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suffixarray: use binary search for both ends of Lookup

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This prevents many unnecessary comparisons when n is large.

R=gri, gri1, rsc
CC=golang-dev
https://golang.org/cl/4068043
This commit is contained in:
Eric Eisner 2011-01-24 13:03:32 -08:00 committed by Robert Griesemer
parent 2aa6cf3e7e
commit ab036abdf0
1 changed files with 17 additions and 11 deletions
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28
src/pkg/index/suffixarray/suffixarray.go
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@ -50,27 +50,33 @@ func (x *Index) at(i int) []byte {
}
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