[antlr-interest] [BUG] Java generator produces useless conditionals

[Marco Hunsicker]

Hello Dr. Parr,

I found a, well, maybe not really a bug, more of a cosmetic issue, but
anyway. 

The following lexer rule (taken from the Java grammar)

<pre>
WS      :       (       ' '
                |       '\t'
                |       '\f'
                        // handle newlines
                |       (       options {generateAmbigWarnings=false;}
                        :       "\r\n"  // Evil DOS
                        |       '\n'    // Unix (the right way)
                        |       '\r'    // Macintosh
                        )
                        { newline(); }
                )+
        ;

</pre>

produces (ANTLR 2.7.2) Java code such as (only the relevant part
shown) 

<pre>
    case '\n':
    case '\r': {
        if (
            (LA(1) == '\r') && (LA(2) == '\n')
                && (true) && (true)) {          // <== USELESS
            match("\r\n");
        } else if ((LA(1) == '\n')) {
            match('\n');
        } else if (
            (LA(1) == '\r') && (true) && (true) // <== USELESS
                && (true)) {                    // <== USELESS
            match('\r');
        } else {
            throw new NoViableAltForCharException(
                (char) LA(1), getFilename(),
                getLine(), getColumn());
        }

        newline();

        break;
</pre>

which contains several useless conditionals. It's a really minor
issue, but as it's easy to fix, I thought I post the code here.

The problem is the method getLookaheadTestExpression(Lookahead[], int)
in the Java CodeGenerator. I changed it to

<pre>
protected String getLookaheadTestExpression(Lookahead[] look, int k) {
    StringBuffer e = new StringBuffer(100);
    boolean first = true;

    e.append("(");

    for (int i = 1; i <= k; i++) {
        BitSet p = look[i].fset;

        // Syn preds can yield <end-of-syn-pred> (epsilon) lookahead.
        // There is no way to predict what that token would be.  Just
        // allow anything instead.
        if (look[i].containsEpsilon()) {
            if (first) {
                e.append(true);
            }
        }
        else {
            if (!first) {
                e.append(") && (");
            }

            e.append(getLookaheadTestTerm(i, p));
        }

        first = false;
    }

    e.append(")");

    return e.toString();
}
</pre>

to get rid of the obsolete tests.


And BTW, wow ;)

-- 
Best regards,
 Marco


 

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