From 8a5b2f001dc7e1a960b3cad76430ed1c3e67143e Mon Sep 17 00:00:00 2001
From: Dave Parker <dave.x.parker@gmail.com>
Date: Wed, 3 Mar 2021 21:53:21 +0000
Subject: [PATCH] POMDP refactoring: new Belief constructors.

---
 prism/src/explicit/Belief.java      | 35 +++++++++++++++++++++++++++--
 prism/src/explicit/POMDPSimple.java |  6 ++---
 2 files changed, 36 insertions(+), 5 deletions(-)

diff --git a/prism/src/explicit/Belief.java b/prism/src/explicit/Belief.java
index 0af35219..797c81c5 100644
--- a/prism/src/explicit/Belief.java
+++ b/prism/src/explicit/Belief.java
@@ -51,8 +51,8 @@ public class Belief implements Comparable<Belief>
 
 	/**
 	 * Constructor
-	 * @param Observable part (index of observation)
-	 * @param Distribution over unobservable part (probability for each unobservation) 
+	 * @param so Observable part (index of observation)
+	 * @param bu Distribution over unobservable part (probability for each unobservation)
 	 */
 	public Belief(int so, double[] bu)
 	{
@@ -60,6 +60,37 @@ public class Belief implements Comparable<Belief>
 		this.bu = bu;
 	}
 
+	/**
+	 * Constructor
+	 * @param dist Distribution over states of a model (probability for each state)
+	 * @param model The (partially observable) model
+	 * If {@code dist} is a not a valid distribution, the resulting belief will be invalid too.
+	 */
+	protected Belief(double[] dist, PartiallyObservableModel model)
+	{
+		so = -1;
+		bu = new double[model.getNumUnobservations()];
+		for (int s = 0; s < dist.length; s++) {
+			if (dist[s] != 0) {
+				so = model.getObservation(s);
+				bu[model.getUnobservation(s)] += dist[s];
+			}
+		}
+	}
+	
+	/**
+	 * Construct a point distribution over a single model stete
+	 * @param s A model state
+	 * @param model The (partially observable) model
+	 */
+	public static Belief pointDistribution(int s, PartiallyObservableModel model)
+	{
+		int so = model.getObservation(s);
+		double[] bu = new double[model.getNumUnobservations()];
+		bu[model.getUnobservation(s)] = 1.0;
+		return new Belief(so, bu);
+	}
+	
 	/**
 	 * Convert to a probability distribution over all model states
 	 * (represented as an array of probabilities). 
diff --git a/prism/src/explicit/POMDPSimple.java b/prism/src/explicit/POMDPSimple.java
index b19dbc48..1ab3769a 100644
--- a/prism/src/explicit/POMDPSimple.java
+++ b/prism/src/explicit/POMDPSimple.java
@@ -378,7 +378,7 @@ public class POMDPSimple extends MDPSimple implements POMDP
 			initialBeliefInDist[i] = 1;
 		}
 		PrismUtils.normalise(initialBeliefInDist);
-		return beliefInDistToBelief(initialBeliefInDist);
+		return new Belief(initialBeliefInDist, this);
 	}
 
 	@Override
@@ -397,7 +397,7 @@ public class POMDPSimple extends MDPSimple implements POMDP
 	{
 		double[] beliefInDist = belief.toDistributionOverStates(this);
 		double[] nextBeliefInDist = getBeliefInDistAfterChoice(beliefInDist, i);
-		return beliefInDistToBelief(nextBeliefInDist);
+		return new Belief(nextBeliefInDist, this);
 	}
 
 	@Override
@@ -423,7 +423,7 @@ public class POMDPSimple extends MDPSimple implements POMDP
 	{
 		double[] beliefInDist = belief.toDistributionOverStates(this);
 		double[] nextBeliefInDist = getBeliefInDistAfterChoiceAndObservation(beliefInDist, i, o);
-		Belief nextBelief = beliefInDistToBelief(nextBeliefInDist);
+		Belief nextBelief = new Belief(nextBeliefInDist, this);
 		assert(nextBelief.so == o);
 		return nextBelief;
 	}