suffixarray: use binary search for both ends of Lookup
This prevents many unnecessary comparisons when n is large.
R=gri, gri1, rsc
CC=golang-dev
https://golang.org/cl/4068043
diff --git a/src/pkg/index/suffixarray/suffixarray.go b/src/pkg/index/suffixarray/suffixarray.go
index 628e000..d8c6fc9 100644
--- a/src/pkg/index/suffixarray/suffixarray.go
+++ b/src/pkg/index/suffixarray/suffixarray.go
@@ -50,27 +50,33 @@
}
-func (x *Index) search(s []byte) int {
- return sort.Search(len(x.sa), func(i int) bool { return bytes.Compare(x.at(i), s) >= 0 })
+// lookupAll returns a slice into the matching region of the index.
+// The runtime is O(log(N)*len(s)).
+func (x *Index) lookupAll(s []byte) []int {
+ // find matching suffix index range [i:j]
+ // find the first index where s would be the prefix
+ i := sort.Search(len(x.sa), func(i int) bool { return bytes.Compare(x.at(i), s) >= 0 })
+ // starting at i, find the first index at which s is not a prefix
+ j := i + sort.Search(len(x.sa)-i, func(j int) bool { return !bytes.HasPrefix(x.at(j+i), s) })
+ return x.sa[i:j]
}
// Lookup returns an unsorted list of at most n indices where the byte string s
// occurs in the indexed data. If n < 0, all occurrences are returned.
// The result is nil if s is empty, s is not found, or n == 0.
-// Lookup time is O((log(N) + len(result))*len(s)) where N is the
+// Lookup time is O(log(N)*len(s) + len(result)) where N is the
// size of the indexed data.
//
func (x *Index) Lookup(s []byte, n int) (result []int) {
if len(s) > 0 && n != 0 {
- // find matching suffix index i
- i := x.search(s)
- // x.at(i-1) < s <= x.at(i)
-
- // collect the following suffixes with matching prefixes
- for (n < 0 || len(result) < n) && i < len(x.sa) && bytes.HasPrefix(x.at(i), s) {
- result = append(result, x.sa[i])
- i++
+ matches := x.lookupAll(s)
+ if len(matches) < n || n < 0 {
+ n = len(matches)
+ }
+ if n > 0 {
+ result = make([]int, n)
+ copy(result, matches)
}
}
return
