src/regexp/exec.go - go - Git at Google

 // 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"
 	"sync"
 )

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

 // An 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 https://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

 	inputs inputs
 }

 type inputs struct {
 	// cached inputs, to avoid allocation
 	bytes  inputBytes
 	string inputString
 	reader inputReader
 }

 func (i *inputs) newBytes(b []byte) input {
 	i.bytes.str = b
 	return &i.bytes
 }

 func (i *inputs) newString(s string) input {
 	i.string.str = s
 	return &i.string
 }

 func (i *inputs) newReader(r io.RuneReader) input {
 	i.reader.r = r
 	i.reader.atEOT = false
 	i.reader.pos = 0
 	return &i.reader
 }

 func (i *inputs) clear() {
 	// We need to clear 1 of these.
 	// Avoid the expense of clearing the others (pointer write barrier).
 	if i.bytes.str != nil {
 		i.bytes.str = nil
 	} else if i.reader.r != nil {
 		i.reader.r = nil
 	} else {
 		i.string.str = ""
 	}
 }

 func (i *inputs) init(r io.RuneReader, b []byte, s string) (input, int) {
 	if r != nil {
 		return i.newReader(r), 0
 	}
 	if b != nil {
 		return i.newBytes(b), len(b)
 	}
 	return i.newString(s), len(s)
 }

 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
 }

 // A lazyFlag is a lazily-evaluated syntax.EmptyOp,
 // for checking zero-width flags like ^ $ \A \z \B \b.
 // It records the pair of relevant runes and does not
 // determine the implied flags until absolutely necessary
 // (most of the time, that means never).
 type lazyFlag uint64

 func newLazyFlag(r1, r2 rune) lazyFlag {
 	return lazyFlag(uint64(r1)<<32 | uint64(uint32(r2)))
 }

 func (f lazyFlag) match(op syntax.EmptyOp) bool {
 	if op == 0 {
 		return true
 	}
 	r1 := rune(f >> 32)
 	if op&syntax.EmptyBeginLine != 0 {
 		if r1 != '\n' && r1 >= 0 {
 			return false
 		}
 		op &^= syntax.EmptyBeginLine
 	}
 	if op&syntax.EmptyBeginText != 0 {
 		if r1 >= 0 {
 			return false
 		}
 		op &^= syntax.EmptyBeginText
 	}
 	if op == 0 {
 		return true
 	}
 	r2 := rune(f)
 	if op&syntax.EmptyEndLine != 0 {
 		if r2 != '\n' && r2 >= 0 {
 			return false
 		}
 		op &^= syntax.EmptyEndLine
 	}
 	if op&syntax.EmptyEndText != 0 {
 		if r2 >= 0 {
 			return false
 		}
 		op &^= syntax.EmptyEndText
 	}
 	if op == 0 {
 		return true
 	}
 	if syntax.IsWordChar(r1) != syntax.IsWordChar(r2) {
 		op &^= syntax.EmptyWordBoundary
 	} else {
 		op &^= syntax.EmptyNoWordBoundary
 	}
 	return op == 0
 }

 // 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 lazyFlag
 	if pos == 0 {
 		flag = newLazyFlag(-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 = newLazyFlag(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.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 *lazyFlag) {
 	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.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.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.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 *lazyFlag, t *thread) *thread {
 Again:
 	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)
 		pc = i.Arg
 		goto Again
 	case syntax.InstEmptyWidth:
 		if cond.match(syntax.EmptyOp(i.Arg)) {
 			pc = i.Out
 			goto Again
 		}
 	case syntax.InstNop:
 		pc = i.Out
 		goto Again
 	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 {
 			pc = i.Out
 			goto Again
 		}
 	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
 }

 type onePassMachine struct {
 	inputs   inputs
 	matchcap []int
 }

 var onePassPool sync.Pool

 func newOnePassMachine() *onePassMachine {
 	m, ok := onePassPool.Get().(*onePassMachine)
 	if !ok {
 		m = new(onePassMachine)
 	}
 	return m
 }

 func freeOnePassMachine(m *onePassMachine) {
 	m.inputs.clear()
 	onePassPool.Put(m)
 }

 // doOnePass implements r.doExecute using the one-pass execution engine.
 func (re *Regexp) doOnePass(ir io.RuneReader, ib []byte, is string, pos, ncap int, dstCap []int) []int {
 	startCond := re.cond
 	if startCond == ^syntax.EmptyOp(0) { // impossible
 		return nil
 	}

 	m := newOnePassMachine()
 	if cap(m.matchcap) < ncap {
 		m.matchcap = make([]int, ncap)
 	} else {
 		m.matchcap = m.matchcap[:ncap]
 	}

 	matched := false
 	for i := range m.matchcap {
 		m.matchcap[i] = -1
 	}

 	i, _ := m.inputs.init(ir, ib, is)

 	r, r1 := endOfText, endOfText
 	width, width1 := 0, 0
 	r, width = i.step(pos)
 	if r != endOfText {
 		r1, width1 = i.step(pos + width)
 	}
 	var flag lazyFlag
 	if pos == 0 {
 		flag = newLazyFlag(-1, r)
 	} else {
 		flag = i.context(pos)
 	}
 	pc := re.onepass.Start
 	inst := &re.onepass.Inst[pc]
 	// If there is a simple literal prefix, skip over it.
 	if pos == 0 && flag.match(syntax.EmptyOp(inst.Arg)) &&
 		len(re.prefix) > 0 && i.canCheckPrefix() {
 		// Match requires literal prefix; fast search for it.
 		if !i.hasPrefix(re) {
 			goto Return
 		}
 		pos += len(re.prefix)
 		r, width = i.step(pos)
 		r1, width1 = i.step(pos + width)
 		flag = i.context(pos)
 		pc = int(re.prefixEnd)
 	}
 	for {
 		inst = &re.onepass.Inst[pc]
 		pc = int(inst.Out)
 		switch inst.Op {
 		default:
 			panic("bad inst")
 		case syntax.InstMatch:
 			matched = true
 			if len(m.matchcap) > 0 {
 				m.matchcap[0] = 0
 				m.matchcap[1] = pos
 			}
 			goto Return
 		case syntax.InstRune:
 			if !inst.MatchRune(r) {
 				goto Return
 			}
 		case syntax.InstRune1:
 			if r != inst.Rune[0] {
 				goto Return
 			}
 		case syntax.InstRuneAny:
 			// Nothing
 		case syntax.InstRuneAnyNotNL:
 			if r == '\n' {
 				goto Return
 			}
 		// peek at the input rune to see which branch of the Alt to take
 		case syntax.InstAlt, syntax.InstAltMatch:
 			pc = int(onePassNext(inst, r))
 			continue
 		case syntax.InstFail:
 			goto Return
 		case syntax.InstNop:
 			continue
 		case syntax.InstEmptyWidth:
 			if !flag.match(syntax.EmptyOp(inst.Arg)) {
 				goto Return
 			}
 			continue
 		case syntax.InstCapture:
 			if int(inst.Arg) < len(m.matchcap) {
 				m.matchcap[inst.Arg] = pos
 			}
 			continue
 		}
 		if width == 0 {
 			break
 		}
 		flag = newLazyFlag(r, r1)
 		pos += width
 		r, width = r1, width1
 		if r != endOfText {
 			r1, width1 = i.step(pos + width)
 		}
 	}

 Return:
 	if !matched {
 		freeOnePassMachine(m)
 		return nil
 	}

 	dstCap = append(dstCap, m.matchcap...)
 	freeOnePassMachine(m)
 	return dstCap
 }

 // doMatch reports whether either r, b or s match the regexp.
 func (re *Regexp) doMatch(r io.RuneReader, b []byte, s string) bool {
 	return re.doExecute(r, b, s, 0, 0, nil) != nil
 }

 // doExecute finds the leftmost match in the input, appends the position
 // of its subexpressions to dstCap and returns dstCap.
 //
 // nil is returned if no matches are found and non-nil if matches are found.
 func (re *Regexp) doExecute(r io.RuneReader, b []byte, s string, pos int, ncap int, dstCap []int) []int {
 	if dstCap == nil {
 		// Make sure 'return dstCap' is non-nil.
 		dstCap = arrayNoInts[:0:0]
 	}

 	if r == nil && len(b)+len(s) < re.minInputLen {
 		return nil
 	}

 	if re.onepass != nil {
 		return re.doOnePass(r, b, s, pos, ncap, dstCap)
 	}
 	if r == nil && len(b)+len(s) < re.maxBitStateLen {
 		return re.backtrack(b, s, pos, ncap, dstCap)
 	}

 	m := re.get()
 	i, _ := m.inputs.init(r, b, s)

 	m.init(ncap)
 	if !m.match(i, pos) {
 		re.put(m)
 		return nil
 	}

 	dstCap = append(dstCap, m.matchcap...)
 	re.put(m)
 	return dstCap
 }

 // arrayNoInts is returned by doExecute match if nil dstCap is passed
 // to it with ncap=0.
 var arrayNoInts [0]int
	// 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"
	"sync"
	)

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

	// An 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 https://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

	inputs inputs
	}

	type inputs struct {
	// cached inputs, to avoid allocation
	bytes inputBytes
	string inputString
	reader inputReader
	}

	func (i *inputs) newBytes(b []byte) input {
	i.bytes.str = b
	return &i.bytes
	}

	func (i *inputs) newString(s string) input {
	i.string.str = s
	return &i.string
	}

	func (i *inputs) newReader(r io.RuneReader) input {
	i.reader.r = r
	i.reader.atEOT = false
	i.reader.pos = 0
	return &i.reader
	}

	func (i *inputs) clear() {
	// We need to clear 1 of these.
	// Avoid the expense of clearing the others (pointer write barrier).
	if i.bytes.str != nil {
	i.bytes.str = nil
	} else if i.reader.r != nil {
	i.reader.r = nil
	} else {
	i.string.str = ""
	}
	}

	func (i *inputs) init(r io.RuneReader, b []byte, s string) (input, int) {
	if r != nil {
	return i.newReader(r), 0
	}
	if b != nil {
	return i.newBytes(b), len(b)
	}
	return i.newString(s), len(s)
	}

	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
	}

	// A lazyFlag is a lazily-evaluated syntax.EmptyOp,
	// for checking zero-width flags like ^ $ \A \z \B \b.
	// It records the pair of relevant runes and does not
	// determine the implied flags until absolutely necessary
	// (most of the time, that means never).
	type lazyFlag uint64

	func newLazyFlag(r1, r2 rune) lazyFlag {
	return lazyFlag(uint64(r1)<<32 \| uint64(uint32(r2)))
	}

	func (f lazyFlag) match(op syntax.EmptyOp) bool {
	if op == 0 {
	return true
	}
	r1 := rune(f >> 32)
	if op&syntax.EmptyBeginLine != 0 {
	if r1 != '\n' && r1 >= 0 {
	return false
	}
	op &^= syntax.EmptyBeginLine
	}
	if op&syntax.EmptyBeginText != 0 {
	if r1 >= 0 {
	return false
	}
	op &^= syntax.EmptyBeginText
	}
	if op == 0 {
	return true
	}
	r2 := rune(f)
	if op&syntax.EmptyEndLine != 0 {
	if r2 != '\n' && r2 >= 0 {
	return false
	}
	op &^= syntax.EmptyEndLine
	}
	if op&syntax.EmptyEndText != 0 {
	if r2 >= 0 {
	return false
	}
	op &^= syntax.EmptyEndText
	}
	if op == 0 {
	return true
	}
	if syntax.IsWordChar(r1) != syntax.IsWordChar(r2) {
	op &^= syntax.EmptyWordBoundary
	} else {
	op &^= syntax.EmptyNoWordBoundary
	}
	return op == 0
	}

	// 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 lazyFlag
	if pos == 0 {
	flag = newLazyFlag(-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 = newLazyFlag(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.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 *lazyFlag) {
	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.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.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.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 lazyFlag, t thread) *thread {
	Again:
	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)
	pc = i.Arg
	goto Again
	case syntax.InstEmptyWidth:
	if cond.match(syntax.EmptyOp(i.Arg)) {
	pc = i.Out
	goto Again
	}
	case syntax.InstNop:
	pc = i.Out
	goto Again
	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 {
	pc = i.Out
	goto Again
	}
	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
	}

	type onePassMachine struct {
	inputs inputs
	matchcap []int
	}

	var onePassPool sync.Pool

	func newOnePassMachine() *onePassMachine {
	m, ok := onePassPool.Get().(*onePassMachine)
	if !ok {
	m = new(onePassMachine)
	}
	return m
	}

	func freeOnePassMachine(m *onePassMachine) {
	m.inputs.clear()
	onePassPool.Put(m)
	}

	// doOnePass implements r.doExecute using the one-pass execution engine.
	func (re *Regexp) doOnePass(ir io.RuneReader, ib []byte, is string, pos, ncap int, dstCap []int) []int {
	startCond := re.cond
	if startCond == ^syntax.EmptyOp(0) { // impossible
	return nil
	}

	m := newOnePassMachine()
	if cap(m.matchcap) < ncap {
	m.matchcap = make([]int, ncap)
	} else {
	m.matchcap = m.matchcap[:ncap]
	}

	matched := false
	for i := range m.matchcap {
	m.matchcap[i] = -1
	}

	i, _ := m.inputs.init(ir, ib, is)

	r, r1 := endOfText, endOfText
	width, width1 := 0, 0
	r, width = i.step(pos)
	if r != endOfText {
	r1, width1 = i.step(pos + width)
	}
	var flag lazyFlag
	if pos == 0 {
	flag = newLazyFlag(-1, r)
	} else {
	flag = i.context(pos)
	}
	pc := re.onepass.Start
	inst := &re.onepass.Inst[pc]
	// If there is a simple literal prefix, skip over it.
	if pos == 0 && flag.match(syntax.EmptyOp(inst.Arg)) &&
	len(re.prefix) > 0 && i.canCheckPrefix() {
	// Match requires literal prefix; fast search for it.
	if !i.hasPrefix(re) {
	goto Return
	}
	pos += len(re.prefix)
	r, width = i.step(pos)
	r1, width1 = i.step(pos + width)
	flag = i.context(pos)
	pc = int(re.prefixEnd)
	}
	for {
	inst = &re.onepass.Inst[pc]
	pc = int(inst.Out)
	switch inst.Op {
	default:
	panic("bad inst")
	case syntax.InstMatch:
	matched = true
	if len(m.matchcap) > 0 {
	m.matchcap[0] = 0
	m.matchcap[1] = pos
	}
	goto Return
	case syntax.InstRune:
	if !inst.MatchRune(r) {
	goto Return
	}
	case syntax.InstRune1:
	if r != inst.Rune[0] {
	goto Return
	}
	case syntax.InstRuneAny:
	// Nothing
	case syntax.InstRuneAnyNotNL:
	if r == '\n' {
	goto Return
	}
	// peek at the input rune to see which branch of the Alt to take
	case syntax.InstAlt, syntax.InstAltMatch:
	pc = int(onePassNext(inst, r))
	continue
	case syntax.InstFail:
	goto Return
	case syntax.InstNop:
	continue
	case syntax.InstEmptyWidth:
	if !flag.match(syntax.EmptyOp(inst.Arg)) {
	goto Return
	}
	continue
	case syntax.InstCapture:
	if int(inst.Arg) < len(m.matchcap) {
	m.matchcap[inst.Arg] = pos
	}
	continue
	}
	if width == 0 {
	break
	}
	flag = newLazyFlag(r, r1)
	pos += width
	r, width = r1, width1
	if r != endOfText {
	r1, width1 = i.step(pos + width)
	}
	}

	Return:
	if !matched {
	freeOnePassMachine(m)
	return nil
	}

	dstCap = append(dstCap, m.matchcap...)
	freeOnePassMachine(m)
	return dstCap
	}

	// doMatch reports whether either r, b or s match the regexp.
	func (re *Regexp) doMatch(r io.RuneReader, b []byte, s string) bool {
	return re.doExecute(r, b, s, 0, 0, nil) != nil
	}

	// doExecute finds the leftmost match in the input, appends the position
	// of its subexpressions to dstCap and returns dstCap.
	//
	// nil is returned if no matches are found and non-nil if matches are found.
	func (re *Regexp) doExecute(r io.RuneReader, b []byte, s string, pos int, ncap int, dstCap []int) []int {
	if dstCap == nil {
	// Make sure 'return dstCap' is non-nil.
	dstCap = arrayNoInts[:0:0]
	}

	if r == nil && len(b)+len(s) < re.minInputLen {
	return nil
	}

	if re.onepass != nil {
	return re.doOnePass(r, b, s, pos, ncap, dstCap)
	}
	if r == nil && len(b)+len(s) < re.maxBitStateLen {
	return re.backtrack(b, s, pos, ncap, dstCap)
	}

	m := re.get()
	i, _ := m.inputs.init(r, b, s)

	m.init(ncap)
	if !m.match(i, pos) {
	re.put(m)
	return nil
	}

	dstCap = append(dstCap, m.matchcap...)
	re.put(m)
	return dstCap
	}

	// arrayNoInts is returned by doExecute match if nil dstCap is passed
	// to it with ncap=0.
	var arrayNoInts [0]int