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|
package grammar
import (
"fmt"
)
type followEntry struct {
symbols map[symbol]struct{}
eof bool
}
func newFollowEntry() *followEntry {
return &followEntry{
symbols: map[symbol]struct{}{},
eof: false,
}
}
func (e *followEntry) add(sym symbol) bool {
if _, ok := e.symbols[sym]; ok {
return false
}
e.symbols[sym] = struct{}{}
return true
}
func (e *followEntry) addEOF() bool {
if !e.eof {
e.eof = true
return true
}
return false
}
func (e *followEntry) merge(fst *firstEntry, flw *followEntry) bool {
changed := false
if fst != nil {
for sym := range fst.symbols {
added := e.add(sym)
if added {
changed = true
}
}
}
if flw != nil {
for sym := range flw.symbols {
added := e.add(sym)
if added {
changed = true
}
}
if flw.eof {
added := e.addEOF()
if added {
changed = true
}
}
}
return changed
}
type followSet struct {
set map[symbol]*followEntry
}
func newFollow(prods *productionSet) *followSet {
flw := &followSet{
set: map[symbol]*followEntry{},
}
for _, prod := range prods.getAllProductions() {
if _, ok := flw.set[prod.lhs]; ok {
continue
}
flw.set[prod.lhs] = newFollowEntry()
}
return flw
}
func (flw *followSet) find(sym symbol) (*followEntry, error) {
e, ok := flw.set[sym]
if !ok {
return nil, fmt.Errorf("an entry of FOLLOW was not found; symbol: %s", sym)
}
return e, nil
}
func genFollowSet(prods *productionSet, first *firstSet) (*followSet, error) {
ntsyms := map[symbol]struct{}{}
for _, prod := range prods.getAllProductions() {
if _, ok := ntsyms[prod.lhs]; ok {
continue
}
ntsyms[prod.lhs] = struct{}{}
}
follow := newFollow(prods)
for {
more := false
for ntsym := range ntsyms {
e, err := follow.find(ntsym)
if err != nil {
return nil, err
}
if ntsym.isStart() {
changed := e.addEOF()
if changed {
more = true
}
}
for _, prod := range prods.getAllProductions() {
for i, sym := range prod.rhs {
if sym != ntsym {
continue
}
fst, err := first.find(prod, i+1)
if err != nil {
return nil, err
}
changed := e.merge(fst, nil)
if changed {
more = true
}
if fst.empty {
flw, err := follow.find(prod.lhs)
if err != nil {
return nil, err
}
changed := e.merge(nil, flw)
if changed {
more = true
}
}
}
}
}
if !more {
break
}
}
return follow, nil
}
|
