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|
package driver
import (
"fmt"
"io"
mldriver "github.com/nihei9/maleeni/driver"
"github.com/nihei9/vartan/spec"
)
type SyntaxError struct {
Row int
Col int
Message string
Token *mldriver.Token
ExpectedTerminals []string
}
type ParserOption func(p *Parser) error
// DisableLAC disables LAC (lookahead correction). When the grammar has the LALR class, LAC is enabled by default.
func DisableLAC() ParserOption {
return func(p *Parser) error {
p.disableLAC = true
return nil
}
}
func SemanticAction(semAct SemanticActionSet) ParserOption {
return func(p *Parser) error {
p.semAct = semAct
return nil
}
}
type Parser struct {
gram *spec.CompiledGrammar
lex *mldriver.Lexer
stateStack *stateStack
semAct SemanticActionSet
disableLAC bool
onError bool
shiftCount int
synErrs []*SyntaxError
}
func NewParser(gram *spec.CompiledGrammar, src io.Reader, opts ...ParserOption) (*Parser, error) {
lex, err := mldriver.NewLexer(gram.LexicalSpecification.Maleeni.Spec, src)
if err != nil {
return nil, err
}
p := &Parser{
gram: gram,
lex: lex,
stateStack: &stateStack{},
}
if p.gram.ParsingTable.Class != "lalr" {
p.disableLAC = true
}
for _, opt := range opts {
err := opt(p)
if err != nil {
return nil, err
}
}
return p, nil
}
func (p *Parser) Parse() error {
p.stateStack.push(p.gram.ParsingTable.InitialState)
tok, err := p.nextToken()
if err != nil {
return err
}
ACTION_LOOP:
for {
act := p.lookupAction(tok)
switch {
case act < 0: // Shift
nextState := act * -1
recovered := false
if p.onError {
p.shiftCount++
// When the parser performs shift three times, the parser recovers from the error state.
if p.shiftCount >= 3 {
p.onError = false
p.shiftCount = 0
recovered = true
}
}
p.shift(nextState)
if p.semAct != nil {
p.semAct.Shift(tok, recovered)
}
tok, err = p.nextToken()
if err != nil {
return err
}
case act > 0: // Reduce
prodNum := act
recovered := false
if p.onError && p.gram.ParsingTable.RecoverProductions[prodNum] != 0 {
p.onError = false
p.shiftCount = 0
recovered = true
}
accepted := p.reduce(prodNum)
if accepted {
if p.semAct != nil {
p.semAct.Accept()
}
return nil
}
if p.semAct != nil {
p.semAct.Reduce(prodNum, recovered)
}
default: // Error
if p.onError {
tok, err = p.nextToken()
if err != nil {
return err
}
if tok.EOF {
return nil
}
continue ACTION_LOOP
}
p.synErrs = append(p.synErrs, &SyntaxError{
Row: tok.Row,
Col: tok.Col,
Message: "unexpected token",
Token: tok,
ExpectedTerminals: p.searchLookahead(p.stateStack.top()),
})
count, ok := p.trapError()
if !ok {
if p.semAct != nil {
p.semAct.MissError()
}
return nil
}
p.onError = true
p.shiftCount = 0
act, err := p.lookupActionOnError()
if err != nil {
return err
}
p.shift(act * -1)
if p.semAct != nil {
p.semAct.TrapAndShiftError(count)
}
}
}
}
// validateLookahead validates whether `term` is a valid lookahead in the current context. When `term` is valid,
// this method returns `true`.
func (p *Parser) validateLookahead(term int) bool {
p.stateStack.enableExploratoryMode()
defer p.stateStack.disableExploratoryMode()
tab := p.gram.ParsingTable
for {
act := tab.Action[p.stateStack.topExploratorily()*tab.TerminalCount+term]
switch {
case act < 0: // Shift
return true
case act > 0: // Reduce
prodNum := act
lhs := tab.LHSSymbols[prodNum]
if lhs == tab.LHSSymbols[tab.StartProduction] {
return true
}
n := tab.AlternativeSymbolCounts[prodNum]
p.stateStack.popExploratorily(n)
state := tab.GoTo[p.stateStack.topExploratorily()*tab.NonTerminalCount+lhs]
p.stateStack.pushExploratorily(state)
default: // Error
return false
}
}
}
func (p *Parser) nextToken() (*mldriver.Token, error) {
skip := p.gram.LexicalSpecification.Maleeni.Skip
for {
// We don't have to check whether the token is invalid because the kind ID of the invalid token is 0,
// and the parsing table doesn't have an entry corresponding to the kind ID 0. Thus we can detect
// a syntax error because the parser cannot find an entry corresponding to the invalid token.
tok, err := p.lex.Next()
if err != nil {
return nil, err
}
if skip[tok.KindID] > 0 {
continue
}
return tok, nil
}
}
func (p *Parser) tokenToTerminal(tok *mldriver.Token) int {
if tok.EOF {
return p.gram.ParsingTable.EOFSymbol
}
return p.gram.LexicalSpecification.Maleeni.KindToTerminal[tok.KindID]
}
func (p *Parser) lookupAction(tok *mldriver.Token) int {
if !p.disableLAC {
term := p.tokenToTerminal(tok)
if !p.validateLookahead(term) {
return 0
}
}
termCount := p.gram.ParsingTable.TerminalCount
term := p.tokenToTerminal(tok)
return p.gram.ParsingTable.Action[p.stateStack.top()*termCount+term]
}
func (p *Parser) lookupActionOnError() (int, error) {
termCount := p.gram.ParsingTable.TerminalCount
errSym := p.gram.ParsingTable.ErrorSymbol
act := p.gram.ParsingTable.Action[p.stateStack.top()*termCount+errSym]
if act >= 0 {
return 0, fmt.Errorf("an entry must be a shift action by the error symbol; entry: %v, state: %v, symbol: %v", act, p.stateStack.top(), p.gram.ParsingTable.Terminals[errSym])
}
return act, nil
}
func (p *Parser) shift(nextState int) {
p.stateStack.push(nextState)
}
func (p *Parser) reduce(prodNum int) bool {
tab := p.gram.ParsingTable
lhs := tab.LHSSymbols[prodNum]
if lhs == tab.LHSSymbols[tab.StartProduction] {
return true
}
n := tab.AlternativeSymbolCounts[prodNum]
p.stateStack.pop(n)
nextState := tab.GoTo[p.stateStack.top()*tab.NonTerminalCount+lhs]
p.stateStack.push(nextState)
return false
}
func (p *Parser) trapError() (int, bool) {
count := 0
for {
if p.gram.ParsingTable.ErrorTrapperStates[p.stateStack.top()] != 0 {
return count, true
}
if p.stateStack.top() != p.gram.ParsingTable.InitialState {
p.stateStack.pop(1)
count++
} else {
return 0, false
}
}
}
func (p *Parser) SyntaxErrors() []*SyntaxError {
return p.synErrs
}
func (p *Parser) searchLookahead(state int) []string {
kinds := []string{}
term2Kind := p.gram.LexicalSpecification.Maleeni.TerminalToKind
kindNames := p.gram.LexicalSpecification.Maleeni.Spec.KindNames
aliases := p.gram.LexicalSpecification.Maleeni.KindAliases
termCount := p.gram.ParsingTable.TerminalCount
base := p.stateStack.top() * termCount
for term := 0; term < termCount; term++ {
if p.disableLAC {
if p.gram.ParsingTable.Action[base+term] == 0 {
continue
}
} else {
if !p.validateLookahead(term) {
continue
}
}
// We don't add the error symbol to the look-ahead symbols because users cannot input the error symbol
// intentionally.
if term == p.gram.ParsingTable.ErrorSymbol {
continue
}
if term == p.gram.ParsingTable.EOFSymbol {
kinds = append(kinds, "<eof>")
continue
}
if alias := aliases[term]; alias != "" {
kinds = append(kinds, alias)
} else {
kinds = append(kinds, kindNames[term2Kind[term]].String())
}
}
return kinds
}
type stateStack struct {
items []int
itemsExp []int
}
func (s *stateStack) enableExploratoryMode() {
s.itemsExp = make([]int, len(s.items))
for i, v := range s.items {
s.itemsExp[i] = v
}
}
func (s *stateStack) disableExploratoryMode() {
s.itemsExp = nil
}
func (s *stateStack) top() int {
return s.items[len(s.items)-1]
}
func (s *stateStack) topExploratorily() int {
return s.itemsExp[len(s.itemsExp)-1]
}
func (s *stateStack) push(state int) {
s.items = append(s.items, state)
}
func (s *stateStack) pushExploratorily(state int) {
s.itemsExp = append(s.itemsExp, state)
}
func (s *stateStack) pop(n int) {
s.items = s.items[:len(s.items)-n]
}
func (s *stateStack) popExploratorily(n int) {
s.itemsExp = s.itemsExp[:len(s.itemsExp)-n]
}
|