compress/flate: reduce memory pressure at cost of additional arithmetic operation.

R=rsc
CC=golang-dev
https://golang.org/cl/5555070

diff --git a/src/pkg/compress/flate/deflate.go b/src/pkg/compress/flate/deflate.go
index 5f5100db830d49f43e22e819c8d7a19db4d9be52..b69fa056e8cd78db24c1f9c55d0d60157981cd4a 100644 (file)
--- a/src/pkg/compress/flate/deflate.go
+++ b/src/pkg/compress/flate/deflate.go
@@ -70,9 +70,10 @@ type compressor struct {
        // 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
+       chainHead  int
+       hashHead   []int
+       hashPrev   []int
+       hashOffset int
 
        // input window: unprocessed data is window[index:windowEnd]
        index         int
@@ -104,20 +105,7 @@ func (d *compressor) fillDeflate(b []byte) int {
                } else {
                        d.blockStart = skipNever
                }
-               for i, h := range d.hashHead {
-                       v := h - windowSize
-                       if v < -1 {
-                               v = -1
-                       }
-                       d.hashHead[i] = v
-               }
-               for i, h := range d.hashPrev {
-                       v := h - windowSize
-                       if v < -1 {
-                               v = -1
-                       }
-                       d.hashPrev[i] = v
-               }
+               d.hashOffset += windowSize
        }
        n := copy(d.window[d.windowEnd:], b)
        d.windowEnd += n
@@ -188,7 +176,7 @@ func (d *compressor) findMatch(pos int, prevHead int, prevLength int, lookahead
                        // hashPrev[i & windowMask] has already been overwritten, so stop now.
                        break
                }
-               if i = d.hashPrev[i&windowMask]; i < minIndex || i < 0 {
+               if i = d.hashPrev[i&windowMask] - d.hashOffset; i < minIndex || i < 0 {
                        break
                }
        }
@@ -207,7 +195,7 @@ func (d *compressor) initDeflate() {
        d.hashHead = make([]int, hashSize)
        d.hashPrev = make([]int, windowSize)
        d.window = make([]byte, 2*windowSize)
-       fillInts(d.hashHead, -1)
+       d.hashOffset = 1
        d.tokens = make([]token, maxFlateBlockTokens, maxFlateBlockTokens+1)
        d.length = minMatchLength - 1
        d.offset = 0
@@ -263,7 +251,7 @@ Loop:
                        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.hashHead[d.hash] = d.index + d.hashOffset
                }
                prevLength := d.length
                prevOffset := d.offset
@@ -274,10 +262,10 @@ Loop:
                        minIndex = 0
                }
 
-               if d.chainHead >= minIndex &&
+               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, minMatchLength-1, lookahead); ok {
+                       if newLength, newOffset, ok := d.findMatch(d.index, d.chainHead-d.hashOffset, minMatchLength-1, lookahead); ok {
                                d.length = newLength
                                d.offset = newOffset
                        }
@@ -310,7 +298,7 @@ Loop:
                                                // 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.hashHead[d.hash] = d.index + d.hashOffset
                                        }
                                }
                                if d.fastSkipHashing == skipNever {