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Diffstat (limited to 'src/index/suffixarray/qsufsort.go')
-rw-r--r-- | src/index/suffixarray/qsufsort.go | 168 |
1 files changed, 168 insertions, 0 deletions
diff --git a/src/index/suffixarray/qsufsort.go b/src/index/suffixarray/qsufsort.go new file mode 100644 index 000000000..9c36a98f8 --- /dev/null +++ b/src/index/suffixarray/qsufsort.go @@ -0,0 +1,168 @@ +// Copyright 2011 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. + +// This algorithm is based on "Faster Suffix Sorting" +// by N. Jesper Larsson and Kunihiko Sadakane +// paper: http://www.larsson.dogma.net/ssrev-tr.pdf +// code: http://www.larsson.dogma.net/qsufsort.c + +// This algorithm computes the suffix array sa by computing its inverse. +// Consecutive groups of suffixes in sa are labeled as sorted groups or +// unsorted groups. For a given pass of the sorter, all suffixes are ordered +// up to their first h characters, and sa is h-ordered. Suffixes in their +// final positions and unambiguously sorted in h-order are in a sorted group. +// Consecutive groups of suffixes with identical first h characters are an +// unsorted group. In each pass of the algorithm, unsorted groups are sorted +// according to the group number of their following suffix. + +// In the implementation, if sa[i] is negative, it indicates that i is +// the first element of a sorted group of length -sa[i], and can be skipped. +// An unsorted group sa[i:k] is given the group number of the index of its +// last element, k-1. The group numbers are stored in the inverse slice (inv), +// and when all groups are sorted, this slice is the inverse suffix array. + +package suffixarray + +import "sort" + +func qsufsort(data []byte) []int { + // initial sorting by first byte of suffix + sa := sortedByFirstByte(data) + if len(sa) < 2 { + return sa + } + // initialize the group lookup table + // this becomes the inverse of the suffix array when all groups are sorted + inv := initGroups(sa, data) + + // the index starts 1-ordered + sufSortable := &suffixSortable{sa: sa, inv: inv, h: 1} + + for sa[0] > -len(sa) { // until all suffixes are one big sorted group + // The suffixes are h-ordered, make them 2*h-ordered + pi := 0 // pi is first position of first group + sl := 0 // sl is negated length of sorted groups + for pi < len(sa) { + if s := sa[pi]; s < 0 { // if pi starts sorted group + pi -= s // skip over sorted group + sl += s // add negated length to sl + } else { // if pi starts unsorted group + if sl != 0 { + sa[pi+sl] = sl // combine sorted groups before pi + sl = 0 + } + pk := inv[s] + 1 // pk-1 is last position of unsorted group + sufSortable.sa = sa[pi:pk] + sort.Sort(sufSortable) + sufSortable.updateGroups(pi) + pi = pk // next group + } + } + if sl != 0 { // if the array ends with a sorted group + sa[pi+sl] = sl // combine sorted groups at end of sa + } + + sufSortable.h *= 2 // double sorted depth + } + + for i := range sa { // reconstruct suffix array from inverse + sa[inv[i]] = i + } + return sa +} + +func sortedByFirstByte(data []byte) []int { + // total byte counts + var count [256]int + for _, b := range data { + count[b]++ + } + // make count[b] equal index of first occurrence of b in sorted array + sum := 0 + for b := range count { + count[b], sum = sum, count[b]+sum + } + // iterate through bytes, placing index into the correct spot in sa + sa := make([]int, len(data)) + for i, b := range data { + sa[count[b]] = i + count[b]++ + } + return sa +} + +func initGroups(sa []int, data []byte) []int { + // label contiguous same-letter groups with the same group number + inv := make([]int, len(data)) + prevGroup := len(sa) - 1 + groupByte := data[sa[prevGroup]] + for i := len(sa) - 1; i >= 0; i-- { + if b := data[sa[i]]; b < groupByte { + if prevGroup == i+1 { + sa[i+1] = -1 + } + groupByte = b + prevGroup = i + } + inv[sa[i]] = prevGroup + if prevGroup == 0 { + sa[0] = -1 + } + } + // Separate out the final suffix to the start of its group. + // This is necessary to ensure the suffix "a" is before "aba" + // when using a potentially unstable sort. + lastByte := data[len(data)-1] + s := -1 + for i := range sa { + if sa[i] >= 0 { + if data[sa[i]] == lastByte && s == -1 { + s = i + } + if sa[i] == len(sa)-1 { + sa[i], sa[s] = sa[s], sa[i] + inv[sa[s]] = s + sa[s] = -1 // mark it as an isolated sorted group + break + } + } + } + return inv +} + +type suffixSortable struct { + sa []int + inv []int + h int + buf []int // common scratch space +} + +func (x *suffixSortable) Len() int { return len(x.sa) } +func (x *suffixSortable) Less(i, j int) bool { return x.inv[x.sa[i]+x.h] < x.inv[x.sa[j]+x.h] } +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 := 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 { + bounds = append(bounds, i) + group = g + } + } + bounds = append(bounds, len(x.sa)) + x.buf = bounds + + // update the group numberings after all new groups are determined + prev := 0 + for _, b := range bounds { + for i := prev; i < b; i++ { + x.inv[x.sa[i]] = offset + b - 1 + } + if b-prev == 1 { + x.sa[prev] = -1 + } + prev = b + } +} |