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// 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 syntax

import (
        "bytes"
        "strconv"
        "unicode"
)

// Compiled program.
// May not belong in this package, but convenient for now.

// A Prog is a compiled regular expression program.
type Prog struct {
        Inst   []Inst
        Start  int // index of start instruction
        NumCap int // number of InstCapture insts in re
}

// An InstOp is an instruction opcode.
type InstOp uint8

const (
        InstAlt InstOp = iota
        InstAltMatch
        InstCapture
        InstEmptyWidth
        InstMatch
        InstFail
        InstNop
        InstRune
        InstRune1
        InstRuneAny
        InstRuneAnyNotNL
)

// An EmptyOp specifies a kind or mixture of zero-width assertions.
type EmptyOp uint8

const (
        EmptyBeginLine EmptyOp = 1 << iota
        EmptyEndLine
        EmptyBeginText
        EmptyEndText
        EmptyWordBoundary
        EmptyNoWordBoundary
)

// EmptyOpContext returns the zero-width assertions
// satisfied at the position between the runes r1 and r2.
// Passing r1 == -1 indicates that the position is
// at the beginning of the text.
// Passing r2 == -1 indicates that the position is
// at the end of the text.
func EmptyOpContext(r1, r2 rune) EmptyOp {
        var op EmptyOp
        if r1 < 0 {
                op |= EmptyBeginText | EmptyBeginLine
        }
        if r1 == '\n' {
                op |= EmptyBeginLine
        }
        if r2 < 0 {
                op |= EmptyEndText | EmptyEndLine
        }
        if r2 == '\n' {
                op |= EmptyEndLine
        }
        if IsWordChar(r1) != IsWordChar(r2) {
                op |= EmptyWordBoundary
        } else {
                op |= EmptyNoWordBoundary
        }
        return op
}

// IsWordChar reports whether r is consider a ``word character''
// during the evaluation of the \b and \B zero-width assertions.
// These assertions are ASCII-only: the word characters are [A-Za-z0-9_].
func IsWordChar(r rune) bool {
        return 'A' <= r && r <= 'Z' || 'a' <= r && r <= 'z' || '0' <= r && r <= '9' || r == '_'
}

// An Inst is a single instruction in a regular expression program.
type Inst struct {
        Op   InstOp
        Out  uint32 // all but InstMatch, InstFail
        Arg  uint32 // InstAlt, InstAltMatch, InstCapture, InstEmptyWidth
        Rune []rune
}

func (p *Prog) String() string {
        var b bytes.Buffer
        dumpProg(&b, p)
        return b.String()
}

// skipNop follows any no-op or capturing instructions
// and returns the resulting pc.
func (p *Prog) skipNop(pc uint32) *Inst {
        i := &p.Inst[pc]
        for i.Op == InstNop || i.Op == InstCapture {
                pc = i.Out
                i = &p.Inst[pc]
        }
        return i
}

// op returns i.Op but merges all the Rune special cases into InstRune
func (i *Inst) op() InstOp {
        op := i.Op
        switch op {
        case InstRune1, InstRuneAny, InstRuneAnyNotNL:
                op = InstRune
        }
        return op
}

// Prefix returns a literal string that all matches for the
// regexp must start with.  Complete is true if the prefix
// is the entire match.
func (p *Prog) Prefix() (prefix string, complete bool) {
        i := p.skipNop(uint32(p.Start))

        // Avoid allocation of buffer if prefix is empty.
        if i.op() != InstRune || len(i.Rune) != 1 {
                return "", i.Op == InstMatch
        }

        // Have prefix; gather characters.
        var buf bytes.Buffer
        for i.op() == InstRune && len(i.Rune) == 1 && Flags(i.Arg)&FoldCase == 0 {
                buf.WriteRune(i.Rune[0])
                i = p.skipNop(i.Out)
        }
        return buf.String(), i.Op == InstMatch
}

// StartCond returns the leading empty-width conditions that must
// be true in any match.  It returns ^EmptyOp(0) if no matches are possible.
func (p *Prog) StartCond() EmptyOp {
        var flag EmptyOp
        pc := uint32(p.Start)
        i := &p.Inst[pc]
Loop:
        for {
                switch i.Op {
                case InstEmptyWidth:
                        flag |= EmptyOp(i.Arg)
                case InstFail:
                        return ^EmptyOp(0)
                case InstCapture, InstNop:
                        // skip
                default:
                        break Loop
                }
                pc = i.Out
                i = &p.Inst[pc]
        }
        return flag
}

// MatchRune returns true if the instruction matches (and consumes) r.
// It should only be called when i.Op == InstRune.
func (i *Inst) MatchRune(r rune) bool {
        rune := i.Rune

        // Special case: single-rune slice is from literal string, not char class.
        if len(rune) == 1 {
                r0 := rune[0]
                if r == r0 {
                        return true
                }
                if Flags(i.Arg)&FoldCase != 0 {
                        for r1 := unicode.SimpleFold(r0); r1 != r0; r1 = unicode.SimpleFold(r1) {
                                if r == r1 {
                                        return true
                                }
                        }
                }
                return false
        }

        // Peek at the first few pairs.
        // Should handle ASCII well.
        for j := 0; j < len(rune) && j <= 8; j += 2 {
                if r < rune[j] {
                        return false
                }
                if r <= rune[j+1] {
                        return true
                }
        }

        // Otherwise binary search.
        lo := 0
        hi := len(rune) / 2
        for lo < hi {
                m := lo + (hi-lo)/2
                if c := rune[2*m]; c <= r {
                        if r <= rune[2*m+1] {
                                return true
                        }
                        lo = m + 1
                } else {
                        hi = m
                }
        }
        return false
}

// As per re2's Prog::IsWordChar. Determines whether rune is an ASCII word char.
// Since we act on runes, it would be easy to support Unicode here.
func wordRune(r rune) bool {
        return r == '_' ||
                ('A' <= r && r <= 'Z') ||
                ('a' <= r && r <= 'z') ||
                ('0' <= r && r <= '9')
}

// MatchEmptyWidth returns true if the instruction matches
// an empty string between the runes before and after.
// It should only be called when i.Op == InstEmptyWidth.
func (i *Inst) MatchEmptyWidth(before rune, after rune) bool {
        switch EmptyOp(i.Arg) {
        case EmptyBeginLine:
                return before == '\n' || before == -1
        case EmptyEndLine:
                return after == '\n' || after == -1
        case EmptyBeginText:
                return before == -1
        case EmptyEndText:
                return after == -1
        case EmptyWordBoundary:
                return wordRune(before) != wordRune(after)
        case EmptyNoWordBoundary:
                return wordRune(before) == wordRune(after)
        }
        panic("unknown empty width arg")
}

func (i *Inst) String() string {
        var b bytes.Buffer
        dumpInst(&b, i)
        return b.String()
}

func bw(b *bytes.Buffer, args ...string) {
        for _, s := range args {
                b.WriteString(s)
        }
}

func dumpProg(b *bytes.Buffer, p *Prog) {
        for j := range p.Inst {
                i := &p.Inst[j]
                pc := strconv.Itoa(j)
                if len(pc) < 3 {
                        b.WriteString("   "[len(pc):])
                }
                if j == p.Start {
                        pc += "*"
                }
                bw(b, pc, "\t")
                dumpInst(b, i)
                bw(b, "\n")
        }
}

func u32(i uint32) string {
        return strconv.FormatUint(uint64(i), 10)
}

func dumpInst(b *bytes.Buffer, i *Inst) {
        switch i.Op {
        case InstAlt:
                bw(b, "alt -> ", u32(i.Out), ", ", u32(i.Arg))
        case InstAltMatch:
                bw(b, "altmatch -> ", u32(i.Out), ", ", u32(i.Arg))
        case InstCapture:
                bw(b, "cap ", u32(i.Arg), " -> ", u32(i.Out))
        case InstEmptyWidth:
                bw(b, "empty ", u32(i.Arg), " -> ", u32(i.Out))
        case InstMatch:
                bw(b, "match")
        case InstFail:
                bw(b, "fail")
        case InstNop:
                bw(b, "nop -> ", u32(i.Out))
        case InstRune:
                if i.Rune == nil {
                        // shouldn't happen
                        bw(b, "rune <nil>")
                }
                bw(b, "rune ", strconv.QuoteToASCII(string(i.Rune)))
                if Flags(i.Arg)&FoldCase != 0 {
                        bw(b, "/i")
                }
                bw(b, " -> ", u32(i.Out))
        case InstRune1:
                bw(b, "rune1 ", strconv.QuoteToASCII(string(i.Rune)), " -> ", u32(i.Out))
        case InstRuneAny:
                bw(b, "any -> ", u32(i.Out))
        case InstRuneAnyNotNL:
                bw(b, "anynotnl -> ", u32(i.Out))
        }
}