/// <summary>
/// Tries to place filters at the earliest point possible i.e. the first point after which all required variables have occurred
/// </summary>
/// <param name="gp">Graph Pattern</param>
/// <param name="filter">Filter to place</param>
/// <returns></returns>
private bool TryPlaceFilter(GraphPattern gp, ISparqlFilter filter)
{
//Firstly we need to find out what variables are needed in the Filter
List<String> variablesNeeded = filter.Variables.Distinct().ToList();
//Then we need to move through the Triple Patterns and find the first place at which all the
//Variables used in the Filter have been used in ordinary Triple Patterns
List<String> variablesUsed = new List<string>();
for (int p = 0; p < gp.TriplePatterns.Count; p++)
{
if (gp.TriplePatterns[p] is TriplePattern || gp.TriplePatterns[p] is IAssignmentPattern)
{
foreach (String var in gp.TriplePatterns[p].Variables)
{
if (!variablesUsed.Contains(var)) variablesUsed.Add(var);
}
//Have all the Variables we need now been used in a Pattern?
if (variablesNeeded.All(v => variablesUsed.Contains(v)))
{
//We can place this Filter after the Pattern we were just looking at
gp.InsertFilter(filter, p + 1);
return true;
}
}
}
//If we reach here then this means that all the Variables used in the Filter did not occur
//in the Triple Patterns which means they likely occur in child graph patterns (or the query
//is malformed). In this case we cannot place the Filter and it has to be applied post-commit
//rather than during Triple Pattern execution
return false;
}