public static PropExpression Parse(string source)
{
var lexer = new PropLogicLexer(source);
lexer.NextToken();
PropExpression result = MatchExpression(lexer);
ExpectToken(lexer, LexerTokenType.End);
return(result);
}
private static PropExpression MatchUnary(PropLogicLexer lexer)
{
if (lexer.CurrentTokenType == LexerTokenType.Not)
{
lexer.NextToken();
PropExpression result = MatchAtom(lexer);
return(new PropNegation(result));
}
return(MatchAtom(lexer));
}
private static PropExpression MatchAndTerm(PropLogicLexer lexer)
{
PropExpression result = MatchUnary(lexer);
while (lexer.CurrentTokenType == LexerTokenType.And)
{
lexer.NextToken();
PropExpression right = MatchUnary(lexer);
result = new PropConjunction(result, right);
}
return(result);
}
private static PropExpression MatchAtom(PropLogicLexer lexer)
{
ExpectToken(lexer, LexerTokenType.Identifier, LexerTokenType.OpenParen);
PropExpression result;
if (lexer.CurrentTokenType == LexerTokenType.Identifier)
{
result = new PropIdentifier(lexer.CurrentTokenValue);
lexer.NextToken();
}
else
{
lexer.NextToken();
result = MatchExpression(lexer);
ExpectToken(lexer, LexerTokenType.CloseParen);
lexer.NextToken();
}
return(result);
}
private static void ExpectToken(PropLogicLexer lexer, params LexerTokenType[] tokens)
{
foreach (LexerTokenType token in tokens)
{
if (lexer.CurrentTokenType == token)
{
return;
}
}
var sb = new StringBuilder();
foreach (LexerTokenType token in tokens)
{
if (sb.Length > 0)
{
sb.Append(", ");
}
sb.Append(TokenTypeToString(token));
throw new ArgumentException(string.Format("Unexpected token in expression, got {0}, expecting one of {1}", TokenTypeToString(lexer.CurrentTokenType), sb));
}
}
// BNF(ish) for the language this parses
// Expression ::== OrTerm
// OrTerm ::== AndTerm ( '||' AndTerm)*
// AndTerm ::== Unary ( '&&' Unary)*
// Unary ::= '!' Atom | Atom
// Atom ::= IDENTIFIER | '(' Expression ')'
// IDENTIFIER ::= [A-Za-z_][A-Za-z0-9_]*
private static PropExpression MatchExpression(PropLogicLexer lexer)
{
PropExpression result = MatchOrTerm(lexer);
return(result);
}
