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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
}