private static Expression ParseHelper(string[] words, int start, int end, Dictionary<string, Operator> operators)
{
Expression value1 = null;
Expression value2 = null;
Operator op = null;
for (int i = start; i <= end; ++i) {
string word = words[i];
// Console.WriteLine ("process " + word);
// ignore parens for now
if (word == "(" || word == ")") continue;
int n = 0;
bool isNumeric = int.TryParse (word, out n);
if (isNumeric) {
if (value1 == null)
value1 = new Value (n);
else if (value2 == null)
value2 = new Value (n);
else
return null;
} else if (op == null) {
if (operators.ContainsKey (word)) {
op = operators [word];
} else
return null;
} else if (value1 != null && value2 != null && operators.ContainsKey(word)) {
Operator newOp = operators [word];
if (newOp.Priority >= op.Priority) {
value1 = new Formula (value1, value2, op);
op = operators [word];
value2 = null;
} else {
value2 = ParseHelper (words, i - 1, end, operators);
break;
}
} else {
return null;
}
}
if (value1 == null || value2 == null || op == null) {
return null;
}
return new Formula (value1, value2, op);
}