suffixarray: generate less garbage during construction

Minorly improves runtime by about 2-3%

R=gri, jeff
CC=golang-dev
https://golang.org/cl/5052045

src/pkg/index/suffixarray/qsufsort.go

@@ -37,7 +37,7 @@ func qsufsort(data []byte) []int32 {
 	inv := initGroups(sa, data)
 
 	// the index starts 1-ordered
-	sufSortable := &suffixSortable{sa, inv, 1}
+	sufSortable := &suffixSortable{sa: sa, inv: inv, h: 1}
 
 	for int(sa[0]) > -len(sa) { // until all suffixes are one big sorted group
 		// The suffixes are h-ordered, make them 2*h-ordered
@@ -135,6 +135,7 @@ type suffixSortable struct {
 	sa  []int32
 	inv []int32
 	h   int32
+	buf []int // common scratch space
 }
 
 func (x *suffixSortable) Len() int           { return len(x.sa) }
@@ -142,7 +143,7 @@ func (x *suffixSortable) Less(i, j int) bool { return x.inv[x.sa[i]+x.h] < x.inv
 func (x *suffixSortable) Swap(i, j int)      { x.sa[i], x.sa[j] = x.sa[j], x.sa[i] }
 
 func (x *suffixSortable) updateGroups(offset int) {
-	bounds := make([]int, 0, 4)
+	bounds := x.buf[0:0]
 	group := x.inv[x.sa[0]+x.h]
 	for i := 1; i < len(x.sa); i++ {
 		if g := x.inv[x.sa[i]+x.h]; g > group {
@@ -151,6 +152,7 @@ func (x *suffixSortable) updateGroups(offset int) {
 		}
 	}
 	bounds = append(bounds, len(x.sa))
+	x.buf = bounds
 
 	// update the group numberings after all new groups are determined
 	prev := 0