Merge pull request #152 from cmaglie/ll1-table-gen-fix

Bug fix in LL1 table generator

Author: DmitrySoshnikov

src/ll/__tests__/grammar1.bnf

@@ -0,0 +1,11 @@
+// https://github.com/DmitrySoshnikov/syntax/issues/151
+
+%%
+
+S
+  : A
+  ;
+A
+  : 'a'
+  | /* empty */
+  ;

src/ll/__tests__/ll-parsing-table-test.js

@@ -0,0 +1,40 @@
+/**
+ * The MIT License (MIT)
+ * Copyright (c) 2015-present Dmitry Soshnikov <dmitry.soshnikov@gmail.com>
+ */
+
+import Grammar from '../../grammar/grammar';
+import {MODES as GRAMMAR_MODE} from '../../grammar/grammar-mode';
+import LLParsingTable from '../ll-parsing-table';
+import LLParser from '../ll-parser';
+
+describe('ll-parsing-table', () => {
+  it('ll1-grammar-1', () => {
+    const grammarFile = __dirname + '/grammar1.bnf';
+    const expectedTable = {
+      S: {
+        "'a'": '1',
+        $: '1',
+      },
+      A: {
+        "'a'": '2',
+        $: '3',
+      },
+    };
+
+    const grammarBySLR = Grammar.fromGrammarFile(grammarFile, {
+      mode: GRAMMAR_MODE.LL1,
+    });
+    expect(new LLParsingTable({grammar: grammarBySLR}).get()).toEqual(
+      expectedTable
+    );
+    expect(new LLParser({grammar: grammarBySLR}).parse('a')).toEqual({
+      status: 'accept',
+      semanticValue: true,
+    });
+    expect(new LLParser({grammar: grammarBySLR}).parse('')).toEqual({
+      status: 'accept',
+      semanticValue: true,
+    });
+  });
+});

src/ll/ll-parsing-table.js

@@ -6,7 +6,7 @@
 import GrammarSymbol from '../grammar/grammar-symbol';
 import SetsGenerator from '../sets-generator';
 import TablePrinter from '../table-printer';
-import {EOF} from '../special-symbols';
+import {EOF, EPSILON} from '../special-symbols';
 import colors from 'colors';
 import debug from '../debug';
 
@@ -175,20 +175,17 @@ export default class LLParsingTable {
         table[lhsSymbol] = {};
       }
 
-      // All productions goes under the terminal column, if
-      // this terminal is not epsilon. Otherwise, an ε-production
-      // goes under the columns from the Follow set of LHS.
-
-      let set = !production.isEpsilon()
-        ? this._setsGenerator.firstOfRHS(rhs)
-        : this._setsGenerator.followOf(lhs);
-
-      const keys = Object.keys(set);
-
-      // Got derived epsilon through indirect production, fallback to
-      // Follow(LHS). Example: A: B; B: ε
-      if (keys.length === 1 && GrammarSymbol.isEpsilon(keys[0])) {
-        set = this._setsGenerator.followOf(lhs);
+      // All productions goes under the terminal column
+      // of the First(RHS) terminals.
+      let set = this._setsGenerator.firstOfRHS(rhs);
+
+      // If First(RHS) has an epsilon terminal it means
+      // that we must check also under the Follow(LHS) column.
+      // (because this production can be eliminated by epsilon).
+      if (set.hasOwnProperty(EPSILON)) {
+        // Compute (First(RHS) - {ε}) + Follow(LHS)
+        delete set[EPSILON];
+        set = Object.assign({}, set, this._setsGenerator.followOf(lhs));
       }
 
       for (let terminal in set) {