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// 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 unambiguouly 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 occurence 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
        }
}