/// <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].PatternType == TriplePatternType.Match || gp.TriplePatterns[p].PatternType == TriplePatternType.BindAssignment || gp.TriplePatterns[p].PatternType == TriplePatternType.LetAssignment)
{
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);
}
