/// <summary>
/// Translates infix expressions into postfix expressions
/// </summary>
/// <param name="query">Infix query that has been split into its tokens</param>
/// <param name="attributes"></param>
public SearchAlgorithm(ArrayList query, ArrayList attributes)
{
// define valid tokens
operators = new ArrayList();
operands = attributes;
// here we hardcode opernads and operators; this can be done in a more elegant manner.
string[] t1 = { "AND", "OR", "(", ")" }; // operators and parentheses
operators.AddRange(t1);
// Create and instantiate a new empty Stack.
Stack rpnStack = new Stack();
// apply dijkstra algorithm using a stack to convert infix to postfix notation (=rpn)
Infix.AddRange(query);
foreach (string infixToken in Infix)
{
// Replace underscores with space for things like "Fiero 2M4"
string token = infixToken;
if (token.Contains("_"))
{
token = token.Replace("_", " ");
}
if (operands.Contains(token))
{ // move operands across to output
Postfix.Add(token);
}
else if (token.Equals("("))
{ // push open parenthesis onto stack
rpnStack.Push(token);
}
else if (token.Equals(")"))
{ // pop all operators off the stack until the mathcing open parenthesis is found
while ((rpnStack.Count > 0) && !((string)rpnStack.Peek()).Equals("("))
{
Postfix.Add(rpnStack.Pop()); // transfer operator to output
if (rpnStack.Count == 0)
{
throw new Exception("Unbalanced parenthesis. Make sure you close any parenthesis and don't have any extras!");
}
}
if (rpnStack.Count == 0)
{
throw new Exception("Unbalanced parenthesis. Make sure you close any parenthesis and don't have any extras!");
}
rpnStack.Pop(); // discard open parenthesis
}
else if (operators.Contains(token))
{ // push operand to the rpn stack after moving to output all higher or equal priority operators
while (rpnStack.Count > 0 && ((string)rpnStack.Peek()).Equals("AND"))
{
Postfix.Add(rpnStack.Pop()); // pop and add to output
}
rpnStack.Push(token); // now pus the operator onto the stack
}
else
{
if (token.Length >= 8 && token.Substring(token.Length - Seat_Suffix_Length) == "-SEATER")
{
throw new Exception(string.Format("No matches found for a {0} vehicle! Try a different number of seats.", token));
}
throw new Exception(string.Format("Unrecognised token '{0}'. Ensure you only use 'AND', 'OR', '(' or ')', in addition to " +
"vehicle attributes only!", token));
}
}
// now copy what's left on the rpnStack
while (rpnStack.Count > 0)
{ // move to the output all remaining operators
if (((string)rpnStack.Peek()).Equals("("))
{
throw new Exception("Unbalanced parenthesis. Make sure you close any parenthesis and don't have any extras!");
}
Postfix.Add(rpnStack.Pop());
}
}