breakable-bottles observation space correction (#93)

Signed-off-by: Scott Johnson <[email protected]>
Co-authored-by: Scott Johnson <[email protected]>

mo_gymnasium/envs/breakable_bottles/breakable_bottles.py

@@ -25,9 +25,13 @@ class BreakableBottles(Env, EzPickle):
     The observation space is a dictionary with 4 keys:
     - location: the current location of the agent
     - bottles_carrying: the number of bottles the agent is currently carrying (0, 1 or 2)
-    - bottles_delivered: the number of bottles the agent has delivered (0 or 1)
+    - bottles_delivered: the number of bottles the agent has delivered (0, 1 or 2)
     - bottles_dropped: for each location, a boolean flag indicating if that location currently contains a bottle
 
+    Note that this observation space is different from that listed in the paper above. In the paper, bottles_delivered's possible values are listed as (0 or 1),
+    rather than (0, 1 or 2). This is because the paper did not take the terminal state, in which 2 bottles have been delivered, into account when calculating
+    the observation space. As such, the observation space of this implementation is larger than specified in the paper, having 360 possible states instead of 240.
+
     ## Reward Space
     The reward space has 3 dimensions:
     - time penalty: -1 for each time step
@@ -96,11 +100,11 @@ def __init__(
             {
                 "location": Discrete(self.size),
                 "bottles_carrying": Discrete(3),
-                "bottles_delivered": Discrete(2),
+                "bottles_delivered": Discrete(3),
                 "bottles_dropped": MultiBinary(self.size - 2),
             }
         )
-        self.num_observations = 240
+        self.num_observations = 360
 
         self.action_space = Discrete(3)  # LEFT, RIGHT, PICKUP
         self.num_actions = 3
@@ -220,7 +224,7 @@ def get_obs_idx(self, obs):
                 *[[bd > 0] for bd in obs["bottles_dropped"]],
             ]
         )
-        return np.ravel_multi_index(multi_index, tuple([self.size, 3, 2, *([2] * (self.size - 2))]))
+        return np.ravel_multi_index(multi_index, tuple([self.size, 3, 3, *([2] * (self.size - 2))]))
 
     def _get_obs(self):
         return {