--- /dev/null
+// 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 int
+ 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
+ reqcap bool // whether any captures are requested
+ input input
+ 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 {
+ 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 len(prog.Inst) > maxBacktrackProg {
+ return notBacktrack
+ }
+ return &bitState{
+ prog: prog,
+ }
+}
+
+// reset resets the state of the backtracker.
+// end is the end position in the input. ncap and reqcap are the number
+// of the machine's capture registers and the number of user-requested
+// captures respectively.
+func (b *bitState) reset(end int, ncap int, reqcap int) {
+ b.end = end
+ b.reqcap = reqcap > 0
+
+ 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 len(b.cap) < ncap {
+ b.cap = make([]int, 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 int) {
+ if b.prog.Inst[pc].Op == syntax.InstFail {
+ return
+ }
+
+ // Only check shouldVisit when arg == 0.
+ // When arg > 0, we are continuing a previous visit.
+ if arg == 0 && !b.shouldVisit(pc, pos) {
+ return
+ }
+
+ 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, 0)
+ 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, 0)
+ // b.push(inst.Arg, pos, 0)
+ // 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==1 as a reminder to push inst.Arg out
+ // later.
+ switch arg {
+ case 0:
+ b.push(pc, pos, 1)
+ pc = inst.Out
+ goto CheckAndLoop
+ case 1:
+ // Finished inst.Out; try inst.Arg.
+ arg = 0
+ pc = inst.Arg
+ goto CheckAndLoop
+ }
+ panic("bad arg in InstAlt")
+
+ 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, 0)
+ pc = inst.Arg
+ pos = b.end
+ goto CheckAndLoop
+ }
+ // inst.Out is the match - non-greedy
+ b.push(inst.Out, b.end, 0)
+ 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:
+ switch arg {
+ case 0:
+ 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], 1) // come back when we're done.
+ b.cap[inst.Arg] = pos
+ }
+ pc = inst.Out
+ goto CheckAndLoop
+ case 1:
+ // Finished inst.Out; restore the old value.
+ b.cap[inst.Arg] = pos
+ continue
+
+ }
+ panic("bad arg in InstCapture")
+ continue
+
+ 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 !b.reqcap {
+ 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.
+ 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
+ }
+ panic("unreachable")
+ }
+
+ 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, reqcap 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, len(m.matchcap), reqcap)
+
+ for i := range m.matchcap {
+ m.matchcap[i] = -1
+ }
+
+ // Anchored search must start at the beginning of the input
+ if startCond&syntax.EmptyBeginText != 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
+ }
+
+ 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
+}