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// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// The suffixarray package implements substring search in logarithmic time
// using an in-memory suffix array.
//
// Example use:
//
// // create index for some data
// index := suffixarray.New(data)
//
// // lookup byte slice s
// offsets1 := index.Lookup(s, -1) // the list of all indices where s occurs in data
// offsets2 := index.Lookup(s, 3) // the list of at most 3 indices where s occurs in data
//
package suffixarray
import (
"bytes"
"container/vector"
"sort"
)
// BUG(gri): For larger data (10MB) which contains very long (say 100000)
// contiguous sequences of identical bytes, index creation time will be extremely slow.
// TODO(gri): Use a more sophisticated algorithm to create the suffix array.
// Index implements a suffix array for fast substring search.
type Index struct {
data []byte
sa []int // suffix array for data
}
// New creates a new Index for data.
// Index creation time is approximately O(N*log(N)) for N = len(data).
//
func New(data []byte) *Index {
sa := make([]int, len(data))
for i, _ := range sa {
sa[i] = i
}
x := &index{data, sa}
sort.Sort(x)
return (*Index)(x)
}
func (x *Index) at(i int) []byte {
return x.data[x.sa[i]:]
}
// Binary search according to "A Method of Programming", E.W. Dijkstra.
func (x *Index) search(s []byte) int {
i, j := 0, len(x.sa)
// i < j for non-empty x
for i+1 < j {
// 0 <= i < j <= len(x.sa) && (x.at(i) <= s < x.at(j) || (s is not in x))
h := i + (j-i)/2 // i < h < j
if bytes.Compare(x.at(h), s) <= 0 {
i = h
} else { // s < x.at(h)
j = h
}
}
// i+1 == j for non-empty x
return i
}
// 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(res))*len(s)) where N is the
// size of the indexed data.
//
func (x *Index) Lookup(s []byte, n int) []int {
var res vector.IntVector
if len(s) > 0 && n != 0 {
// find matching suffix index i
i := x.search(s)
// x.at(i) <= s < x.at(i+1)
// ignore the first suffix if it is < s
if i < len(x.sa) && bytes.Compare(x.at(i), s) < 0 {
i++
}
// collect the following suffixes with matching prefixes
for (n < 0 || len(res) < n) && i < len(x.sa) && bytes.HasPrefix(x.at(i), s) {
res.Push(x.sa[i])
i++
}
}
return res
}
// index is like Index; it is only used to hide the sort.Interface methods
type index struct {
data []byte
sa []int
}
// index implements sort.Interface
func (x *index) Len() int { return len(x.sa) }
func (x *index) Less(i, j int) bool { return bytes.Compare(x.at(i), x.at(j)) < 0 }
func (x *index) Swap(i, j int) { x.sa[i], x.sa[j] = x.sa[j], x.sa[i] }
func (a *index) at(i int) []byte { return a.data[a.sa[i]:] }
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