flate: Increase level 7+8 speed and compression (#473)

Keep expensive check for level 9.

flate/deflate.go

@@ -73,8 +73,8 @@ var levels = []compressionLevel{
 	{0, 0, 0, 0, 0, 6},
 	// Levels 7-9 use increasingly more lazy matching
 	// and increasingly stringent conditions for "good enough".
-	{6, 10, 12, 16, skipNever, 7},
-	{10, 24, 32, 64, skipNever, 8},
+	{8, 12, 16, 24, skipNever, 7},
+	{16, 30, 40, 64, skipNever, 8},
 	{32, 258, 258, 1024, skipNever, 9},
 }
 
@@ -295,11 +295,38 @@ func (d *compressor) findMatch(pos int, prevHead int, lookahead int) (length, of
 	}
 	offset = 0
 
+	cGain := 0
+	if d.chain < 100 {
+		for i := prevHead; tries > 0; tries-- {
+			if wEnd == win[i+length] {
+				n := matchLen(win[i:i+minMatchLook], wPos)
+				if n > length {
+					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
+			}
+			i = int(d.state.hashPrev[i&windowMask]) - d.state.hashOffset
+			if i < minIndex {
+				break
+			}
+		}
+		return
+	}
+
 	// Some like it higher (CSV), some like it lower (JSON)
 	const baseCost = 6
 	// Base is 4 bytes at with an additional cost.
 	// Matches must be better than this.
-	cGain := 0
 	for i := prevHead; tries > 0; tries-- {
 		if wEnd == win[i+length] {
 			n := matchLen(win[i:i+minMatchLook], wPos)
@@ -392,7 +419,7 @@ func (d *compressor) deflateLazy() {
 	if d.windowEnd-s.index < minMatchLength+maxMatchLength && !d.sync {
 		return
 	}
-	if d.windowEnd != s.index {
+	if d.windowEnd != s.index && d.chain > 100 {
 		// Get literal huffman coder.
 		if d.h == nil {
 			d.h = newHuffmanEncoder(maxFlateBlockTokens)
@@ -402,7 +429,6 @@ func (d *compressor) deflateLazy() {
 			tmp[v]++
 		}
 		d.h.generate(tmp[:], 15)
-		//s.estBitsPerByte = d.h.bitLength(tmp[:])/(d.windowEnd-s.index) + 1
 	}
 
 	s.maxInsertIndex = d.windowEnd - (minMatchLength - 1)