Fixes problems in mdp.py (aimacode#918)

* Added MDP2 class

* Updated loop termination condition in value_iteration

Author: ad71

mdp.py

@@ -104,6 +104,19 @@ def check_consistency(self):
                 assert abs(s - 1) < 0.001
 
 
+class MDP2(MDP):
+
+    """Inherits from MDP. Handles terminal states, and transitions to and from terminal states better."""
+    def __init__(self, init, actlist, terminals, transitions, reward=None, gamma=0.9):
+        MDP.__init__(self, init, actlist, terminals, transitions, reward, gamma=gamma)
+
+    def T(self, state, action):
+        if action is None:
+            return [(0.0, state)]
+        else:
+            return self.transitions[state][action]
+
+
 class GridMDP(MDP):
 
     """A two-dimensional grid MDP, as in [Figure 17.1]. All you have to do is
@@ -186,7 +199,7 @@ def value_iteration(mdp, epsilon=0.001):
             U1[s] = R(s) + gamma * max(sum(p*U[s1] for (p, s1) in T(s, a))
                                                    for a in mdp.actions(s))
             delta = max(delta, abs(U1[s] - U[s]))
-        if delta < epsilon*(1 - gamma)/gamma:
+        if delta <= epsilon*(1 - gamma)/gamma:
             return U