|
static private BuildLookupTable ( IEnumerable |
||
| productions | IEnumerable |
|
| return | ICollection | |
private IEnumerable <Production> RemoveUnits(IEnumerable <Production> productions)
{
var productionTable = new Dictionary <Production, Production>(new ProductionComparer());
foreach (var production in productions)
{
productionTable[production] = production;
}
var oldSumOfProbability = double.MaxValue;
// we keep looping until the probability of any unit production has been driven to zero
// as an invariant, we make sure that the sum of the unit probabilities goes down each iteration
while (oldSumOfProbability > 0)
{
// TODO: don't need to build this table every round
var productionsByNonterminal = GrammarHelpers.BuildLookupTable(productionTable.Keys);
var newSumOfProbability = 0.0;
var toAdd = new List <Production>();
var toRemove = new List <Production>();
// find all the unit productions and replace them with equivalent rules
// X -> Y gets replaced with rules X -> Z for all Y -> Z
foreach (var production in productionTable.Keys)
{
if (production.IsUnit())
{
var thisProb = GetProbability(production, productionsByNonterminal);
if (double.IsNaN(thisProb))
{
continue;
}
newSumOfProbability += thisProb;
var replacements = UnitReplacementProductions(production, productionsByNonterminal);
toAdd.AddRange(replacements);
toRemove.Add(production);
}
}
if (oldSumOfProbability < newSumOfProbability)
{
throw new Exception("Invariant didn't hold, we want probability sums to decrease every iteration");
}
oldSumOfProbability = newSumOfProbability;
foreach (var production in toRemove)
{
production.Weight = 0.0;
}
MergeProductions(productionTable, toAdd);
}
return(productionTable.Keys.Where((p) => p.Weight > 0.0));
}