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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

import (
        "io"
        "math"
)

const (
        NoCompression      = 0
        BestSpeed          = 1
        fastCompression    = 3
        BestCompression    = 9
        DefaultCompression = -1
        logWindowSize      = 15
        windowSize         = 1 << logWindowSize
        windowMask         = windowSize - 1
        logMaxOffsetSize   = 15  // Standard DEFLATE
        minMatchLength     = 3   // The smallest match that the compressor looks for
        maxMatchLength     = 258 // The longest match for the compressor
        minOffsetSize      = 1   // The shortest offset that makes any sence

        // The maximum number of tokens we put into a single flat block, just too
        // stop things from getting too large.
        maxFlateBlockTokens = 1 << 14
        maxStoreBlockSize   = 65535
        hashBits            = 17
        hashSize            = 1 << hashBits
        hashMask            = (1 << hashBits) - 1
        hashShift           = (hashBits + minMatchLength - 1) / minMatchLength

        skipNever = math.MaxInt32
)

type compressionLevel struct {
        good, lazy, nice, chain, fastSkipHashing int
}

var levels = []compressionLevel{
        {}, // 0
        // For levels 1-3 we don't bother trying with lazy matches
        {3, 0, 8, 4, 4},
        {3, 0, 16, 8, 5},
        {3, 0, 32, 32, 6},
        // Levels 4-9 use increasingly more lazy matching
        // and increasingly stringent conditions for "good enough".
        {4, 4, 16, 16, skipNever},
        {8, 16, 32, 32, skipNever},
        {8, 16, 128, 128, skipNever},
        {8, 32, 128, 256, skipNever},
        {32, 128, 258, 1024, skipNever},
        {32, 258, 258, 4096, skipNever},
}

type compressor struct {
        compressionLevel

        w *huffmanBitWriter

        // compression algorithm
        fill func(*compressor, []byte) int // copy data to window
        step func(*compressor)             // process window
        sync bool                          // requesting flush

        // Input hash chains
        // hashHead[hashValue] contains the largest inputIndex with the specified hash value
        // If hashHead[hashValue] is within the current window, then
        // hashPrev[hashHead[hashValue] & windowMask] contains the previous index
        // with the same hash value.
        chainHead  int
        hashHead   []int
        hashPrev   []int
        hashOffset int

        // input window: unprocessed data is window[index:windowEnd]
        index         int
        window        []byte
        windowEnd     int
        blockStart    int  // window index where current tokens start
        byteAvailable bool // if true, still need to process window[index-1].

        // queued output tokens
        tokens []token

        // deflate state
        length         int
        offset         int
        hash           int
        maxInsertIndex int
        err            error
}

func (d *compressor) fillDeflate(b []byte) int {
        if d.index >= 2*windowSize-(minMatchLength+maxMatchLength) {
                // shift the window by windowSize
                copy(d.window, d.window[windowSize:2*windowSize])
                d.index -= windowSize
                d.windowEnd -= windowSize
                if d.blockStart >= windowSize {
                        d.blockStart -= windowSize
                } else {
                        d.blockStart = math.MaxInt32
                }
                d.hashOffset += windowSize
        }
        n := copy(d.window[d.windowEnd:], b)
        d.windowEnd += n
        return n
}

func (d *compressor) writeBlock(tokens []token, index int, eof bool) error {
        if index > 0 || eof {
                var window []byte
                if d.blockStart <= index {
                        window = d.window[d.blockStart:index]
                }
                d.blockStart = index
                d.w.writeBlock(tokens, eof, window)
                return d.w.err
        }
        return nil
}

// Try to find a match starting at index whose length is greater than prevSize.
// We only look at chainCount possibilities before giving up.
func (d *compressor) findMatch(pos int, prevHead int, prevLength int, lookahead int) (length, offset int, ok bool) {
        minMatchLook := maxMatchLength
        if lookahead < minMatchLook {
                minMatchLook = lookahead
        }

        win := d.window[0 : pos+minMatchLook]

        // We quit when we get a match that's at least nice long
        nice := len(win) - pos
        if d.nice < nice {
                nice = d.nice
        }

        // If we've got a match that's good enough, only look in 1/4 the chain.
        tries := d.chain
        length = prevLength
        if length >= d.good {
                tries >>= 2
        }

        w0 := win[pos]
        w1 := win[pos+1]
        wEnd := win[pos+length]
        minIndex := pos - windowSize

        for i := prevHead; tries > 0; tries-- {
                if w0 == win[i] && w1 == win[i+1] && wEnd == win[i+length] {
                        // The hash function ensures that if win[i] and win[i+1] match, win[i+2] matches

                        n := 3
                        for pos+n < len(win) && win[i+n] == win[pos+n] {
                                n++
                        }
                        if n > length && (n > 3 || pos-i <= 4096) {
                                length = n
                                offset = pos - i
                                ok = true
                                if n >= nice {
                                        // The match is good enough that we don't try to find a better one.
                                        break
                                }
                                wEnd = win[pos+n]
                        }
                }
                if i == minIndex {
                        // hashPrev[i & windowMask] has already been overwritten, so stop now.
                        break
                }
                if i = d.hashPrev[i&windowMask] - d.hashOffset; i < minIndex || i < 0 {
                        break
                }
        }
        return
}

func (d *compressor) writeStoredBlock(buf []byte) error {
        if d.w.writeStoredHeader(len(buf), false); d.w.err != nil {
                return d.w.err
        }
        d.w.writeBytes(buf)
        return d.w.err
}

func (d *compressor) initDeflate() {
        d.hashHead = make([]int, hashSize)
        d.hashPrev = make([]int, windowSize)
        d.window = make([]byte, 2*windowSize)
        d.hashOffset = 1
        d.tokens = make([]token, 0, maxFlateBlockTokens+1)
        d.length = minMatchLength - 1
        d.offset = 0
        d.byteAvailable = false
        d.index = 0
        d.hash = 0
        d.chainHead = -1
}

func (d *compressor) deflate() {
        if d.windowEnd-d.index < minMatchLength+maxMatchLength && !d.sync {
                return
        }

        d.maxInsertIndex = d.windowEnd - (minMatchLength - 1)
        if d.index < d.maxInsertIndex {
                d.hash = int(d.window[d.index])<<hashShift + int(d.window[d.index+1])
        }

Loop:
        for {
                if d.index > d.windowEnd {
                        panic("index > windowEnd")
                }
                lookahead := d.windowEnd - d.index
                if lookahead < minMatchLength+maxMatchLength {
                        if !d.sync {
                                break Loop
                        }
                        if d.index > d.windowEnd {
                                panic("index > windowEnd")
                        }
                        if lookahead == 0 {
                                // Flush current output block if any.
                                if d.byteAvailable {
                                        // There is still one pending token that needs to be flushed
                                        d.tokens = append(d.tokens, literalToken(uint32(d.window[d.index-1])))
                                        d.byteAvailable = false
                                }
                                if len(d.tokens) > 0 {
                                        if d.err = d.writeBlock(d.tokens, d.index, false); d.err != nil {
                                                return
                                        }
                                        d.tokens = d.tokens[:0]
                                }
                                break Loop
                        }
                }
                if d.index < d.maxInsertIndex {
                        // Update the hash
                        d.hash = (d.hash<<hashShift + int(d.window[d.index+2])) & hashMask
                        d.chainHead = d.hashHead[d.hash]
                        d.hashPrev[d.index&windowMask] = d.chainHead
                        d.hashHead[d.hash] = d.index + d.hashOffset
                }
                prevLength := d.length
                prevOffset := d.offset
                d.length = minMatchLength - 1
                d.offset = 0
                minIndex := d.index - windowSize
                if minIndex < 0 {
                        minIndex = 0
                }

                if d.chainHead-d.hashOffset >= minIndex &&
                        (d.fastSkipHashing != skipNever && lookahead > minMatchLength-1 ||
                                d.fastSkipHashing == skipNever && lookahead > prevLength && prevLength < d.lazy) {
                        if newLength, newOffset, ok := d.findMatch(d.index, d.chainHead-d.hashOffset, minMatchLength-1, lookahead); ok {
                                d.length = newLength
                                d.offset = newOffset
                        }
                }
                if d.fastSkipHashing != skipNever && d.length >= minMatchLength ||
                        d.fastSkipHashing == skipNever && prevLength >= minMatchLength && d.length <= prevLength {
                        // There was a match at the previous step, and the current match is
                        // not better. Output the previous match.
                        if d.fastSkipHashing != skipNever {
                                d.tokens = append(d.tokens, matchToken(uint32(d.length-minMatchLength), uint32(d.offset-minOffsetSize)))
                        } else {
                                d.tokens = append(d.tokens, matchToken(uint32(prevLength-minMatchLength), uint32(prevOffset-minOffsetSize)))
                        }
                        // Insert in the hash table all strings up to the end of the match.
                        // index and index-1 are already inserted. If there is not enough
                        // lookahead, the last two strings are not inserted into the hash
                        // table.
                        if d.length <= d.fastSkipHashing {
                                var newIndex int
                                if d.fastSkipHashing != skipNever {
                                        newIndex = d.index + d.length
                                } else {
                                        newIndex = d.index + prevLength - 1
                                }
                                for d.index++; d.index < newIndex; d.index++ {
                                        if d.index < d.maxInsertIndex {
                                                d.hash = (d.hash<<hashShift + int(d.window[d.index+2])) & hashMask
                                                // Get previous value with the same hash.
                                                // Our chain should point to the previous value.
                                                d.hashPrev[d.index&windowMask] = d.hashHead[d.hash]
                                                // Set the head of the hash chain to us.
                                                d.hashHead[d.hash] = d.index + d.hashOffset
                                        }
                                }
                                if d.fastSkipHashing == skipNever {
                                        d.byteAvailable = false
                                        d.length = minMatchLength - 1
                                }
                        } else {
                                // For matches this long, we don't bother inserting each individual
                                // item into the table.
                                d.index += d.length
                                if d.index < d.maxInsertIndex {
                                        d.hash = (int(d.window[d.index])<<hashShift + int(d.window[d.index+1]))
                                }
                        }
                        if len(d.tokens) == maxFlateBlockTokens {
                                // The block includes the current character
                                if d.err = d.writeBlock(d.tokens, d.index, false); d.err != nil {
                                        return
                                }
                                d.tokens = d.tokens[:0]
                        }
                } else {
                        if d.fastSkipHashing != skipNever || d.byteAvailable {
                                i := d.index - 1
                                if d.fastSkipHashing != skipNever {
                                        i = d.index
                                }
                                d.tokens = append(d.tokens, literalToken(uint32(d.window[i])))
                                if len(d.tokens) == maxFlateBlockTokens {
                                        if d.err = d.writeBlock(d.tokens, i+1, false); d.err != nil {
                                                return
                                        }
                                        d.tokens = d.tokens[:0]
                                }
                        }
                        d.index++
                        if d.fastSkipHashing == skipNever {
                                d.byteAvailable = true
                        }
                }
        }
}

func (d *compressor) fillStore(b []byte) int {
        n := copy(d.window[d.windowEnd:], b)
        d.windowEnd += n
        return n
}

func (d *compressor) store() {
        if d.windowEnd > 0 {
                d.err = d.writeStoredBlock(d.window[:d.windowEnd])
        }
        d.windowEnd = 0
}

func (d *compressor) write(b []byte) (n int, err error) {
        n = len(b)
        b = b[d.fill(d, b):]
        for len(b) > 0 {
                d.step(d)
                b = b[d.fill(d, b):]
        }
        return n, d.err
}

func (d *compressor) syncFlush() error {
        d.sync = true
        d.step(d)
        if d.err == nil {
                d.w.writeStoredHeader(0, false)
                d.w.flush()
                d.err = d.w.err
        }
        d.sync = false
        return d.err
}

func (d *compressor) init(w io.Writer, level int) (err error) {
        d.w = newHuffmanBitWriter(w)

        switch {
        case level == NoCompression:
                d.window = make([]byte, maxStoreBlockSize)
                d.fill = (*compressor).fillStore
                d.step = (*compressor).store
        case level == DefaultCompression:
                level = 6
                fallthrough
        case 1 <= level && level <= 9:
                d.compressionLevel = levels[level]
                d.initDeflate()
                d.fill = (*compressor).fillDeflate
                d.step = (*compressor).deflate
        default:
                return WrongValueError{"level", 0, 9, int32(level)}
        }
        return nil
}

func (d *compressor) close() error {
        d.sync = true
        d.step(d)
        if d.err != nil {
                return d.err
        }
        if d.w.writeStoredHeader(0, true); d.w.err != nil {
                return d.w.err
        }
        d.w.flush()
        return d.w.err
}

// NewWriter returns a new Writer compressing
// data at the given level.  Following zlib, levels
// range from 1 (BestSpeed) to 9 (BestCompression);
// higher levels typically run slower but compress more.
// Level 0 (NoCompression) does not attempt any
// compression; it only adds the necessary DEFLATE framing.
func NewWriter(w io.Writer, level int) *Writer {
        const logWindowSize = logMaxOffsetSize
        var dw Writer
        dw.d.init(w, level)
        return &dw
}

// NewWriterDict is like NewWriter but initializes the new
// Writer with a preset dictionary.  The returned Writer behaves
// as if the dictionary had been written to it without producing
// any compressed output.  The compressed data written to w
// can only be decompressed by a Reader initialized with the
// same dictionary.
func NewWriterDict(w io.Writer, level int, dict []byte) *Writer {
        dw := &dictWriter{w, false}
        zw := NewWriter(dw, level)
        zw.Write(dict)
        zw.Flush()
        dw.enabled = true
        return zw
}

type dictWriter struct {
        w       io.Writer
        enabled bool
}

func (w *dictWriter) Write(b []byte) (n int, err error) {
        if w.enabled {
                return w.w.Write(b)
        }
        return len(b), nil
}

// A Writer takes data written to it and writes the compressed
// form of that data to an underlying writer (see NewWriter).
type Writer struct {
        d compressor
}

// Write writes data to w, which will eventually write the
// compressed form of data to its underlying writer.
func (w *Writer) Write(data []byte) (n int, err error) {
        return w.d.write(data)
}

// Flush flushes any pending compressed data to the underlying writer.
// It is useful mainly in compressed network protocols, to ensure that
// a remote reader has enough data to reconstruct a packet.
// Flush does not return until the data has been written.
// If the underlying writer returns an error, Flush returns that error.
//
// In the terminology of the zlib library, Flush is equivalent to Z_SYNC_FLUSH.
func (w *Writer) Flush() error {
        // For more about flushing:
        // http://www.bolet.org/~pornin/deflate-flush.html
        return w.d.syncFlush()
}

// Close flushes and closes the writer.
func (w *Writer) Close() error {
        return w.d.close()
}