/// <summary>
/// Parses an addition expression.
/// If the top-level expression is not an addition,
/// the proper type of expression will be parsed and returned.
/// </summary>
/// <param name="tokens">Input list of tokens.</param>
/// <param name="start">Index of first token of the expression.</param>
/// <param name="length">Output variable for expression length (number of tokens used).</param>
/// <returns>Returns the parsed expression.</returns>
private static Expression ParseAddition(List <Token> tokens, int start, out int length)
{
// Create list for expressions belonging to this addition.
List <Expression> parts = new List <Expression>()
{
// Read the first part / sub-expression of the addition.
ParseMultiplication(tokens, start, out int firstLen)
};
// Index of last processed token's end.
int end = start + firstLen;
// Keep going while sub-expressions are being added (or subtracted)
// and while there are more tokens to process.
while (end < tokens.Count && tokens[end] is SpecialToken special && (special.Addition || special.Subtraction))
{
// Parse next part / sub-expression of the addition.
Expression nextPart = ParseMultiplication(tokens, end + 1, out int nextLen);
// Convert subtraction into addition of negated expression.
parts.Add(special.Subtraction ? NegatedExpression.Build(nextPart) : nextPart);
// Add the length of the operator and the sub-expression to the end token index.
end += 1 + nextLen;
}
// Calculate the total length of the addition.
length = end - start;
return(AdditionExpression.Build(parts));
}
public static Expression Build(Expression expr)
{
if (expr is NegatedExpression negated)
{
return(negated.Expression);
}
else if (expr is ConstantExpression constant)
{
return(-constant.Value);
}
else if (expr is AdditionExpression addition)
{
return(AdditionExpression.Build(-addition.Constant, addition.VariableParts.Select(x => NegatedExpression.Build(x))));
}
else if (expr is MultiplicationExpression multiplication)
{
return(MultiplicationExpression.Build(-multiplication.Coefficient, multiplication.VariableParts));
}
else
{
return(new NegatedExpression(expr));
}
}
/// <summary> /// Unary subtraction (negation) operation of an expression. /// </summary> /// <param name="expr">Expression to be negated.</param> /// <returns>Returns a negated expression of the input expression.</returns> public static Expression operator -(Expression expr) => NegatedExpression.Build(expr);