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|
package grammar
import (
"crypto/sha256"
"encoding/binary"
"fmt"
"sort"
"strconv"
"urubu/grammar/symbol"
)
type lrItemID [32]byte
func (id lrItemID) String() string {
return fmt.Sprintf("%x", id.num())
}
func (id lrItemID) num() uint32 {
return binary.LittleEndian.Uint32(id[:])
}
type lookAhead struct {
symbols map[symbol.Symbol]struct{}
// When propagation is true, an item propagates look-ahead symbols to other items.
propagation bool
}
type lrItem struct {
id lrItemID
prod productionID
// E → E + T
//
// Dot | Dotted Symbol | Item
// ----+---------------+------------
// 0 | E | E →・E + T
// 1 | + | E → E・+ T
// 2 | T | E → E +・T
// 3 | Nil | E → E + T・
dot int
dottedSymbol symbol.Symbol
// When initial is true, the LHS of the production is the augmented start symbol and dot is 0.
// It looks like S' →・S.
initial bool
// When reducible is true, the item looks like E → E + T・.
reducible bool
// When kernel is true, the item is kernel item.
kernel bool
// lookAhead stores look-ahead symbols, and they are terminal symbols.
// The item is reducible only when the look-ahead symbols appear as the next input symbol.
lookAhead lookAhead
}
func newLR0Item(prod *production, dot int) (*lrItem, error) {
if prod == nil {
return nil, fmt.Errorf("production must be non-nil")
}
if dot < 0 || dot > prod.rhsLen {
return nil, fmt.Errorf("dot must be between 0 and %v", prod.rhsLen)
}
var id lrItemID
{
b := []byte{}
b = append(b, prod.id[:]...)
bDot := make([]byte, 8)
binary.LittleEndian.PutUint64(bDot, uint64(dot))
b = append(b, bDot...)
id = sha256.Sum256(b)
}
dottedSymbol := symbol.SymbolNil
if dot < prod.rhsLen {
dottedSymbol = prod.rhs[dot]
}
initial := false
if prod.lhs.IsStart() && dot == 0 {
initial = true
}
reducible := false
if dot == prod.rhsLen {
reducible = true
}
kernel := false
if initial || dot > 0 {
kernel = true
}
item := &lrItem{
id: id,
prod: prod.id,
dot: dot,
dottedSymbol: dottedSymbol,
initial: initial,
reducible: reducible,
kernel: kernel,
}
return item, nil
}
type kernelID [32]byte
func (id kernelID) String() string {
return fmt.Sprintf("%x", binary.LittleEndian.Uint32(id[:]))
}
type kernel struct {
id kernelID
items []*lrItem
}
func newKernel(items []*lrItem) (*kernel, error) {
if len(items) == 0 {
return nil, fmt.Errorf("a kernel need at least one item")
}
// Remove duplicates from items.
var sortedItems []*lrItem
{
m := map[lrItemID]*lrItem{}
for _, item := range items {
if !item.kernel {
return nil, fmt.Errorf("not a kernel item: %v", item)
}
m[item.id] = item
}
sortedItems = []*lrItem{}
for _, item := range m {
sortedItems = append(sortedItems, item)
}
sort.Slice(sortedItems, func(i, j int) bool {
return sortedItems[i].id.num() < sortedItems[j].id.num()
})
}
var id kernelID
{
b := []byte{}
for _, item := range sortedItems {
b = append(b, item.id[:]...)
}
id = sha256.Sum256(b)
}
return &kernel{
id: id,
items: sortedItems,
}, nil
}
type stateNum int
const stateNumInitial = stateNum(0)
func (n stateNum) Int() int {
return int(n)
}
func (n stateNum) String() string {
return strconv.Itoa(int(n))
}
func (n stateNum) next() stateNum {
return stateNum(n + 1)
}
type lrState struct {
*kernel
num stateNum
next map[symbol.Symbol]kernelID
reducible map[productionID]struct{}
// emptyProdItems stores items that have an empty production like `p → ε` and is reducible.
// Thus the items emptyProdItems stores are like `p → ・ε`. emptyProdItems is needed to store
// look-ahead symbols because the kernel items don't include these items.
//
// For instance, we have the following productions, and A is a terminal symbol.
//
// s' → s
// s → A | ε
//
// CLOSURE({s' → ・s}) generates the following closure, but the kernel of this closure doesn't
// include `s → ・ε`.
//
// s' → ・s
// s → ・A
// s → ・ε
emptyProdItems []*lrItem
// When isErrorTrapper is `true`, the item can shift the `error` symbol. The item has the following form.
// The `α` and `β` can be empty.
//
// A → α・error β
isErrorTrapper bool
}
|
