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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.

package regexp

import (
        "io"
        "regexp/syntax"
)

// A queue is a 'sparse array' holding pending threads of execution.
// See http://research.swtch.com/2008/03/using-uninitialized-memory-for-fun-and.html
type queue struct {
        sparse []uint32
        dense  []entry
}

// A entry is an entry on a queue.
// It holds both the instruction pc and the actual thread.
// Some queue entries are just place holders so that the machine
// knows it has considered that pc.  Such entries have t == nil.
type entry struct {
        pc uint32
        t  *thread
}

// A thread is the state of a single path through the machine:
// an instruction and a corresponding capture array.
// See http://swtch.com/~rsc/regexp/regexp2.html
type thread struct {
        inst *syntax.Inst
        cap  []int
}

// A machine holds all the state during an NFA simulation for p.
type machine struct {
        re       *Regexp      // corresponding Regexp
        p        *syntax.Prog // compiled program
        q0, q1   queue        // two queues for runq, nextq
        pool     []*thread    // pool of available threads
        matched  bool         // whether a match was found
        matchcap []int        // capture information for the match

        // cached inputs, to avoid allocation
        inputBytes  inputBytes
        inputString inputString
        inputReader inputReader
}

func (m *machine) newInputBytes(b []byte) input {
        m.inputBytes.str = b
        return &m.inputBytes
}

func (m *machine) newInputString(s string) input {
        m.inputString.str = s
        return &m.inputString
}

func (m *machine) newInputReader(r io.RuneReader) input {
        m.inputReader.r = r
        m.inputReader.atEOT = false
        m.inputReader.pos = 0
        return &m.inputReader
}

// progMachine returns a new machine running the prog p.
func progMachine(p *syntax.Prog) *machine {
        m := &machine{p: p}
        n := len(m.p.Inst)
        m.q0 = queue{make([]uint32, n), make([]entry, 0, n)}
        m.q1 = queue{make([]uint32, n), make([]entry, 0, n)}
        ncap := p.NumCap
        if ncap < 2 {
                ncap = 2
        }
        m.matchcap = make([]int, ncap)
        return m
}

func (m *machine) init(ncap int) {
        for _, t := range m.pool {
                t.cap = t.cap[:ncap]
        }
        m.matchcap = m.matchcap[:ncap]
}

// alloc allocates a new thread with the given instruction.
// It uses the free pool if possible.
func (m *machine) alloc(i *syntax.Inst) *thread {
        var t *thread
        if n := len(m.pool); n > 0 {
                t = m.pool[n-1]
                m.pool = m.pool[:n-1]
        } else {
                t = new(thread)
                t.cap = make([]int, len(m.matchcap), cap(m.matchcap))
        }
        t.inst = i
        return t
}

// free returns t to the free pool.
func (m *machine) free(t *thread) {
        m.inputBytes.str = nil
        m.inputString.str = ""
        m.inputReader.r = nil
        m.pool = append(m.pool, t)
}

// match runs the machine over the input starting at pos.
// It reports whether a match was found.
// If so, m.matchcap holds the submatch information.
func (m *machine) match(i input, pos int) bool {
        startCond := m.re.cond
        if startCond == ^syntax.EmptyOp(0) { // impossible
                return false
        }
        m.matched = false
        for i := range m.matchcap {
                m.matchcap[i] = -1
        }
        runq, nextq := &m.q0, &m.q1
        r, r1 := endOfText, endOfText
        width, width1 := 0, 0
        r, width = i.step(pos)
        if r != endOfText {
                r1, width1 = i.step(pos + width)
        }
        var flag syntax.EmptyOp
        if pos == 0 {
                flag = syntax.EmptyOpContext(-1, r)
        } else {
                flag = i.context(pos)
        }
        for {
                if len(runq.dense) == 0 {
                        if startCond&syntax.EmptyBeginText != 0 && pos != 0 {
                                // Anchored match, past beginning of text.
                                break
                        }
                        if m.matched {
                                // Have match; finished exploring alternatives.
                                break
                        }
                        if len(m.re.prefix) > 0 && r1 != m.re.prefixRune && i.canCheckPrefix() {
                                // Match requires literal prefix; fast search for it.
                                advance := i.index(m.re, pos)
                                if advance < 0 {
                                        break
                                }
                                pos += advance
                                r, width = i.step(pos)
                                r1, width1 = i.step(pos + width)
                        }
                }
                if !m.matched {
                        if len(m.matchcap) > 0 {
                                m.matchcap[0] = pos
                        }
                        m.add(runq, uint32(m.p.Start), pos, m.matchcap, flag, nil)
                }
                flag = syntax.EmptyOpContext(r, r1)
                m.step(runq, nextq, pos, pos+width, r, flag)
                if width == 0 {
                        break
                }
                if len(m.matchcap) == 0 && m.matched {
                        // Found a match and not paying attention
                        // to where it is, so any match will do.
                        break
                }
                pos += width
                r, width = r1, width1
                if r != endOfText {
                        r1, width1 = i.step(pos + width)
                }
                runq, nextq = nextq, runq
        }
        m.clear(nextq)
        return m.matched
}

// clear frees all threads on the thread queue.
func (m *machine) clear(q *queue) {
        for _, d := range q.dense {
                if d.t != nil {
                        // m.free(d.t)
                        m.pool = append(m.pool, d.t)
                }
        }
        q.dense = q.dense[:0]
}

// step executes one step of the machine, running each of the threads
// on runq and appending new threads to nextq.
// The step processes the rune c (which may be endOfText),
// which starts at position pos and ends at nextPos.
// nextCond gives the setting for the empty-width flags after c.
func (m *machine) step(runq, nextq *queue, pos, nextPos int, c rune, nextCond syntax.EmptyOp) {
        longest := m.re.longest
        for j := 0; j < len(runq.dense); j++ {
                d := &runq.dense[j]
                t := d.t
                if t == nil {
                        continue
                }
                if longest && m.matched && len(t.cap) > 0 && m.matchcap[0] < t.cap[0] {
                        // m.free(t)
                        m.pool = append(m.pool, t)
                        continue
                }
                i := t.inst
                add := false
                switch i.Op {
                default:
                        panic("bad inst")

                case syntax.InstMatch:
                        if len(t.cap) > 0 && (!longest || !m.matched || m.matchcap[1] < pos) {
                                t.cap[1] = pos
                                copy(m.matchcap, t.cap)
                        }
                        if !longest {
                                // First-match mode: cut off all lower-priority threads.
                                for _, d := range runq.dense[j+1:] {
                                        if d.t != nil {
                                                // m.free(d.t)
                                                m.pool = append(m.pool, d.t)
                                        }
                                }
                                runq.dense = runq.dense[:0]
                        }
                        m.matched = true

                case syntax.InstRune:
                        add = i.MatchRune(c)
                case syntax.InstRune1:
                        add = c == i.Rune[0]
                case syntax.InstRuneAny:
                        add = true
                case syntax.InstRuneAnyNotNL:
                        add = c != '\n'
                }
                if add {
                        t = m.add(nextq, i.Out, nextPos, t.cap, nextCond, t)
                }
                if t != nil {
                        // m.free(t)
                        m.pool = append(m.pool, t)
                }
        }
        runq.dense = runq.dense[:0]
}

// add adds an entry to q for pc, unless the q already has such an entry.
// It also recursively adds an entry for all instructions reachable from pc by following
// empty-width conditions satisfied by cond.  pos gives the current position
// in the input.
func (m *machine) add(q *queue, pc uint32, pos int, cap []int, cond syntax.EmptyOp, t *thread) *thread {
        if pc == 0 {
                return t
        }
        if j := q.sparse[pc]; j < uint32(len(q.dense)) && q.dense[j].pc == pc {
                return t
        }

        j := len(q.dense)
        q.dense = q.dense[:j+1]
        d := &q.dense[j]
        d.t = nil
        d.pc = pc
        q.sparse[pc] = uint32(j)

        i := &m.p.Inst[pc]
        switch i.Op {
        default:
                panic("unhandled")
        case syntax.InstFail:
                // nothing
        case syntax.InstAlt, syntax.InstAltMatch:
                t = m.add(q, i.Out, pos, cap, cond, t)
                t = m.add(q, i.Arg, pos, cap, cond, t)
        case syntax.InstEmptyWidth:
                if syntax.EmptyOp(i.Arg)&^cond == 0 {
                        t = m.add(q, i.Out, pos, cap, cond, t)
                }
        case syntax.InstNop:
                t = m.add(q, i.Out, pos, cap, cond, t)
        case syntax.InstCapture:
                if int(i.Arg) < len(cap) {
                        opos := cap[i.Arg]
                        cap[i.Arg] = pos
                        m.add(q, i.Out, pos, cap, cond, nil)
                        cap[i.Arg] = opos
                } else {
                        t = m.add(q, i.Out, pos, cap, cond, t)
                }
        case syntax.InstMatch, syntax.InstRune, syntax.InstRune1, syntax.InstRuneAny, syntax.InstRuneAnyNotNL:
                if t == nil {
                        t = m.alloc(i)
                } else {
                        t.inst = i
                }
                if len(cap) > 0 && &t.cap[0] != &cap[0] {
                        copy(t.cap, cap)
                }
                d.t = t
                t = nil
        }
        return t
}

// empty is a non-nil 0-element slice,
// so doExecute can avoid an allocation
// when 0 captures are requested from a successful match.
var empty = make([]int, 0)

// doExecute finds the leftmost match in the input and returns
// the position of its subexpressions.
func (re *Regexp) doExecute(r io.RuneReader, b []byte, s string, pos int, ncap int) []int {
        m := re.get()
        var i input
        if r != nil {
                i = m.newInputReader(r)
        } else if b != nil {
                i = m.newInputBytes(b)
        } else {
                i = m.newInputString(s)
        }
        m.init(ncap)
        if !m.match(i, pos) {
                re.put(m)
                return nil
        }
        if ncap == 0 {
                re.put(m)
                return empty // empty but not nil
        }
        cap := make([]int, ncap)
        copy(cap, m.matchcap)
        re.put(m)
        return cap
}