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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.

// Package suffixarray 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"

        "encoding/binary"

        "io"

        "regexp"

        "sort"

)

// Index implements a suffix array for fast substring search.

type Index struct {

        data []byte

        sa   []int // suffix array for data; len(sa) == len(data)

}

// New creates a new Index for data.

// Index creation time is O(N*log(N)) for N = len(data).

func New(data []byte) *Index {

        return &Index{data, qsufsort(data)}

}

// writeInt writes an int x to w using buf to buffer the write.

func writeInt(w io.Writer, buf []byte, x int) error {

        binary.PutVarint(buf, int64(x))

        _, err := w.Write(buf[0:binary.MaxVarintLen64])

        return err

}

// readInt reads an int x from r using buf to buffer the read and returns x.

func readInt(r io.Reader, buf []byte) (int, error) {

        _, err := io.ReadFull(r, buf[0:binary.MaxVarintLen64]) // ok to continue with error

        x, _ := binary.Varint(buf)

        return int(x), err

}

// writeSlice writes data[:n] to w and returns n.

// It uses buf to buffer the write.

func writeSlice(w io.Writer, buf []byte, data []int) (n int, err error) {

        // encode as many elements as fit into buf

        p := binary.MaxVarintLen64

        for ; n < len(data) && p+binary.MaxVarintLen64 <= len(buf); n++ {

                p += binary.PutUvarint(buf[p:], uint64(data[n]))

        }

        // update buffer size

        binary.PutVarint(buf, int64(p))

        // write buffer

        _, err = w.Write(buf[0:p])

        return

}

// readSlice reads data[:n] from r and returns n.

// It uses buf to buffer the read.

func readSlice(r io.Reader, buf []byte, data []int) (n int, err error) {

        // read buffer size

        var size int

        size, err = readInt(r, buf)

        if err != nil {

                return

        }

        // read buffer w/o the size

        if _, err = io.ReadFull(r, buf[binary.MaxVarintLen64:size]); err != nil {

                return

        }

        // decode as many elements as present in buf

        for p := binary.MaxVarintLen64; p < size; n++ {

                x, w := binary.Uvarint(buf[p:])

                data[n] = int(x)

                p += w

        }

        return

}

const bufSize = 16 << 10 // reasonable for BenchmarkSaveRestore

// Read reads the index from r into x; x must not be nil.

func (x *Index) Read(r io.Reader) error {

        // buffer for all reads

        buf := make([]byte, bufSize)

        // read length

        n, err := readInt(r, buf)

        if err != nil {

                return err

        }

        // allocate space

        if 2*n < cap(x.data) || cap(x.data) < n {

                // new data is significantly smaller or larger then

                // existing buffers - allocate new ones

                x.data = make([]byte, n)

                x.sa = make([]int, n)

        } else {

                // re-use existing buffers

                x.data = x.data[0:n]

                x.sa = x.sa[0:n]

        }

        // read data

        if _, err := io.ReadFull(r, x.data); err != nil {

                return err

        }

        // read index

        for sa := x.sa; len(sa) > 0; {

                n, err := readSlice(r, buf, sa)

                if err != nil {

                        return err

                }

                sa = sa[n:]

        }

        return nil

}

// Write writes the index x to w.

func (x *Index) Write(w io.Writer) error {

        // buffer for all writes

        buf := make([]byte, bufSize)

        // write length

        if err := writeInt(w, buf, len(x.data)); err != nil {

                return err

        }

        // write data

        if _, err := w.Write(x.data); err != nil {

                return err

        }

        // write index

        for sa := x.sa; len(sa) > 0; {

                n, err := writeSlice(w, buf, sa)

                if err != nil {

                        return err

                }

                sa = sa[n:]

        }

        return nil

}

// Bytes returns the data over which the index was created.

// It must not be modified.

//

func (x *Index) Bytes() []byte {

        return x.data

}

func (x *Index) at(i int) []byte {

        return x.data[x.sa[i]:]

}

// 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(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 {

                matches := x.lookupAll(s)

                if n < 0 || len(matches) < n {

                        n = len(matches)

                }

                // 0 <= n <= len(matches)

                if n > 0 {

                        result = make([]int, n)

                        copy(result, matches)

                }

        }

        return

}

// FindAllIndex returns a sorted list of non-overlapping matches of the

// regular expression r, where a match is a pair of indices specifying

// the matched slice of x.Bytes(). If n < 0, all matches are returned

// in successive order. Otherwise, at most n matches are returned and

// they may not be successive. The result is nil if there are no matches,

// or if n == 0.

//

func (x *Index) FindAllIndex(r *regexp.Regexp, n int) (result [][]int) {

        // a non-empty literal prefix is used to determine possible

        // match start indices with Lookup

        prefix, complete := r.LiteralPrefix()

        lit := []byte(prefix)

        // worst-case scenario: no literal prefix

        if prefix == "" {

                return r.FindAllIndex(x.data, n)

        }

        // if regexp is a literal just use Lookup and convert its

        // result into match pairs

        if complete {

                // Lookup returns indices that may belong to overlapping matches.

                // After eliminating them, we may end up with fewer than n matches.

                // If we don't have enough at the end, redo the search with an

                // increased value n1, but only if Lookup returned all the requested

                // indices in the first place (if it returned fewer than that then

                // there cannot be more).

                for n1 := n; ; n1 += 2 * (n - len(result)) /* overflow ok */ {

                        indices := x.Lookup(lit, n1)

                        if len(indices) == 0 {

                                return

                        }

                        sort.Ints(indices)

                        pairs := make([]int, 2*len(indices))

                        result = make([][]int, len(indices))

                        count := 0

                        prev := 0

                        for _, i := range indices {

                                if count == n {

                                        break

                                }

                                // ignore indices leading to overlapping matches

                                if prev <= i {

                                        j := 2 * count

                                        pairs[j+0] = i

                                        pairs[j+1] = i + len(lit)

                                        result[count] = pairs[j : j+2]

                                        count++

                                        prev = i + len(lit)

                                }

                        }

                        result = result[0:count]

                        if len(result) >= n || len(indices) != n1 {

                                // found all matches or there's no chance to find more

                                // (n and n1 can be negative)

                                break

                        }

                }

                if len(result) == 0 {

                        result = nil

                }

                return

        }

        // regexp has a non-empty literal prefix; Lookup(lit) computes

        // the indices of possible complete matches; use these as starting

        // points for anchored searches

        // (regexp "^" matches beginning of input, not beginning of line)

        r = regexp.MustCompile("^" + r.String()) // compiles because r compiled

        // same comment about Lookup applies here as in the loop above

        for n1 := n; ; n1 += 2 * (n - len(result)) /* overflow ok */ {

                indices := x.Lookup(lit, n1)

                if len(indices) == 0 {

                        return

                }

                sort.Ints(indices)

                result = result[0:0]

                prev := 0

                for _, i := range indices {

                        if len(result) == n {

                                break

                        }

                        m := r.FindIndex(x.data[i:]) // anchored search - will not run off

                        // ignore indices leading to overlapping matches

                        if m != nil && prev <= i {

                                m[0] = i // correct m

                                m[1] += i

                                result = append(result, m)

                                prev = m[1]

                        }

                }

                if len(result) >= n || len(indices) != n1 {

                        // found all matches or there's no chance to find more

                        // (n and n1 can be negative)

                        break

                }

        }

        if len(result) == 0 {

                result = nil

        }

        return

}