Practice Activity 2 simplified Nim (Monte Carlo solver)

practice_activity2.py

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+# Practice Activity 2 for Principles of Computing class, by k., 07/07/2014
+# simplified Nim (Monte Carlo solver) (see https://class.coursera.org/principlescomputing-001/wiki/view?page=nim_mc )
+# template: http://www.codeskulptor.org/#poc_nim_mc_template.py
+# official solution: http://www.codeskulptor.org/#poc_nim_mc_student.py
+
+'''
+a simple Monte Carlo solver for Nim (http://en.wikipedia.org/wiki/Nim#The_21_game )
+'''
+
+import random
+#import codeskulptor
+#codeskulptor.set_timeout(20)
+
+
+MAX_REMOVE = 3
+TRIALS = 10000
+
+def evaluate_position(num_items):
+    '''
+    Monte Carlo evalation method for Nim
+    '''
+    max_computer_wins = float('-inf')
+    for move in range(1, (MAX_REMOVE) + 1):
+        computer_wins = 0
+        for trial in range(TRIALS):
+            initial_move = move
+            current_items = num_items - initial_move
+
+            while True:
+                # play & evaluate player's move
+                generated_move = random.choice(range(1, (MAX_REMOVE) + 1))
+                current_items -= generated_move
+                if current_items <= 0:
+                    break
+                # play & evaluate computer's move
+                generated_move = random.choice(range(1, (MAX_REMOVE) + 1))
+                current_items -= generated_move
+                if current_items <= 0:
+                    computer_wins += 1
+                    break
+
+        # keeping track of best value amongst (1, 2, 3) initial moves
+        if max_computer_wins < computer_wins:
+            result = move
+            max_computer_wins = computer_wins
+
+        #print 'For', move, 'Computer won:', computer_wins, 'Player won:', TRIALS - computer_wins
+    return result
+
+#print evaluate_position(10)
+
+# optimal strategy: when your turn to move, remove exactly enough items so that the number of items
+# remaining in the heap has remainder zero when divided by four     
+
+
+def play_game(start_items):
+    '''
+    play a game of Nim against Monte Carlo bot
+    '''
+
+    current_items = start_items
+    print 'Starting game with value', current_items
+    while True:
+        comp_move = evaluate_position(current_items)
+        current_items -= comp_move
+        print 'Computer choose', comp_move, ', current value is', current_items
+        if current_items <= 0:
+            print 'Computer wins'
+            break
+        player_move = int(input('Enter your current move: '))
+        current_items -= player_move
+        print 'Player choose', player_move, ', current value is', current_items
+        if current_items <= 0:
+            print 'Player wins'
+            break
+
+#play_game(21)
+# quick test
+#play_game(10)