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Diffstat (limited to 'libgo/go/regexp/backtrack.go')
-rw-r--r-- | libgo/go/regexp/backtrack.go | 366 |
1 files changed, 366 insertions, 0 deletions
diff --git a/libgo/go/regexp/backtrack.go b/libgo/go/regexp/backtrack.go new file mode 100644 index 00000000000..fd95604fe44 --- /dev/null +++ b/libgo/go/regexp/backtrack.go @@ -0,0 +1,366 @@ +// 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 + 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 { + 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 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 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 + } + 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, 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 +} |