// Copyright 2009 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.

// The testing package implements a simple regular expression library.
// It is a reduced version of the regular expression package suitable
// for use in tests; it avoids many dependencies.
//
// The syntax of the regular expressions accepted is:
//
//	regexp:
//		concatenation { '|' concatenation }
//	concatenation:
//		{ closure }
//	closure:
//		term [ '*' | '+' | '?' ]
//	term:
//		'^'
//		'$'
//		'.'
//		character
//		'[' [ '^' ] character-ranges ']'
//		'(' regexp ')'
//

package testing

import (
	"utf8"
)

var debug = false

// Error codes returned by failures to parse an expression.
var (
	ErrInternal            = "internal error"
	ErrUnmatchedLpar       = "unmatched '('"
	ErrUnmatchedRpar       = "unmatched ')'"
	ErrUnmatchedLbkt       = "unmatched '['"
	ErrUnmatchedRbkt       = "unmatched ']'"
	ErrBadRange            = "bad range in character class"
	ErrExtraneousBackslash = "extraneous backslash"
	ErrBadClosure          = "repeated closure (**, ++, etc.)"
	ErrBareClosure         = "closure applies to nothing"
	ErrBadBackslash        = "illegal backslash escape"
)

// An instruction executed by the NFA
type instr interface {
	kind() int   // the type of this instruction: _CHAR, _ANY, etc.
	next() instr // the instruction to execute after this one
	setNext(i instr)
	index() int
	setIndex(i int)
	print()
}

// Fields and methods common to all instructions
type common struct {
	_next  instr
	_index int
}

func (c *common) next() instr     { return c._next }
func (c *common) setNext(i instr) { c._next = i }
func (c *common) index() int      { return c._index }
func (c *common) setIndex(i int)  { c._index = i }

// The representation of a compiled regular expression.
// The public interface is entirely through methods.
type Regexp struct {
	expr  string // the original expression
	inst  []instr
	start instr
	nbra  int // number of brackets in expression, for subexpressions
}

const (
	_START     = iota // beginning of program
	_END              // end of program: success
	_BOT              // '^' beginning of text
	_EOT              // '$' end of text
	_CHAR             // 'a' regular character
	_CHARCLASS        // [a-z] character class
	_ANY              // '.' any character including newline
	_NOTNL            // [^\n] special case: any character but newline
	_BRA              // '(' parenthesized expression
	_EBRA             // ')'; end of '(' parenthesized expression
	_ALT              // '|' alternation
	_NOP              // do nothing; makes it easy to link without patching
)

// --- START start of program
type _Start struct {
	common
}

func (start *_Start) kind() int { return _START }
func (start *_Start) print()    { print("start") }

// --- END end of program
type _End struct {
	common
}

func (end *_End) kind() int { return _END }
func (end *_End) print()    { print("end") }

// --- BOT beginning of text
type _Bot struct {
	common
}

func (bot *_Bot) kind() int { return _BOT }
func (bot *_Bot) print()    { print("bot") }

// --- EOT end of text
type _Eot struct {
	common
}

func (eot *_Eot) kind() int { return _EOT }
func (eot *_Eot) print()    { print("eot") }

// --- CHAR a regular character
type _Char struct {
	common
	char int
}

func (char *_Char) kind() int { return _CHAR }
func (char *_Char) print()    { print("char ", string(char.char)) }

func newChar(char int) *_Char {
	c := new(_Char)
	c.char = char
	return c
}

// --- CHARCLASS [a-z]

type _CharClass struct {
	common
	char   int
	negate bool // is character class negated? ([^a-z])
	// stored pairwise: [a-z] is (a,z); x is (x,x):
	ranges []int
}

func (cclass *_CharClass) kind() int { return _CHARCLASS }

func (cclass *_CharClass) print() {
	print("charclass")
	if cclass.negate {
		print(" (negated)")
	}
	for i := 0; i < len(cclass.ranges); i += 2 {
		l := cclass.ranges[i]
		r := cclass.ranges[i+1]
		if l == r {
			print(" [", string(l), "]")
		} else {
			print(" [", string(l), "-", string(r), "]")
		}
	}
}

func (cclass *_CharClass) addRange(a, b int) {
	// range is a through b inclusive
	n := len(cclass.ranges)
	if n >= cap(cclass.ranges) {
		nr := make([]int, n, 2*n)
		for i, j := range nr {
			nr[i] = j
		}
		cclass.ranges = nr
	}
	cclass.ranges = cclass.ranges[0 : n+2]
	cclass.ranges[n] = a
	n++
	cclass.ranges[n] = b
	n++
}

func (cclass *_CharClass) matches(c int) bool {
	for i := 0; i < len(cclass.ranges); i = i + 2 {
		min := cclass.ranges[i]
		max := cclass.ranges[i+1]
		if min <= c && c <= max {
			return !cclass.negate
		}
	}
	return cclass.negate
}

func newCharClass() *_CharClass {
	c := new(_CharClass)
	c.ranges = make([]int, 0, 20)
	return c
}

// --- ANY any character
type _Any struct {
	common
}

func (any *_Any) kind() int { return _ANY }
func (any *_Any) print()    { print("any") }

// --- NOTNL any character but newline
type _NotNl struct {
	common
}

func (notnl *_NotNl) kind() int { return _NOTNL }
func (notnl *_NotNl) print()    { print("notnl") }

// --- BRA parenthesized expression
type _Bra struct {
	common
	n int // subexpression number
}

func (bra *_Bra) kind() int { return _BRA }
func (bra *_Bra) print()    { print("bra", bra.n) }

// --- EBRA end of parenthesized expression
type _Ebra struct {
	common
	n int // subexpression number
}

func (ebra *_Ebra) kind() int { return _EBRA }
func (ebra *_Ebra) print()    { print("ebra ", ebra.n) }

// --- ALT alternation
type _Alt struct {
	common
	left instr // other branch
}

func (alt *_Alt) kind() int { return _ALT }
func (alt *_Alt) print()    { print("alt(", alt.left.index(), ")") }

// --- NOP no operation
type _Nop struct {
	common
}

func (nop *_Nop) kind() int { return _NOP }
func (nop *_Nop) print()    { print("nop") }

func (re *Regexp) add(i instr) instr {
	n := len(re.inst)
	i.setIndex(len(re.inst))
	if n >= cap(re.inst) {
		ni := make([]instr, n, 2*n)
		for i, j := range re.inst {
			ni[i] = j
		}
		re.inst = ni
	}
	re.inst = re.inst[0 : n+1]
	re.inst[n] = i
	return i
}

type parser struct {
	re    *Regexp
	nlpar int // number of unclosed lpars
	pos   int
	ch    int
}

func (p *parser) error(err string) {
	panic(err)
}

const endOfFile = -1

func (p *parser) c() int { return p.ch }

func (p *parser) nextc() int {
	if p.pos >= len(p.re.expr) {
		p.ch = endOfFile
	} else {
		c, w := utf8.DecodeRuneInString(p.re.expr[p.pos:])
		p.ch = c
		p.pos += w
	}
	return p.ch
}

func newParser(re *Regexp) *parser {
	p := new(parser)
	p.re = re
	p.nextc() // load p.ch
	return p
}

func special(c int) bool {
	s := `\.+*?()|[]^$`
	for i := 0; i < len(s); i++ {
		if c == int(s[i]) {
			return true
		}
	}
	return false
}

func specialcclass(c int) bool {
	s := `\-[]`
	for i := 0; i < len(s); i++ {
		if c == int(s[i]) {
			return true
		}
	}
	return false
}

func (p *parser) charClass() instr {
	cc := newCharClass()
	if p.c() == '^' {
		cc.negate = true
		p.nextc()
	}
	left := -1
	for {
		switch c := p.c(); c {
		case ']', endOfFile:
			if left >= 0 {
				p.error(ErrBadRange)
			}
			// Is it [^\n]?
			if cc.negate && len(cc.ranges) == 2 &&
				cc.ranges[0] == '\n' && cc.ranges[1] == '\n' {
				nl := new(_NotNl)
				p.re.add(nl)
				return nl
			}
			p.re.add(cc)
			return cc
		case '-': // do this before backslash processing
			p.error(ErrBadRange)
		case '\\':
			c = p.nextc()
			switch {
			case c == endOfFile:
				p.error(ErrExtraneousBackslash)
			case c == 'n':
				c = '\n'
			case specialcclass(c):
			// c is as delivered
			default:
				p.error(ErrBadBackslash)
			}
			fallthrough
		default:
			p.nextc()
			switch {
			case left < 0: // first of pair
				if p.c() == '-' { // range
					p.nextc()
					left = c
				} else { // single char
					cc.addRange(c, c)
				}
			case left <= c: // second of pair
				cc.addRange(left, c)
				left = -1
			default:
				p.error(ErrBadRange)
			}
		}
	}
	return nil
}

func (p *parser) term() (start, end instr) {
	switch c := p.c(); c {
	case '|', endOfFile:
		return nil, nil
	case '*', '+':
		p.error(ErrBareClosure)
		return
	case ')':
		if p.nlpar == 0 {
			p.error(ErrUnmatchedRpar)
		}
		return nil, nil
	case ']':
		p.error(ErrUnmatchedRbkt)
	case '^':
		p.nextc()
		start = p.re.add(new(_Bot))
		return start, start
	case '$':
		p.nextc()
		start = p.re.add(new(_Eot))
		return start, start
	case '.':
		p.nextc()
		start = p.re.add(new(_Any))
		return start, start
	case '[':
		p.nextc()
		start = p.charClass()
		if p.c() != ']' {
			p.error(ErrUnmatchedLbkt)
		}
		p.nextc()
		return start, start
	case '(':
		p.nextc()
		p.nlpar++
		p.re.nbra++ // increment first so first subexpr is \1
		nbra := p.re.nbra
		start, end = p.regexp()
		if p.c() != ')' {
			p.error(ErrUnmatchedLpar)
		}
		p.nlpar--
		p.nextc()
		bra := new(_Bra)
		p.re.add(bra)
		ebra := new(_Ebra)
		p.re.add(ebra)
		bra.n = nbra
		ebra.n = nbra
		if start == nil {
			if end == nil {
				p.error(ErrInternal)
			}
			start = ebra
		} else {
			end.setNext(ebra)
		}
		bra.setNext(start)
		return bra, ebra
	case '\\':
		c = p.nextc()
		switch {
		case c == endOfFile:
			p.error(ErrExtraneousBackslash)
			return
		case c == 'n':
			c = '\n'
		case special(c):
		// c is as delivered
		default:
			p.error(ErrBadBackslash)
		}
		fallthrough
	default:
		p.nextc()
		start = newChar(c)
		p.re.add(start)
		return start, start
	}
	panic("unreachable")
}

func (p *parser) closure() (start, end instr) {
	start, end = p.term()
	if start == nil {
		return
	}
	switch p.c() {
	case '*':
		// (start,end)*:
		alt := new(_Alt)
		p.re.add(alt)
		end.setNext(alt) // after end, do alt
		alt.left = start // alternate brach: return to start
		start = alt      // alt becomes new (start, end)
		end = alt
	case '+':
		// (start,end)+:
		alt := new(_Alt)
		p.re.add(alt)
		end.setNext(alt) // after end, do alt
		alt.left = start // alternate brach: return to start
		end = alt        // start is unchanged; end is alt
	case '?':
		// (start,end)?:
		alt := new(_Alt)
		p.re.add(alt)
		nop := new(_Nop)
		p.re.add(nop)
		alt.left = start // alternate branch is start
		alt.setNext(nop) // follow on to nop
		end.setNext(nop) // after end, go to nop
		start = alt      // start is now alt
		end = nop        // end is nop pointed to by both branches
	default:
		return
	}
	switch p.nextc() {
	case '*', '+', '?':
		p.error(ErrBadClosure)
	}
	return
}

func (p *parser) concatenation() (start, end instr) {
	for {
		nstart, nend := p.closure()
		switch {
		case nstart == nil: // end of this concatenation
			if start == nil { // this is the empty string
				nop := p.re.add(new(_Nop))
				return nop, nop
			}
			return
		case start == nil: // this is first element of concatenation
			start, end = nstart, nend
		default:
			end.setNext(nstart)
			end = nend
		}
	}
	panic("unreachable")
}

func (p *parser) regexp() (start, end instr) {
	start, end = p.concatenation()
	for {
		switch p.c() {
		default:
			return
		case '|':
			p.nextc()
			nstart, nend := p.concatenation()
			alt := new(_Alt)
			p.re.add(alt)
			alt.left = start
			alt.setNext(nstart)
			nop := new(_Nop)
			p.re.add(nop)
			end.setNext(nop)
			nend.setNext(nop)
			start, end = alt, nop
		}
	}
	panic("unreachable")
}

func unNop(i instr) instr {
	for i.kind() == _NOP {
		i = i.next()
	}
	return i
}

func (re *Regexp) eliminateNops() {
	for i := 0; i < len(re.inst); i++ {
		inst := re.inst[i]
		if inst.kind() == _END {
			continue
		}
		inst.setNext(unNop(inst.next()))
		if inst.kind() == _ALT {
			alt := inst.(*_Alt)
			alt.left = unNop(alt.left)
		}
	}
}

func (re *Regexp) doParse() {
	p := newParser(re)
	start := new(_Start)
	re.add(start)
	s, e := p.regexp()
	start.setNext(s)
	re.start = start
	e.setNext(re.add(new(_End)))
	re.eliminateNops()
}

// CompileRegexp parses a regular expression and returns, if successful, a Regexp
// object that can be used to match against text.
func CompileRegexp(str string) (regexp *Regexp, error string) {
	regexp = new(Regexp)
	// doParse will panic if there is a parse error.
	defer func() {
		if e := recover(); e != nil {
			regexp = nil
			error = e.(string) // Will re-panic if error was not a string, e.g. nil-pointer exception
		}
	}()
	regexp.expr = str
	regexp.inst = make([]instr, 0, 20)
	regexp.doParse()
	return
}

// MustCompileRegexp is like CompileRegexp but panics if the expression cannot be parsed.
// It simplifies safe initialization of global variables holding compiled regular
// expressions.
func MustCompile(str string) *Regexp {
	regexp, error := CompileRegexp(str)
	if error != "" {
		panic(`regexp: compiling "` + str + `": ` + error)
	}
	return regexp
}

type state struct {
	inst  instr // next instruction to execute
	match []int // pairs of bracketing submatches. 0th is start,end
}

// Append new state to to-do list.  Leftmost-longest wins so avoid
// adding a state that's already active.
func addState(s []state, inst instr, match []int) []state {
	index := inst.index()
	l := len(s)
	pos := match[0]
	// TODO: Once the state is a vector and we can do insert, have inputs always
	// go in order correctly and this "earlier" test is never necessary,
	for i := 0; i < l; i++ {
		if s[i].inst.index() == index && // same instruction
			s[i].match[0] < pos { // earlier match already going; lefmost wins
			return s
		}
	}
	if l == cap(s) {
		s1 := make([]state, 2*l)[0:l]
		for i := 0; i < l; i++ {
			s1[i] = s[i]
		}
		s = s1
	}
	s = s[0 : l+1]
	s[l].inst = inst
	s[l].match = match
	return s
}

// Accepts either string or bytes - the logic is identical either way.
// If bytes == nil, scan str.
func (re *Regexp) doExecute(str string, bytes []byte, pos int) []int {
	var s [2][]state // TODO: use a vector when state values (not ptrs) can be vector elements
	s[0] = make([]state, 10)[0:0]
	s[1] = make([]state, 10)[0:0]
	in, out := 0, 1
	var final state
	found := false
	end := len(str)
	if bytes != nil {
		end = len(bytes)
	}
	for pos <= end {
		if !found {
			// prime the pump if we haven't seen a match yet
			match := make([]int, 2*(re.nbra+1))
			for i := 0; i < len(match); i++ {
				match[i] = -1 // no match seen; catches cases like "a(b)?c" on "ac"
			}
			match[0] = pos
			s[out] = addState(s[out], re.start.next(), match)
		}
		in, out = out, in    // old out state is new in state
		s[out] = s[out][0:0] // clear out state
		if len(s[in]) == 0 {
			// machine has completed
			break
		}
		charwidth := 1
		c := endOfFile
		if pos < end {
			if bytes == nil {
				c, charwidth = utf8.DecodeRuneInString(str[pos:end])
			} else {
				c, charwidth = utf8.DecodeRune(bytes[pos:end])
			}
		}
		for i := 0; i < len(s[in]); i++ {
			st := s[in][i]
			switch s[in][i].inst.kind() {
			case _BOT:
				if pos == 0 {
					s[in] = addState(s[in], st.inst.next(), st.match)
				}
			case _EOT:
				if pos == end {
					s[in] = addState(s[in], st.inst.next(), st.match)
				}
			case _CHAR:
				if c == st.inst.(*_Char).char {
					s[out] = addState(s[out], st.inst.next(), st.match)
				}
			case _CHARCLASS:
				if st.inst.(*_CharClass).matches(c) {
					s[out] = addState(s[out], st.inst.next(), st.match)
				}
			case _ANY:
				if c != endOfFile {
					s[out] = addState(s[out], st.inst.next(), st.match)
				}
			case _NOTNL:
				if c != endOfFile && c != '\n' {
					s[out] = addState(s[out], st.inst.next(), st.match)
				}
			case _BRA:
				n := st.inst.(*_Bra).n
				st.match[2*n] = pos
				s[in] = addState(s[in], st.inst.next(), st.match)
			case _EBRA:
				n := st.inst.(*_Ebra).n
				st.match[2*n+1] = pos
				s[in] = addState(s[in], st.inst.next(), st.match)
			case _ALT:
				s[in] = addState(s[in], st.inst.(*_Alt).left, st.match)
				// give other branch a copy of this match vector
				s1 := make([]int, 2*(re.nbra+1))
				for i := 0; i < len(s1); i++ {
					s1[i] = st.match[i]
				}
				s[in] = addState(s[in], st.inst.next(), s1)
			case _END:
				// choose leftmost longest
				if !found || // first
					st.match[0] < final.match[0] || // leftmost
					(st.match[0] == final.match[0] && pos > final.match[1]) { // longest
					final = st
					final.match[1] = pos
				}
				found = true
			default:
				st.inst.print()
				panic("unknown instruction in execute")
			}
		}
		pos += charwidth
	}
	return final.match
}


// ExecuteString matches the Regexp against the string s.
// The return value is an array of integers, in pairs, identifying the positions of
// substrings matched by the expression.
//    s[a[0]:a[1]] is the substring matched by the entire expression.
//    s[a[2*i]:a[2*i+1]] for i > 0 is the substring matched by the ith parenthesized subexpression.
// A negative value means the subexpression did not match any element of the string.
// An empty array means "no match".
func (re *Regexp) ExecuteString(s string) (a []int) {
	return re.doExecute(s, nil, 0)
}


// Execute matches the Regexp against the byte slice b.
// The return value is an array of integers, in pairs, identifying the positions of
// subslices matched by the expression.
//    b[a[0]:a[1]] is the subslice matched by the entire expression.
//    b[a[2*i]:a[2*i+1]] for i > 0 is the subslice matched by the ith parenthesized subexpression.
// A negative value means the subexpression did not match any element of the slice.
// An empty array means "no match".
func (re *Regexp) Execute(b []byte) (a []int) { return re.doExecute("", b, 0) }


// MatchString returns whether the Regexp matches the string s.
// The return value is a boolean: true for match, false for no match.
func (re *Regexp) MatchString(s string) bool { return len(re.doExecute(s, nil, 0)) > 0 }


// Match returns whether the Regexp matches the byte slice b.
// The return value is a boolean: true for match, false for no match.
func (re *Regexp) Match(b []byte) bool { return len(re.doExecute("", b, 0)) > 0 }


// MatchStrings matches the Regexp against the string s.
// The return value is an array of strings matched by the expression.
//    a[0] is the substring matched by the entire expression.
//    a[i] for i > 0 is the substring matched by the ith parenthesized subexpression.
// An empty array means ``no match''.
func (re *Regexp) MatchStrings(s string) (a []string) {
	r := re.doExecute(s, nil, 0)
	if r == nil {
		return nil
	}
	a = make([]string, len(r)/2)
	for i := 0; i < len(r); i += 2 {
		if r[i] != -1 { // -1 means no match for this subexpression
			a[i/2] = s[r[i]:r[i+1]]
		}
	}
	return
}

// MatchSlices matches the Regexp against the byte slice b.
// The return value is an array of subslices matched by the expression.
//    a[0] is the subslice matched by the entire expression.
//    a[i] for i > 0 is the subslice matched by the ith parenthesized subexpression.
// An empty array means ``no match''.
func (re *Regexp) MatchSlices(b []byte) (a [][]byte) {
	r := re.doExecute("", b, 0)
	if r == nil {
		return nil
	}
	a = make([][]byte, len(r)/2)
	for i := 0; i < len(r); i += 2 {
		if r[i] != -1 { // -1 means no match for this subexpression
			a[i/2] = b[r[i]:r[i+1]]
		}
	}
	return
}

// MatchString checks whether a textual regular expression
// matches a string.  More complicated queries need
// to use Compile and the full Regexp interface.
func MatchString(pattern string, s string) (matched bool, error string) {
	re, err := CompileRegexp(pattern)
	if err != "" {
		return false, err
	}
	return re.MatchString(s), ""
}

// Match checks whether a textual regular expression
// matches a byte slice.  More complicated queries need
// to use Compile and the full Regexp interface.
func Match(pattern string, b []byte) (matched bool, error string) {
	re, err := CompileRegexp(pattern)
	if err != "" {
		return false, err
	}
	return re.Match(b), ""
}