package regexp

import "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
}

// 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.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(i input, pos int, ncap int) []int {
	m := re.get()
	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
}