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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package flate implements the DEFLATE compressed data format, described in
// RFC 1951.  The gzip and zlib packages implement access to DEFLATE-based file
// formats.
package flate

import (
        "bufio"
        "io"
        "strconv"
)

const (
        maxCodeLen = 16    // max length of Huffman code
        maxHist    = 32768 // max history required
        maxLit     = 286
        maxDist    = 32
        numCodes   = 19 // number of codes in Huffman meta-code
)

// A CorruptInputError reports the presence of corrupt input at a given offset.
type CorruptInputError int64

func (e CorruptInputError) Error() string {
        return "flate: corrupt input before offset " + strconv.FormatInt(int64(e), 10)
}

// An InternalError reports an error in the flate code itself.
type InternalError string

func (e InternalError) Error() string { return "flate: internal error: " + string(e) }

// A ReadError reports an error encountered while reading input.
type ReadError struct {
        Offset int64 // byte offset where error occurred
        Err    error // error returned by underlying Read
}

func (e *ReadError) Error() string {
        return "flate: read error at offset " + strconv.FormatInt(e.Offset, 10) + ": " + e.Err.Error()
}

// A WriteError reports an error encountered while writing output.
type WriteError struct {
        Offset int64 // byte offset where error occurred
        Err    error // error returned by underlying Write
}

func (e *WriteError) Error() string {
        return "flate: write error at offset " + strconv.FormatInt(e.Offset, 10) + ": " + e.Err.Error()
}

// Huffman decoder is based on
// J. Brian Connell, ``A Huffman-Shannon-Fano Code,''
// Proceedings of the IEEE, 61(7) (July 1973), pp 1046-1047.
type huffmanDecoder struct {
        // min, max code length
        min, max int

        // limit[i] = largest code word of length i
        // Given code v of length n,
        // need more bits if v > limit[n].
        limit [maxCodeLen + 1]int

        // base[i] = smallest code word of length i - seq number
        base [maxCodeLen + 1]int

        // codes[seq number] = output code.
        // Given code v of length n, value is
        // codes[v - base[n]].
        codes []int
}

// Initialize Huffman decoding tables from array of code lengths.
func (h *huffmanDecoder) init(bits []int) bool {
        // Count number of codes of each length,
        // compute min and max length.
        var count [maxCodeLen + 1]int
        var min, max int
        for _, n := range bits {
                if n == 0 {
                        continue
                }
                if min == 0 || n < min {
                        min = n
                }
                if n > max {
                        max = n
                }
                count[n]++
        }
        if max == 0 {
                return false
        }

        h.min = min
        h.max = max

        // For each code range, compute
        // nextcode (first code of that length),
        // limit (last code of that length), and
        // base (offset from first code to sequence number).
        code := 0
        seq := 0
        var nextcode [maxCodeLen]int
        for i := min; i <= max; i++ {
                n := count[i]
                nextcode[i] = code
                h.base[i] = code - seq
                code += n
                seq += n
                h.limit[i] = code - 1
                code <<= 1
        }

        // Make array mapping sequence numbers to codes.
        if len(h.codes) < len(bits) {
                h.codes = make([]int, len(bits))
        }
        for i, n := range bits {
                if n == 0 {
                        continue
                }
                code := nextcode[n]
                nextcode[n]++
                seq := code - h.base[n]
                h.codes[seq] = i
        }
        return true
}

// Hard-coded Huffman tables for DEFLATE algorithm.
// See RFC 1951, section 3.2.6.
var fixedHuffmanDecoder = huffmanDecoder{
        7, 9,
        [maxCodeLen + 1]int{7: 23, 199, 511},
        [maxCodeLen + 1]int{7: 0, 24, 224},
        []int{
                // length 7: 256-279
                256, 257, 258, 259, 260, 261, 262,
                263, 264, 265, 266, 267, 268, 269,
                270, 271, 272, 273, 274, 275, 276,
                277, 278, 279,

                // length 8: 0-143
                0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,
                12, 13, 14, 15, 16, 17, 18, 19, 20, 21,
                22, 23, 24, 25, 26, 27, 28, 29, 30, 31,
                32, 33, 34, 35, 36, 37, 38, 39, 40, 41,
                42, 43, 44, 45, 46, 47, 48, 49, 50, 51,
                52, 53, 54, 55, 56, 57, 58, 59, 60, 61,
                62, 63, 64, 65, 66, 67, 68, 69, 70, 71,
                72, 73, 74, 75, 76, 77, 78, 79, 80, 81,
                82, 83, 84, 85, 86, 87, 88, 89, 90, 91,
                92, 93, 94, 95, 96, 97, 98, 99, 100,
                101, 102, 103, 104, 105, 106, 107, 108,
                109, 110, 111, 112, 113, 114, 115, 116,
                117, 118, 119, 120, 121, 122, 123, 124,
                125, 126, 127, 128, 129, 130, 131, 132,
                133, 134, 135, 136, 137, 138, 139, 140,
                141, 142, 143,

                // length 8: 280-287
                280, 281, 282, 283, 284, 285, 286, 287,

                // length 9: 144-255
                144, 145, 146, 147, 148, 149, 150, 151,
                152, 153, 154, 155, 156, 157, 158, 159,
                160, 161, 162, 163, 164, 165, 166, 167,
                168, 169, 170, 171, 172, 173, 174, 175,
                176, 177, 178, 179, 180, 181, 182, 183,
                184, 185, 186, 187, 188, 189, 190, 191,
                192, 193, 194, 195, 196, 197, 198, 199,
                200, 201, 202, 203, 204, 205, 206, 207,
                208, 209, 210, 211, 212, 213, 214, 215,
                216, 217, 218, 219, 220, 221, 222, 223,
                224, 225, 226, 227, 228, 229, 230, 231,
                232, 233, 234, 235, 236, 237, 238, 239,
                240, 241, 242, 243, 244, 245, 246, 247,
                248, 249, 250, 251, 252, 253, 254, 255,
        },
}

// The actual read interface needed by NewReader.
// If the passed in io.Reader does not also have ReadByte,
// the NewReader will introduce its own buffering.
type Reader interface {
        io.Reader
        ReadByte() (c byte, err error)
}

// Decompress state.
type decompressor struct {
        // Input source.
        r       Reader
        roffset int64
        woffset int64

        // Input bits, in top of b.
        b  uint32
        nb uint

        // Huffman decoders for literal/length, distance.
        h1, h2 huffmanDecoder

        // Length arrays used to define Huffman codes.
        bits     [maxLit + maxDist]int
        codebits [numCodes]int

        // Output history, buffer.
        hist  [maxHist]byte
        hp    int  // current output position in buffer
        hw    int  // have written hist[0:hw] already
        hfull bool // buffer has filled at least once

        // Temporary buffer (avoids repeated allocation).
        buf [4]byte

        // Next step in the decompression,
        // and decompression state.
        step     func(*decompressor)
        final    bool
        err      error
        toRead   []byte
        hl, hd   *huffmanDecoder
        copyLen  int
        copyDist int
}

func (f *decompressor) nextBlock() {
        if f.final {
                if f.hw != f.hp {
                        f.flush((*decompressor).nextBlock)
                        return
                }
                f.err = io.EOF
                return
        }
        for f.nb < 1+2 {
                if f.err = f.moreBits(); f.err != nil {
                        return
                }
        }
        f.final = f.b&1 == 1
        f.b >>= 1
        typ := f.b & 3
        f.b >>= 2
        f.nb -= 1 + 2
        switch typ {
        case 0:
                f.dataBlock()
        case 1:
                // compressed, fixed Huffman tables
                f.hl = &fixedHuffmanDecoder
                f.hd = nil
                f.huffmanBlock()
        case 2:
                // compressed, dynamic Huffman tables
                if f.err = f.readHuffman(); f.err != nil {
                        break
                }
                f.hl = &f.h1
                f.hd = &f.h2
                f.huffmanBlock()
        default:
                // 3 is reserved.
                f.err = CorruptInputError(f.roffset)
        }
}

func (f *decompressor) Read(b []byte) (int, error) {
        for {
                if len(f.toRead) > 0 {
                        n := copy(b, f.toRead)
                        f.toRead = f.toRead[n:]
                        return n, nil
                }
                if f.err != nil {
                        return 0, f.err
                }
                f.step(f)
        }
        panic("unreachable")
}

func (f *decompressor) Close() error {
        if f.err == io.EOF {
                return nil
        }
        return f.err
}

// RFC 1951 section 3.2.7.
// Compression with dynamic Huffman codes

var codeOrder = [...]int{16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15}

func (f *decompressor) readHuffman() error {
        // HLIT[5], HDIST[5], HCLEN[4].
        for f.nb < 5+5+4 {
                if err := f.moreBits(); err != nil {
                        return err
                }
        }
        nlit := int(f.b&0x1F) + 257
        f.b >>= 5
        ndist := int(f.b&0x1F) + 1
        f.b >>= 5
        nclen := int(f.b&0xF) + 4
        f.b >>= 4
        f.nb -= 5 + 5 + 4

        // (HCLEN+4)*3 bits: code lengths in the magic codeOrder order.
        for i := 0; i < nclen; i++ {
                for f.nb < 3 {
                        if err := f.moreBits(); err != nil {
                                return err
                        }
                }
                f.codebits[codeOrder[i]] = int(f.b & 0x7)
                f.b >>= 3
                f.nb -= 3
        }
        for i := nclen; i < len(codeOrder); i++ {
                f.codebits[codeOrder[i]] = 0
        }
        if !f.h1.init(f.codebits[0:]) {
                return CorruptInputError(f.roffset)
        }

        // HLIT + 257 code lengths, HDIST + 1 code lengths,
        // using the code length Huffman code.
        for i, n := 0, nlit+ndist; i < n; {
                x, err := f.huffSym(&f.h1)
                if err != nil {
                        return err
                }
                if x < 16 {
                        // Actual length.
                        f.bits[i] = x
                        i++
                        continue
                }
                // Repeat previous length or zero.
                var rep int
                var nb uint
                var b int
                switch x {
                default:
                        return InternalError("unexpected length code")
                case 16:
                        rep = 3
                        nb = 2
                        if i == 0 {
                                return CorruptInputError(f.roffset)
                        }
                        b = f.bits[i-1]
                case 17:
                        rep = 3
                        nb = 3
                        b = 0
                case 18:
                        rep = 11
                        nb = 7
                        b = 0
                }
                for f.nb < nb {
                        if err := f.moreBits(); err != nil {
                                return err
                        }
                }
                rep += int(f.b & uint32(1<<nb-1))
                f.b >>= nb
                f.nb -= nb
                if i+rep > n {
                        return CorruptInputError(f.roffset)
                }
                for j := 0; j < rep; j++ {
                        f.bits[i] = b
                        i++
                }
        }

        if !f.h1.init(f.bits[0:nlit]) || !f.h2.init(f.bits[nlit:nlit+ndist]) {
                return CorruptInputError(f.roffset)
        }

        return nil
}

// Decode a single Huffman block from f.
// hl and hd are the Huffman states for the lit/length values
// and the distance values, respectively.  If hd == nil, using the
// fixed distance encoding associated with fixed Huffman blocks.
func (f *decompressor) huffmanBlock() {
        for {
                v, err := f.huffSym(f.hl)
                if err != nil {
                        f.err = err
                        return
                }
                var n uint // number of bits extra
                var length int
                switch {
                case v < 256:
                        f.hist[f.hp] = byte(v)
                        f.hp++
                        if f.hp == len(f.hist) {
                                // After the flush, continue this loop.
                                f.flush((*decompressor).huffmanBlock)
                                return
                        }
                        continue
                case v == 256:
                        // Done with huffman block; read next block.
                        f.step = (*decompressor).nextBlock
                        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
                default:
                        length = 258
                        n = 0
                }
                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 >= 30:
                        f.err = CorruptInputError(f.roffset)
                        return
                default:
                        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
                }

                // Copy history[-dist:-dist+length] into output.
                if dist > len(f.hist) {
                        f.err = InternalError("bad history distance")
                        return
                }

                // No check on length; encoding can be prescient.
                if !f.hfull && dist > f.hp {
                        f.err = CorruptInputError(f.roffset)
                        return
                }

                p := f.hp - dist
                if p < 0 {
                        p += len(f.hist)
                }
                for i := 0; i < length; i++ {
                        f.hist[f.hp] = f.hist[p]
                        f.hp++
                        p++
                        if f.hp == len(f.hist) {
                                // After flush continue copying out of history.
                                f.copyLen = length - (i + 1)
                                f.copyDist = dist
                                f.flush((*decompressor).copyHuff)
                                return
                        }
                        if p == len(f.hist) {
                                p = 0
                        }
                }
        }
        panic("unreached")
}

func (f *decompressor) copyHuff() {
        length := f.copyLen
        dist := f.copyDist
        p := f.hp - dist
        if p < 0 {
                p += len(f.hist)
        }
        for i := 0; i < length; i++ {
                f.hist[f.hp] = f.hist[p]
                f.hp++
                p++
                if f.hp == len(f.hist) {
                        f.copyLen = length - (i + 1)
                        f.flush((*decompressor).copyHuff)
                        return
                }
                if p == len(f.hist) {
                        p = 0
                }
        }

        // Continue processing Huffman block.
        f.huffmanBlock()
}

// 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 {
                f.err = &ReadError{f.roffset, 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 {
                // 0-length block means sync
                f.flush((*decompressor).nextBlock)
                return
        }

        f.copyLen = n
        f.copyData()
}

func (f *decompressor) copyData() {
        // Read f.dataLen bytes into history,
        // pausing for reads as history fills.
        n := f.copyLen
        for n > 0 {
                m := len(f.hist) - f.hp
                if m > n {
                        m = n
                }
                m, err := io.ReadFull(f.r, f.hist[f.hp:f.hp+m])
                f.roffset += int64(m)
                if err != nil {
                        f.err = &ReadError{f.roffset, err}
                        return
                }
                n -= m
                f.hp += m
                if f.hp == len(f.hist) {
                        f.copyLen = n
                        f.flush((*decompressor).copyData)
                        return
                }
        }
        f.step = (*decompressor).nextBlock
}

func (f *decompressor) setDict(dict []byte) {
        if len(dict) > len(f.hist) {
                // Will only remember the tail.
                dict = dict[len(dict)-len(f.hist):]
        }

        f.hp = copy(f.hist[:], dict)
        if f.hp == len(f.hist) {
                f.hp = 0
                f.hfull = true
        }
        f.hw = f.hp
}

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) {
        for n := uint(h.min); n <= uint(h.max); n++ {
                lim := h.limit[n]
                if lim == -1 {
                        continue
                }
                for f.nb < n {
                        if err := f.moreBits(); err != nil {
                                return 0, err
                        }
                }
                v := int(f.b & uint32(1<<n-1))
                v <<= 16 - n
                v = int(reverseByte[v>>8]) | int(reverseByte[v&0xFF])<<8 // reverse bits
                if v <= lim {
                        f.b >>= n
                        f.nb -= n
                        return h.codes[v-h.base[n]], nil
                }
        }
        return 0, CorruptInputError(f.roffset)
}

// Flush any buffered output to the underlying writer.
func (f *decompressor) flush(step func(*decompressor)) {
        f.toRead = f.hist[f.hw:f.hp]
        f.woffset += int64(f.hp - f.hw)
        f.hw = f.hp
        if f.hp == len(f.hist) {
                f.hp = 0
                f.hw = 0
                f.hfull = true
        }
        f.step = step
}

func makeReader(r io.Reader) Reader {
        if rr, ok := r.(Reader); ok {
                return rr
        }
        return bufio.NewReader(r)
}

// NewReader returns a new ReadCloser that can be used
// to read the uncompressed version of r.  It is the caller's
// responsibility to call Close on the ReadCloser when
// finished reading.
func NewReader(r io.Reader) io.ReadCloser {
        var f decompressor
        f.r = makeReader(r)
        f.step = (*decompressor).nextBlock
        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.
func NewReaderDict(r io.Reader, dict []byte) io.ReadCloser {
        var f decompressor
        f.setDict(dict)
        f.r = makeReader(r)
        f.step = (*decompressor).nextBlock
        return &f
}