private static IEnumerable <Token> ToPostfix(ExpressionLexer lexer)
{
// Shunting yard algorithm to transform the infix expression into postfix for easier interpretation.
var operatorStack = new Stack <Token>();
while (lexer.HasNext())
{
var current = lexer.Next();
switch (current.Terminal)
{
case Terminal.Decimal:
case Terminal.Hexadecimal:
case Terminal.Flag:
case Terminal.Register:
yield return(current);
break;
case Terminal.LPar:
operatorStack.Push(current);
break;
case Terminal.RPar:
while (operatorStack.Peek().Terminal != Terminal.LPar)
{
yield return(operatorStack.Pop());
}
operatorStack.Pop(); // Pop LPar
break;
default:
while (operatorStack.Count > 0)
{
var lastOperator = operatorStack.Peek();
int lastPrecedence = Array.IndexOf(OperatorPrecedence, lastOperator.Terminal);
int currentPrecedence = Array.IndexOf(OperatorPrecedence, current.Terminal);
if (lastPrecedence <= currentPrecedence)
{
yield return(operatorStack.Pop());
}
else
{
break;
}
}
operatorStack.Push(current);
break;
}
}
while (operatorStack.Count > 0)
{
yield return(operatorStack.Pop());
}
}
public static Predicate <GameBoyCpu> CompileExpression(string code)
{
var lexer = new ExpressionLexer(new StringReader(code));
var stack = new Stack <Expression>();
foreach (var token in ToPostfix(lexer))
{
switch (token.Terminal)
{
case Terminal.Register:
stack.Push(Expression.Convert(Expression.PropertyOrField(
Expression.Property(CpuParameter, "Registers"),
token.Text.ToUpperInvariant()), typeof(ushort)));
break;
case Terminal.Hexadecimal:
var matches = Regex.Matches(token.Text, @"[\da-fA-F]+");
stack.Push(Expression.Constant(ushort.Parse(matches[matches.Count - 1].Value,
NumberStyles.HexNumber)));
break;
case Terminal.Decimal:
stack.Push(Expression.Constant(ushort.Parse(token.Text)));
break;
case Terminal.Equals:
stack.Push(Expression.Equal(stack.Pop(), stack.Pop()));
break;
case Terminal.NotEquals:
stack.Push(Expression.NotEqual(stack.Pop(), stack.Pop()));
break;
case Terminal.GreaterThan:
// Mirrored operator to accomodate for stack order.
stack.Push(Expression.LessThan(stack.Pop(), stack.Pop()));
break;
case Terminal.GreaterThanOrEqual:
// Mirrored operator to accomodate for stack order.
stack.Push(Expression.LessThanOrEqual(stack.Pop(), stack.Pop()));
break;
case Terminal.LessThan:
// Mirrored operator to accomodate for stack order.
stack.Push(Expression.GreaterThan(stack.Pop(), stack.Pop()));
break;
case Terminal.LessThanOrEqual:
// Mirrored operator to accomodate for stack order.
stack.Push(Expression.GreaterThanOrEqual(stack.Pop(), stack.Pop()));
break;
case Terminal.BitwiseAnd:
stack.Push(Expression.And(stack.Pop(), stack.Pop()));
break;
case Terminal.BitwiseOr:
stack.Push(Expression.Or(stack.Pop(), stack.Pop()));
break;
case Terminal.BooleanAnd:
var v2 = stack.Pop();
var v1 = stack.Pop();
stack.Push(Expression.AndAlso(v1, v2));
break;
case Terminal.BooleanOr:
v2 = stack.Pop();
v1 = stack.Pop();
stack.Push(Expression.OrElse(v1, v2));
break;
}
}
if (stack.Count >= 2)
{
throw new SyntaxErrorException("Expression contains unused terms.");
}
var final = stack.Pop();
var lambda = Expression.Lambda <Predicate <GameBoyCpu> >(final, CpuParameter);
return(lambda.Compile());
}