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

 // Copyright 2015 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.

 // backtrack is a regular expression search with submatch
 // tracking for small regular expressions and texts. It allocates
 // a bit vector with (length of input) * (length of prog) bits,
 // to make sure it never explores the same (character position, instruction)
 // state multiple times. This limits the search to run in time linear in
 // the length of the test.
 //
 // backtrack is a fast replacement for the NFA code on small
 // regexps when onepass cannot be used.

 package regexp

 import "regexp/syntax"

 // A job is an entry on the backtracker's job stack. It holds
 // the instruction pc and the position in the input.
 type job struct {
 	pc  uint32
 	arg bool
 	pos int
 }

 const (
 	visitedBits        = 32
 	maxBacktrackProg   = 500        // len(prog.Inst) <= max
 	maxBacktrackVector = 256 * 1024 // bit vector size <= max (bits)
 )

 // bitState holds state for the backtracker.
 type bitState struct {
 	prog *syntax.Prog

 	end     int
 	cap     []int
 	jobs    []job
 	visited []uint32
 }

 var notBacktrack *bitState = nil

 // maxBitStateLen returns the maximum length of a string to search with
 // the backtracker using prog.
 func maxBitStateLen(prog *syntax.Prog) int {
 	if !shouldBacktrack(prog) {
 		return 0
 	}
 	return maxBacktrackVector / len(prog.Inst)
 }

 // newBitState returns a new bitState for the given prog,
 // or notBacktrack if the size of the prog exceeds the maximum size that
 // the backtracker will be run for.
 func newBitState(prog *syntax.Prog) *bitState {
 	if !shouldBacktrack(prog) {
 		return notBacktrack
 	}
 	return &bitState{
 		prog: prog,
 	}
 }

 // shouldBacktrack reports whether the program is too
 // long for the backtracker to run.
 func shouldBacktrack(prog *syntax.Prog) bool {
 	return len(prog.Inst) <= maxBacktrackProg
 }

 // reset resets the state of the backtracker.
 // end is the end position in the input.
 // ncap is the number of captures.
 func (b *bitState) reset(end int, ncap int) {
 	b.end = end

 	if cap(b.jobs) == 0 {
 		b.jobs = make([]job, 0, 256)
 	} else {
 		b.jobs = b.jobs[:0]
 	}

 	visitedSize := (len(b.prog.Inst)*(end+1) + visitedBits - 1) / visitedBits
 	if cap(b.visited) < visitedSize {
 		b.visited = make([]uint32, visitedSize, maxBacktrackVector/visitedBits)
 	} else {
 		b.visited = b.visited[:visitedSize]
 		for i := range b.visited {
 			b.visited[i] = 0
 		}
 	}

 	if cap(b.cap) < ncap {
 		b.cap = make([]int, ncap)
 	} else {
 		b.cap = b.cap[:ncap]
 	}
 	for i := range b.cap {
 		b.cap[i] = -1
 	}
 }

 // shouldVisit reports whether the combination of (pc, pos) has not
 // been visited yet.
 func (b *bitState) shouldVisit(pc uint32, pos int) bool {
 	n := uint(int(pc)*(b.end+1) + pos)
 	if b.visited[n/visitedBits]&(1<<(n&(visitedBits-1))) != 0 {
 		return false
 	}
 	b.visited[n/visitedBits] |= 1 << (n & (visitedBits - 1))
 	return true
 }

 // push pushes (pc, pos, arg) onto the job stack if it should be
 // visited.
 func (b *bitState) push(pc uint32, pos int, arg bool) {
 	// Only check shouldVisit when arg is false.
 	// When arg is true, we are continuing a previous visit.
 	if b.prog.Inst[pc].Op != syntax.InstFail && (arg || b.shouldVisit(pc, pos)) {
 		b.jobs = append(b.jobs, job{pc: pc, arg: arg, pos: pos})
 	}
 }

 // tryBacktrack runs a backtracking search starting at pos.
 func (m *machine) tryBacktrack(b *bitState, i input, pc uint32, pos int) bool {
 	longest := m.re.longest
 	m.matched = false

 	b.push(pc, pos, false)
 	for len(b.jobs) > 0 {
 		l := len(b.jobs) - 1
 		// Pop job off the stack.
 		pc := b.jobs[l].pc
 		pos := b.jobs[l].pos
 		arg := b.jobs[l].arg
 		b.jobs = b.jobs[:l]

 		// Optimization: rather than push and pop,
 		// code that is going to Push and continue
 		// the loop simply updates ip, p, and arg
 		// and jumps to CheckAndLoop. We have to
 		// do the ShouldVisit check that Push
 		// would have, but we avoid the stack
 		// manipulation.
 		goto Skip
 	CheckAndLoop:
 		if !b.shouldVisit(pc, pos) {
 			continue
 		}
 	Skip:

 		inst := b.prog.Inst[pc]

 		switch inst.Op {
 		default:
 			panic("bad inst")
 		case syntax.InstFail:
 			panic("unexpected InstFail")
 		case syntax.InstAlt:
 			// Cannot just
 			//   b.push(inst.Out, pos, false)
 			//   b.push(inst.Arg, pos, false)
 			// If during the processing of inst.Out, we encounter
 			// inst.Arg via another path, we want to process it then.
 			// Pushing it here will inhibit that. Instead, re-push
 			// inst with arg==true as a reminder to push inst.Arg out
 			// later.
 			if arg {
 				// Finished inst.Out; try inst.Arg.
 				arg = false
 				pc = inst.Arg
 				goto CheckAndLoop
 			} else {
 				b.push(pc, pos, true)
 				pc = inst.Out
 				goto CheckAndLoop
 			}

 		case syntax.InstAltMatch:
 			// One opcode consumes runes; the other leads to match.
 			switch b.prog.Inst[inst.Out].Op {
 			case syntax.InstRune, syntax.InstRune1, syntax.InstRuneAny, syntax.InstRuneAnyNotNL:
 				// inst.Arg is the match.
 				b.push(inst.Arg, pos, false)
 				pc = inst.Arg
 				pos = b.end
 				goto CheckAndLoop
 			}
 			// inst.Out is the match - non-greedy
 			b.push(inst.Out, b.end, false)
 			pc = inst.Out
 			goto CheckAndLoop

 		case syntax.InstRune:
 			r, width := i.step(pos)
 			if !inst.MatchRune(r) {
 				continue
 			}
 			pos += width
 			pc = inst.Out
 			goto CheckAndLoop

 		case syntax.InstRune1:
 			r, width := i.step(pos)
 			if r != inst.Rune[0] {
 				continue
 			}
 			pos += width
 			pc = inst.Out
 			goto CheckAndLoop

 		case syntax.InstRuneAnyNotNL:
 			r, width := i.step(pos)
 			if r == '\n' || r == endOfText {
 				continue
 			}
 			pos += width
 			pc = inst.Out
 			goto CheckAndLoop

 		case syntax.InstRuneAny:
 			r, width := i.step(pos)
 			if r == endOfText {
 				continue
 			}
 			pos += width
 			pc = inst.Out
 			goto CheckAndLoop

 		case syntax.InstCapture:
 			if arg {
 				// Finished inst.Out; restore the old value.
 				b.cap[inst.Arg] = pos
 				continue
 			} else {
 				if 0 <= inst.Arg && inst.Arg < uint32(len(b.cap)) {
 					// Capture pos to register, but save old value.
 					b.push(pc, b.cap[inst.Arg], true) // come back when we're done.
 					b.cap[inst.Arg] = pos
 				}
 				pc = inst.Out
 				goto CheckAndLoop
 			}

 		case syntax.InstEmptyWidth:
 			if syntax.EmptyOp(inst.Arg)&^i.context(pos) != 0 {
 				continue
 			}
 			pc = inst.Out
 			goto CheckAndLoop

 		case syntax.InstNop:
 			pc = inst.Out
 			goto CheckAndLoop

 		case syntax.InstMatch:
 			// We found a match. If the caller doesn't care
 			// where the match is, no point going further.
 			if len(b.cap) == 0 {
 				m.matched = true
 				return m.matched
 			}

 			// Record best match so far.
 			// Only need to check end point, because this entire
 			// call is only considering one start position.
 			if len(b.cap) > 1 {
 				b.cap[1] = pos
 			}
 			if !m.matched || (longest && pos > 0 && pos > m.matchcap[1]) {
 				copy(m.matchcap, b.cap)
 			}
 			m.matched = true

 			// If going for first match, we're done.
 			if !longest {
 				return m.matched
 			}

 			// If we used the entire text, no longer match is possible.
 			if pos == b.end {
 				return m.matched
 			}

 			// Otherwise, continue on in hope of a longer match.
 			continue
 		}
 	}

 	return m.matched
 }

 // backtrack runs a backtracking search of prog on the input starting at pos.
 func (m *machine) backtrack(i input, pos int, end int, ncap int) bool {
 	if !i.canCheckPrefix() {
 		panic("backtrack called for a RuneReader")
 	}

 	startCond := m.re.cond
 	if startCond == ^syntax.EmptyOp(0) { // impossible
 		return false
 	}
 	if startCond&syntax.EmptyBeginText != 0 && pos != 0 {
 		// Anchored match, past beginning of text.
 		return false
 	}

 	b := m.b
 	b.reset(end, ncap)

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

 	// Anchored search must start at the beginning of the input
 	if startCond&syntax.EmptyBeginText != 0 {
 		if len(b.cap) > 0 {
 			b.cap[0] = pos
 		}
 		return m.tryBacktrack(b, i, uint32(m.p.Start), pos)
 	}

 	// Unanchored search, starting from each possible text position.
 	// Notice that we have to try the empty string at the end of
 	// the text, so the loop condition is pos <= end, not pos < end.
 	// This looks like it's quadratic in the size of the text,
 	// but we are not clearing visited between calls to TrySearch,
 	// so no work is duplicated and it ends up still being linear.
 	width := -1
 	for ; pos <= end && width != 0; pos += width {
 		if len(m.re.prefix) > 0 {
 			// Match requires literal prefix; fast search for it.
 			advance := i.index(m.re, pos)
 			if advance < 0 {
 				return false
 			}
 			pos += advance
 		}

 		if len(b.cap) > 0 {
 			b.cap[0] = pos
 		}
 		if m.tryBacktrack(b, i, uint32(m.p.Start), pos) {
 			// Match must be leftmost; done.
 			return true
 		}
 		_, width = i.step(pos)
 	}
 	return false
 }
	// Copyright 2015 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.

	// backtrack is a regular expression search with submatch
	// tracking for small regular expressions and texts. It allocates
	// a bit vector with (length of input) * (length of prog) bits,
	// to make sure it never explores the same (character position, instruction)
	// state multiple times. This limits the search to run in time linear in
	// the length of the test.
	//
	// backtrack is a fast replacement for the NFA code on small
	// regexps when onepass cannot be used.

	package regexp

	import "regexp/syntax"

	// A job is an entry on the backtracker's job stack. It holds
	// the instruction pc and the position in the input.
	type job struct {
	pc uint32
	arg bool
	pos int
	}

	const (
	visitedBits = 32
	maxBacktrackProg = 500 // len(prog.Inst) <= max
	maxBacktrackVector = 256 * 1024 // bit vector size <= max (bits)
	)

	// bitState holds state for the backtracker.
	type bitState struct {
	prog *syntax.Prog

	end int
	cap []int
	jobs []job
	visited []uint32
	}

	var notBacktrack *bitState = nil

	// maxBitStateLen returns the maximum length of a string to search with
	// the backtracker using prog.
	func maxBitStateLen(prog *syntax.Prog) int {
	if !shouldBacktrack(prog) {
	return 0
	}
	return maxBacktrackVector / len(prog.Inst)
	}

	// newBitState returns a new bitState for the given prog,
	// or notBacktrack if the size of the prog exceeds the maximum size that
	// the backtracker will be run for.
	func newBitState(prog syntax.Prog) bitState {
	if !shouldBacktrack(prog) {
	return notBacktrack
	}
	return &bitState{
	prog: prog,
	}
	}

	// shouldBacktrack reports whether the program is too
	// long for the backtracker to run.
	func shouldBacktrack(prog *syntax.Prog) bool {
	return len(prog.Inst) <= maxBacktrackProg
	}

	// reset resets the state of the backtracker.
	// end is the end position in the input.
	// ncap is the number of captures.
	func (b *bitState) reset(end int, ncap int) {
	b.end = end

	if cap(b.jobs) == 0 {
	b.jobs = make([]job, 0, 256)
	} else {
	b.jobs = b.jobs[:0]
	}

	visitedSize := (len(b.prog.Inst)*(end+1) + visitedBits - 1) / visitedBits
	if cap(b.visited) < visitedSize {
	b.visited = make([]uint32, visitedSize, maxBacktrackVector/visitedBits)
	} else {
	b.visited = b.visited[:visitedSize]
	for i := range b.visited {
	b.visited[i] = 0
	}
	}

	if cap(b.cap) < ncap {
	b.cap = make([]int, ncap)
	} else {
	b.cap = b.cap[:ncap]
	}
	for i := range b.cap {
	b.cap[i] = -1
	}
	}

	// shouldVisit reports whether the combination of (pc, pos) has not
	// been visited yet.
	func (b *bitState) shouldVisit(pc uint32, pos int) bool {
	n := uint(int(pc)*(b.end+1) + pos)
	if b.visited[n/visitedBits]&(1<<(n&(visitedBits-1))) != 0 {
	return false
	}
	b.visited[n/visitedBits] \|= 1 << (n & (visitedBits - 1))
	return true
	}

	// push pushes (pc, pos, arg) onto the job stack if it should be
	// visited.
	func (b *bitState) push(pc uint32, pos int, arg bool) {
	// Only check shouldVisit when arg is false.
	// When arg is true, we are continuing a previous visit.
	if b.prog.Inst[pc].Op != syntax.InstFail && (arg \|\| b.shouldVisit(pc, pos)) {
	b.jobs = append(b.jobs, job{pc: pc, arg: arg, pos: pos})
	}
	}

	// tryBacktrack runs a backtracking search starting at pos.
	func (m machine) tryBacktrack(b bitState, i input, pc uint32, pos int) bool {
	longest := m.re.longest
	m.matched = false

	b.push(pc, pos, false)
	for len(b.jobs) > 0 {
	l := len(b.jobs) - 1
	// Pop job off the stack.
	pc := b.jobs[l].pc
	pos := b.jobs[l].pos
	arg := b.jobs[l].arg
	b.jobs = b.jobs[:l]

	// Optimization: rather than push and pop,
	// code that is going to Push and continue
	// the loop simply updates ip, p, and arg
	// and jumps to CheckAndLoop. We have to
	// do the ShouldVisit check that Push
	// would have, but we avoid the stack
	// manipulation.
	goto Skip
	CheckAndLoop:
	if !b.shouldVisit(pc, pos) {
	continue
	}
	Skip:

	inst := b.prog.Inst[pc]

	switch inst.Op {
	default:
	panic("bad inst")
	case syntax.InstFail:
	panic("unexpected InstFail")
	case syntax.InstAlt:
	// Cannot just
	// b.push(inst.Out, pos, false)
	// b.push(inst.Arg, pos, false)
	// If during the processing of inst.Out, we encounter
	// inst.Arg via another path, we want to process it then.
	// Pushing it here will inhibit that. Instead, re-push
	// inst with arg==true as a reminder to push inst.Arg out
	// later.
	if arg {
	// Finished inst.Out; try inst.Arg.
	arg = false
	pc = inst.Arg
	goto CheckAndLoop
	} else {
	b.push(pc, pos, true)
	pc = inst.Out
	goto CheckAndLoop
	}

	case syntax.InstAltMatch:
	// One opcode consumes runes; the other leads to match.
	switch b.prog.Inst[inst.Out].Op {
	case syntax.InstRune, syntax.InstRune1, syntax.InstRuneAny, syntax.InstRuneAnyNotNL:
	// inst.Arg is the match.
	b.push(inst.Arg, pos, false)
	pc = inst.Arg
	pos = b.end
	goto CheckAndLoop
	}
	// inst.Out is the match - non-greedy
	b.push(inst.Out, b.end, false)
	pc = inst.Out
	goto CheckAndLoop

	case syntax.InstRune:
	r, width := i.step(pos)
	if !inst.MatchRune(r) {
	continue
	}
	pos += width
	pc = inst.Out
	goto CheckAndLoop

	case syntax.InstRune1:
	r, width := i.step(pos)
	if r != inst.Rune[0] {
	continue
	}
	pos += width
	pc = inst.Out
	goto CheckAndLoop

	case syntax.InstRuneAnyNotNL:
	r, width := i.step(pos)
	if r == '\n' \|\| r == endOfText {
	continue
	}
	pos += width
	pc = inst.Out
	goto CheckAndLoop

	case syntax.InstRuneAny:
	r, width := i.step(pos)
	if r == endOfText {
	continue
	}
	pos += width
	pc = inst.Out
	goto CheckAndLoop

	case syntax.InstCapture:
	if arg {
	// Finished inst.Out; restore the old value.
	b.cap[inst.Arg] = pos
	continue
	} else {
	if 0 <= inst.Arg && inst.Arg < uint32(len(b.cap)) {
	// Capture pos to register, but save old value.
	b.push(pc, b.cap[inst.Arg], true) // come back when we're done.
	b.cap[inst.Arg] = pos
	}
	pc = inst.Out
	goto CheckAndLoop
	}

	case syntax.InstEmptyWidth:
	if syntax.EmptyOp(inst.Arg)&^i.context(pos) != 0 {
	continue
	}
	pc = inst.Out
	goto CheckAndLoop

	case syntax.InstNop:
	pc = inst.Out
	goto CheckAndLoop

	case syntax.InstMatch:
	// We found a match. If the caller doesn't care
	// where the match is, no point going further.
	if len(b.cap) == 0 {
	m.matched = true
	return m.matched
	}

	// Record best match so far.
	// Only need to check end point, because this entire
	// call is only considering one start position.
	if len(b.cap) > 1 {
	b.cap[1] = pos
	}
	if !m.matched \|\| (longest && pos > 0 && pos > m.matchcap[1]) {
	copy(m.matchcap, b.cap)
	}
	m.matched = true

	// If going for first match, we're done.
	if !longest {
	return m.matched
	}

	// If we used the entire text, no longer match is possible.
	if pos == b.end {
	return m.matched
	}

	// Otherwise, continue on in hope of a longer match.
	continue
	}
	}

	return m.matched
	}

	// backtrack runs a backtracking search of prog on the input starting at pos.
	func (m *machine) backtrack(i input, pos int, end int, ncap int) bool {
	if !i.canCheckPrefix() {
	panic("backtrack called for a RuneReader")
	}

	startCond := m.re.cond
	if startCond == ^syntax.EmptyOp(0) { // impossible
	return false
	}
	if startCond&syntax.EmptyBeginText != 0 && pos != 0 {
	// Anchored match, past beginning of text.
	return false
	}

	b := m.b
	b.reset(end, ncap)

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

	// Anchored search must start at the beginning of the input
	if startCond&syntax.EmptyBeginText != 0 {
	if len(b.cap) > 0 {
	b.cap[0] = pos
	}
	return m.tryBacktrack(b, i, uint32(m.p.Start), pos)
	}

	// Unanchored search, starting from each possible text position.
	// Notice that we have to try the empty string at the end of
	// the text, so the loop condition is pos <= end, not pos < end.
	// This looks like it's quadratic in the size of the text,
	// but we are not clearing visited between calls to TrySearch,
	// so no work is duplicated and it ends up still being linear.
	width := -1
	for ; pos <= end && width != 0; pos += width {
	if len(m.re.prefix) > 0 {
	// Match requires literal prefix; fast search for it.
	advance := i.index(m.re, pos)
	if advance < 0 {
	return false
	}
	pos += advance
	}

	if len(b.cap) > 0 {
	b.cap[0] = pos
	}
	if m.tryBacktrack(b, i, uint32(m.p.Start), pos) {
	// Match must be leftmost; done.
	return true
	}
	_, width = i.step(pos)
	}
	return false
	}