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|
package grammar
import (
"fmt"
"github.com/nihei9/vartan/grammar/symbol"
)
type firstEntry struct {
symbols map[symbol.Symbol]struct{}
empty bool
}
func newFirstEntry() *firstEntry {
return &firstEntry{
symbols: map[symbol.Symbol]struct{}{},
empty: false,
}
}
func (e *firstEntry) add(sym symbol.Symbol) bool {
if _, ok := e.symbols[sym]; ok {
return false
}
e.symbols[sym] = struct{}{}
return true
}
func (e *firstEntry) addEmpty() bool {
if !e.empty {
e.empty = true
return true
}
return false
}
func (e *firstEntry) mergeExceptEmpty(target *firstEntry) bool {
if target == nil {
return false
}
changed := false
for sym := range target.symbols {
added := e.add(sym)
if added {
changed = true
}
}
return changed
}
type firstSet struct {
set map[symbol.Symbol]*firstEntry
}
func newFirstSet(prods *productionSet) *firstSet {
fst := &firstSet{
set: map[symbol.Symbol]*firstEntry{},
}
for _, prod := range prods.getAllProductions() {
if _, ok := fst.set[prod.lhs]; ok {
continue
}
fst.set[prod.lhs] = newFirstEntry()
}
return fst
}
func (fst *firstSet) find(prod *production, head int) (*firstEntry, error) {
entry := newFirstEntry()
if prod.rhsLen <= head {
entry.addEmpty()
return entry, nil
}
for _, sym := range prod.rhs[head:] {
if sym.IsTerminal() {
entry.add(sym)
return entry, nil
}
e := fst.findBySymbol(sym)
if e == nil {
return nil, fmt.Errorf("an entry of FIRST was not found; symbol: %s", sym)
}
for s := range e.symbols {
entry.add(s)
}
if !e.empty {
return entry, nil
}
}
entry.addEmpty()
return entry, nil
}
func (fst *firstSet) findBySymbol(sym symbol.Symbol) *firstEntry {
return fst.set[sym]
}
type firstComContext struct {
first *firstSet
}
func newFirstComContext(prods *productionSet) *firstComContext {
return &firstComContext{
first: newFirstSet(prods),
}
}
func genFirstSet(prods *productionSet) (*firstSet, error) {
cc := newFirstComContext(prods)
for {
more := false
for _, prod := range prods.getAllProductions() {
e := cc.first.findBySymbol(prod.lhs)
changed, err := genProdFirstEntry(cc, e, prod)
if err != nil {
return nil, err
}
if changed {
more = true
}
}
if !more {
break
}
}
return cc.first, nil
}
func genProdFirstEntry(cc *firstComContext, acc *firstEntry, prod *production) (bool, error) {
if prod.isEmpty() {
return acc.addEmpty(), nil
}
for _, sym := range prod.rhs {
if sym.IsTerminal() {
return acc.add(sym), nil
}
e := cc.first.findBySymbol(sym)
changed := acc.mergeExceptEmpty(e)
if !e.empty {
return changed, nil
}
}
return acc.addEmpty(), nil
}
|
