1 files changed, 86 insertions, 0 deletions
diff --git a/gi/posterior-regularisation/prjava/src/util/LogSummer.java b/gi/posterior-regularisation/prjava/src/util/LogSummer.java
new file mode 100644
index 00000000..117393b9
--- /dev/null
+++ b/gi/posterior-regularisation/prjava/src/util/LogSummer.java
@@ -0,0 +1,86 @@
+package util;
+
+import java.lang.Math;
+
+/*
+ * Math tool for computing logs of sums, when the terms of the sum are already in log form.
+ * (Useful if the terms of the sum are very small numbers.)
+ */
+public class LogSummer {
+	
+	private LogSummer() {
+	}
+		
+	/**
+	 * Given log(a) and log(b), computes log(a + b).
+	 * 
+	 * @param  loga log of first sum term
+	 * @param  logb log of second sum term
+	 * @return     log(sum), where sum = a + b
+	 */
+	public static double sum(double loga, double logb) {
+		assert(!Double.isNaN(loga));
+		assert(!Double.isNaN(logb));
+		
+		if(Double.isInfinite(loga))
+			return logb;
+		if(Double.isInfinite(logb))
+			return loga;
+
+		double maxLog;
+		double difference;
+		if(loga > logb) {
+			difference = logb - loga;
+			maxLog = loga;
+		}
+		else {
+			difference = loga - logb;
+			maxLog = logb;
+		}
+
+		return Math.log1p(Math.exp(difference)) + maxLog;
+	}
+
+	/**
+	 * Computes log(exp(array[index]) + b), and
+	 * modifies array[index] to contain this new value.
+	 * 
+	 * @param array array to modify
+	 * @param index index at which to modify
+	 * @param logb  log of the second sum term
+	 */
+	public static void sum(double[] array, int index, double logb) {
+		array[index] = sum(array[index], logb);
+	}
+	
+	/**
+	 * Computes log(a + b + c + ...) from log(a), log(b), log(c), ...
+	 * by recursively splitting the input and delegating to the sum method.
+	 * 
+	 * @param  terms an array containing the log of all the terms for the sum
+	 * @return log(sum), where sum = exp(terms[0]) + exp(terms[1]) + ...
+	 */
+	public static double sumAll(double... terms) {
+		return sumAllHelper(terms, 0, terms.length);
+	}
+	
+	/**
+	 * Computes log(a_0 + a_1 + ...) from a_0 = exp(terms[begin]),
+	 * a_1 = exp(terms[begin + 1]), ..., a_{end - 1 - begin} = exp(terms[end - 1]).
+	 * 
+	 * @param  terms an array containing the log of all the terms for the sum,
+	 *               and possibly some other terms that will not go into the sum
+	 * @return log of the sum of the elements in the [begin, end) region of the terms array
+	 */
+	private static double sumAllHelper(final double[] terms, final int begin, final int end) {
+		int length = end - begin;
+		switch(length) {
+			case 0: return Double.NEGATIVE_INFINITY;
+			case 1: return terms[begin];
+			default:
+				int midIndex = begin + length/2;
+				return sum(sumAllHelper(terms, begin, midIndex), sumAllHelper(terms, midIndex, end));
+		}
+	}
+
+}
+\ No newline at end of file