by NewReader.
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// If the passed in io.Reader does not also have ReadByte,
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// the NewReader will introduce its own buffering.
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type Reader interface {
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io.Reader
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io.ByteReader
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}
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// Decompress state.
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type decompressor struct {
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// Input source.
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r Reader
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roffset int64
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// Input bits, in top of b.
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b uint32
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nb uint
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// Huffman decoders for literal/length, distance.
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h1, h2 huffmanDecoder
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// Length arrays used to define Huffman codes.
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bits *[maxNumLit + maxNumDist]int
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codebits *[numCodes]int
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// Output history, buffer.
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dict dictDecoder
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// Temporary buffer (avoids repeated allocation).
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buf [4]byte
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// Next step in the decompression,
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// and decompression state.
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step func(*decompressor)
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stepState int
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final bool
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err error
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toRead []byte
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hl, hd *huffmanDecoder
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copyLen int
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copyDist int
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}
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func (f *decompressor) nextBlock() {
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for f.nb < 1+2 {
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if f.err = f.moreBits(); f.err != nil {
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return
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}
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}
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f.final = f.b&1 == 1
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f.b >>= 1
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typ := f.b & 3
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f.b >>= 2
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f.nb -= 1 + 2
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switch typ {
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case 0:
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f.dataBlock()
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case 1:
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// compressed, fixed Huffman tables
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f.hl = &fixedHuffmanDecoder
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f.hd = nil
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f.huffmanBlock()
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case 2:
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// compressed, dynamic Huffman tables
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if f.err = f.readHuffman(); f.err != nil {
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break
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}
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f.hl = &f.h1
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f.hd = &f.h2
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f.huffmanBlock()
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default:
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// 3 is reserved.
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f.err = CorruptInputError(f.roffset)
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}
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}
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func (f *decompressor) Read(b []byte) (int, error) {
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for {
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if len(f.toRead) > 0 {
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n := copy(b, f.toRead)
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f.toRead = f.toRead[n:]
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if len(f.toRead) == 0 {
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return n, f.err
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}
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return n, nil
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}
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if f.err != nil {
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return 0, f.err
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}
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f.step(f)
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if f.err != nil && len(f.toRead) == 0 {
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f.toRead = f.dict.readFlush() // Flush what's left in case of error
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}
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}
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}
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// Support the io.WriteTo interface for io.Copy and friends.
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func (f *decompressor) WriteTo(w io.Writer) (int64, error) {
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total := int64(0)
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flushed := false
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for {
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if len(f.toRead) > 0 {
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n, err := w.Write(f.toRead)
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total += int64(n)
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if err != nil {
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f.err = err
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return total, err
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}
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if n != len(f.toRead) {
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return total, io.ErrShortWrite
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}
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f.toRead = f.toRead[:0]
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}
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if f.err != nil && flushed {
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if f.err == io.EOF {
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return total, nil
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}
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return total, f.err
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}
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if f.err == nil {
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f.step(f)
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}
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if len(f.toRead) == 0 && f.err != nil && !flushed {
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f.toRead = f.dict.readFlush() // Flush what's left in case of error
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flushed = true
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}
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}
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}
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func (f *decompressor) Close() error {
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if f.err == io.EOF {
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return nil
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}
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return f.err
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}
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// RFC 1951 section 3.2.7.
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// Compression with dynamic Huffman codes
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var codeOrder = [...]int{16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15}
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func (f *decompressor) readHuffman() error {
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// HLIT[5], HDIST[5], HCLEN[4].
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for f.nb < 5+5+4 {
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if err := f.moreBits(); err != nil {
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return err
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}
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}
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nlit := int(f.b&0x1F) + 257
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if nlit > maxNumLit {
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return CorruptInputError(f.roffset)
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}
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f.b >>= 5
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ndist := int(f.b&0x1F) + 1
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if ndist > maxNumDist {
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return CorruptInputError(f.roffset)
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}
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f.b >>= 5
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nclen := int(f.b&0xF) + 4
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// numCodes is 19, so nclen is always valid.
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f.b >>= 4
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f.nb -= 5 + 5 + 4
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// (HCLEN+4)*3 bits: code lengths in the magic codeOrder order.
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for i := 0; i < nclen; i++ {
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for f.nb < 3 {
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if err := f.moreBits(); err != nil {
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return err
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}
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}
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f.codebits[codeOrder[i]] = int(f.b & 0x7)
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f.b >>= 3
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f.nb -= 3
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}
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for i := nclen; i < len(codeOrder); i++ {
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f.codebits[codeOrder[i]] = 0
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}
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if !f.h1.init(f.codebits[0:]) {
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return CorruptInputError(f.roffset)
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}
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// HLIT + 257 code lengths, HDIST + 1 code lengths,
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// using the code length Huffman code.
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for i, n := 0, nlit+ndist; i < n; {
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x, err := f.huffSym(&f.h1)
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if err != nil {
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return err
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}
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if x < 16 {
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// Actual length.
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f.bits[i] = x
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i++
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continue
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}
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// Repeat previous length or zero.
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var rep int
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var nb uint
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var b int
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switch x {
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default:
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return InternalError("unexpected length code")
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case 16:
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rep = 3
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nb = 2
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if i == 0 {
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return CorruptInputError(f.roffset)
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}
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b = f.bits[i-1]
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case 17:
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rep = 3
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nb = 3
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b = 0
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case 18:
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rep = 11
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nb = 7
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b = 0
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}
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for f.nb < nb {
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if err := f.moreBits(); err != nil {
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return err
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}
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}
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rep += int(f.b & uint32(1<<nb-1))
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f.b >>= nb
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f.nb -= nb
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if i+rep > n {
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return CorruptInputError(f.roffset)
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}
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for j := 0; j < rep; j++ {
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f.bits[i] = b
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i++
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}
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}
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if !f.h1.init(f.bits[0:nlit]) || !f.h2.init(f.bits[nlit:nlit+ndist]) {
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return CorruptInputError(f.roffset)
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}
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// As an optimization, we can initialize the min bits to read at a time
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// for the HLIT tree to the length of the EOB marker since we know that
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// every block must terminate with one. This preserves the property that
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// we never read any extra bytes after the end of the DEFLATE stream.
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if f.h1.min < f.bits[endBlockMarker] {
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f.h1.min = f.bits[endBlockMarker]
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}
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return nil
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}
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|
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// Decode a single Huffman block from f.
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// hl and hd are the Huffman states for the lit/length values
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// and the distance values, respectively. If hd == nil, using the
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// fixed distance encoding associated with fixed Huffman blocks.
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func (f *decompressor) huffmanBlock() {
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const (
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stateInit = iota // Zero value must be stateInit
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stateDict
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)
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switch f.stepState {
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case stateInit:
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goto readLiteral
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case stateDict:
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goto copyHistory
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}
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readLiteral:
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// Read literal and/or (length, distance) according to RFC section 3.2.3.
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{
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v, err := f.huffSym(f.hl)
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if err != nil {
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f.err = err
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return
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}
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var n uint // number of bits extra
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var length int
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switch {
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case v < 256:
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f.dict.writeByte(byte(v))
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if f.dict.availWrite() == 0 {
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f.toRead = f.dict.readFlush()
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f.step = (*decompressor).huffmanBlock
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f.stepState = stateInit
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return
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}
|
|
goto readLiteral
|
|
case v == 256:
|
|
f.finishBlock()
|
|
return
|
|
// otherwise, reference to older data
|
|
case v < 265:
|
|
length = v - (257 - 3)
|
|
n = 0
|
|
case v < 269:
|
|
length = v*2 - (265*2 - 11)
|
|
n = 1
|
|
case v < 273:
|
|
length = v*4 - (269*4 - 19)
|
|
n = 2
|
|
case v < 277:
|
|
length = v*8 - (273*8 - 35)
|
|
n = 3
|
|
case v < 281:
|
|
length = v*16 - (277*16 - 67)
|
|
n = 4
|
|
case v < 285:
|
|
length = v*32 - (281*32 - 131)
|
|
n = 5
|
|
case v < maxNumLit:
|
|
length = 258
|
|
n = 0
|
|
default:
|
|
f.err = CorruptInputError(f.roffset)
|
|
return
|
|
}
|
|
if n > 0 {
|
|
for f.nb < n {
|
|
if err = f.moreBits(); err != nil {
|
|
f.err = err
|
|
return
|
|
}
|
|
}
|
|
length += int(f.b & uint32(1<<n-1))
|
|
f.b >>= n
|
|
f.nb -= n
|
|
}
|
|
|
|
var dist int
|
|
if f.hd == nil {
|
|
for f.nb < 5 {
|
|
if err = f.moreBits(); err != nil {
|
|
f.err = err
|
|
return
|
|
}
|
|
}
|
|
dist = int(reverseByte[(f.b&0x1F)<<3])
|
|
f.b >>= 5
|
|
f.nb -= 5
|
|
} else {
|
|
if dist, err = f.huffSym(f.hd); err != nil {
|
|
f.err = err
|
|
return
|
|
}
|
|
}
|
|
|
|
switch {
|
|
case dist < 4:
|
|
dist++
|
|
case dist < maxNumDist:
|
|
nb := uint(dist-2) >> 1
|
|
// have 1 bit in bottom of dist, need nb more.
|
|
extra := (dist & 1) << nb
|
|
for f.nb < nb {
|
|
if err = f.moreBits(); err != nil {
|
|
f.err = err
|
|
return
|
|
}
|
|
}
|
|
extra |= int(f.b & uint32(1<<nb-1))
|
|
f.b >>= nb
|
|
f.nb -= nb
|
|
dist = 1<<(nb+1) + 1 + extra
|
|
default:
|
|
f.err = CorruptInputError(f.roffset)
|
|
return
|
|
}
|
|
|
|
// No check on length; encoding can be prescient.
|
|
if dist > f.dict.histSize() {
|
|
f.err = CorruptInputError(f.roffset)
|
|
return
|
|
}
|
|
|
|
f.copyLen, f.copyDist = length, dist
|
|
goto copyHistory
|
|
}
|
|
|
|
copyHistory:
|
|
// Perform a backwards copy according to RFC section 3.2.3.
|
|
{
|
|
cnt := f.dict.tryWriteCopy(f.copyDist, f.copyLen)
|
|
if cnt == 0 {
|
|
cnt = f.dict.writeCopy(f.copyDist, f.copyLen)
|
|
}
|
|
f.copyLen -= cnt
|
|
|
|
if f.dict.availWrite() == 0 || f.copyLen > 0 {
|
|
f.toRead = f.dict.readFlush()
|
|
f.step = (*decompressor).huffmanBlock // We need to continue this work
|
|
f.stepState = stateDict
|
|
return
|
|
}
|
|
goto readLiteral
|
|
}
|
|
}
|
|
|
|
// Copy a single uncompressed data block from input to output.
|
|
func (f *decompressor) dataBlock() {
|
|
// Uncompressed.
|
|
// Discard current half-byte.
|
|
f.nb = 0
|
|
f.b = 0
|
|
|
|
// Length then ones-complement of length.
|
|
nr, err := io.ReadFull(f.r, f.buf[0:4])
|
|
f.roffset += int64(nr)
|
|
if err != nil {
|
|
if err == io.EOF {
|
|
err = io.ErrUnexpectedEOF
|
|
}
|
|
f.err = err
|
|
return
|
|
}
|
|
n := int(f.buf[0]) | int(f.buf[1])<<8
|
|
nn := int(f.buf[2]) | int(f.buf[3])<<8
|
|
if uint16(nn) != uint16(^n) {
|
|
f.err = CorruptInputError(f.roffset)
|
|
return
|
|
}
|
|
|
|
if n == 0 {
|
|
f.toRead = f.dict.readFlush()
|
|
f.finishBlock()
|
|
return
|
|
}
|
|
|
|
f.copyLen = n
|
|
f.copyData()
|
|
}
|
|
|
|
// copyData copies f.copyLen bytes from the underlying reader into f.hist.
|
|
// It pauses for reads when f.hist is full.
|
|
func (f *decompressor) copyData() {
|
|
buf := f.dict.writeSlice()
|
|
if len(buf) > f.copyLen {
|
|
buf = buf[:f.copyLen]
|
|
}
|
|
|
|
cnt, err := io.ReadFull(f.r, buf)
|
|
f.roffset += int64(cnt)
|
|
f.copyLen -= cnt
|
|
f.dict.writeMark(cnt)
|
|
if err != nil {
|
|
if err == io.EOF {
|
|
err = io.ErrUnexpectedEOF
|
|
}
|
|
f.err = err
|
|
return
|
|
}
|
|
|
|
if f.dict.availWrite() == 0 || f.copyLen > 0 {
|
|
f.toRead = f.dict.readFlush()
|
|
f.step = (*decompressor).copyData
|
|
return
|
|
}
|
|
f.finishBlock()
|
|
}
|
|
|
|
func (f *decompressor) finishBlock() {
|
|
if f.final {
|
|
if f.dict.availRead() > 0 {
|
|
f.toRead = f.dict.readFlush()
|
|
}
|
|
f.err = io.EOF
|
|
}
|
|
f.step = (*decompressor).nextBlock
|
|
}
|
|
|
|
func (f *decompressor) moreBits() error {
|
|
c, err := f.r.ReadByte()
|
|
if err != nil {
|
|
if err == io.EOF {
|
|
err = io.ErrUnexpectedEOF
|
|
}
|
|
return err
|
|
}
|
|
f.roffset++
|
|
f.b |= uint32(c) << f.nb
|
|
f.nb += 8
|
|
return nil
|
|
}
|
|
|
|
// Read the next Huffman-encoded symbol from f according to h.
|
|
func (f *decompressor) huffSym(h *huffmanDecoder) (int, error) {
|
|
// Since a huffmanDecoder can be empty or be composed of a degenerate tree
|
|
// with single element, huffSym must error on these two edge cases. In both
|
|
// cases, the chunks slice will be 0 for the invalid sequence, leading it
|
|
// satisfy the n == 0 check below.
|
|
n := uint(h.min)
|
|
for {
|
|
for f.nb < n {
|
|
if err := f.moreBits(); err != nil {
|
|
return 0, err
|
|
}
|
|
}
|
|
chunk := h.chunks[f.b&(huffmanNumChunks-1)]
|
|
n = uint(chunk & huffmanCountMask)
|
|
if n > huffmanChunkBits {
|
|
chunk = h.links[chunk>>huffmanValueShift][(f.b>>huffmanChunkBits)&h.linkMask]
|
|
n = uint(chunk & huffmanCountMask)
|
|
}
|
|
if n <= f.nb {
|
|
if n == 0 {
|
|
f.err = CorruptInputError(f.roffset)
|
|
return 0, f.err
|
|
}
|
|
f.b >>= n
|
|
f.nb -= n
|
|
return int(chunk >> huffmanValueShift), nil
|
|
}
|
|
}
|
|
}
|
|
|
|
func makeReader(r io.Reader) Reader {
|
|
if rr, ok := r.(Reader); ok {
|
|
return rr
|
|
}
|
|
return bufio.NewReader(r)
|
|
}
|
|
|
|
func fixedHuffmanDecoderInit() {
|
|
fixedOnce.Do(func() {
|
|
// These come from the RFC section 3.2.6.
|
|
var bits [288]int
|
|
for i := 0; i < 144; i++ {
|
|
bits[i] = 8
|
|
}
|
|
for i := 144; i < 256; i++ {
|
|
bits[i] = 9
|
|
}
|
|
for i := 256; i < 280; i++ {
|
|
bits[i] = 7
|
|
}
|
|
for i := 280; i < 288; i++ {
|
|
bits[i] = 8
|
|
}
|
|
fixedHuffmanDecoder.init(bits[:])
|
|
})
|
|
}
|
|
|
|
func (f *decompressor) Reset(r io.Reader, dict []byte) error {
|
|
*f = decompressor{
|
|
r: makeReader(r),
|
|
bits: f.bits,
|
|
codebits: f.codebits,
|
|
dict: f.dict,
|
|
step: (*decompressor).nextBlock,
|
|
}
|
|
f.dict.init(maxMatchOffset, dict)
|
|
return nil
|
|
}
|
|
|
|
// NewReader returns a new ReadCloser that can be used
|
|
// to read the uncompressed version of r.
|
|
// If r does not also implement io.ByteReader,
|
|
// the decompressor may read more data than necessary from r.
|
|
// It is the caller's responsibility to call Close on the ReadCloser
|
|
// when finished reading.
|
|
//
|
|
// The ReadCloser returned by NewReader also implements Resetter.
|
|
func NewReader(r io.Reader) io.ReadCloser {
|
|
fixedHuffmanDecoderInit()
|
|
|
|
var f decompressor
|
|
f.r = makeReader(r)
|
|
f.bits = new([maxNumLit + maxNumDist]int)
|
|
f.codebits = new([numCodes]int)
|
|
f.step = (*decompressor).nextBlock
|
|
f.dict.init(maxMatchOffset, nil)
|
|
return &f
|
|
}
|
|
|
|
// NewReaderDict is like NewReader but initializes the reader
|
|
// with a preset dictionary. The returned Reader behaves as if
|
|
// the uncompressed data stream started with the given dictionary,
|
|
// which has already been read. NewReaderDict is typically used
|
|
// to read data compressed by NewWriterDict.
|
|
//
|
|
// The ReadCloser returned by NewReader also implements Resetter.
|
|
func NewReaderDict(r io.Reader, dict []byte) io.ReadCloser {
|
|
fixedHuffmanDecoderInit()
|
|
|
|
var f decompressor
|
|
f.r = makeReader(r)
|
|
f.bits = new([maxNumLit + maxNumDist]int)
|
|
f.codebits = new([numCodes]int)
|
|
f.step = (*decompressor).nextBlock
|
|
f.dict.init(maxMatchOffset, dict)
|
|
return &f
|
|
}
|