CFGLib.GrammarHelpers.GetProductionProbability C# (CSharp) Method

		private static double GetProductionProbability(Production production, Dictionary<Nonterminal, int> nonterminalToIndex, double[] previousEstimates) {
			if (production.Rhs.Count == 0) {
				return 1.0;
			}
			// if it contains a terminal, then it always is non-empty
			if (!production.Rhs.OnlyNonterminals()) {
				return 0.0;
			}

			var product = 1.0;
			foreach (var word in production.Rhs) {
				var nt = (Nonterminal)word;
				var rhsIndex = nonterminalToIndex[nt];
				var previous = previousEstimates[rhsIndex];
				// if this is the first iteration, we assume that the previous values were 100% chance of yielding null
				if (previous == _magicStartProbability) {
					previous = 1.0;
				}
				product *= previous;
			}
			if (double.IsNaN(product)) {
				throw new Exception("Didn't expect to get NaN probability");
			}
			return product;
		}