This PR adds the Momentum strategy (#1469)

* Add Momentum strategy

Author: dongwonmoon

axelrod/classifier.py

@@ -185,7 +185,7 @@ def __getitem__(
             raise KeyError("Unknown classifier")
 
         def classify_player_for_this_classifier(
-            player: Union[Player, Type[Player]]
+            player: Union[Player, Type[Player]],
         ) -> Any:
             def try_lookup() -> Any:
                 try:

axelrod/makes_use_of.py

@@ -36,7 +36,7 @@ def makes_use_of(player: Type[Player]) -> Set[Text]:
 
 
 def makes_use_of_variant(
-    player_or_method: Union[Callable, Type[Player]]
+    player_or_method: Union[Callable, Type[Player]],
 ) -> Set[Text]:
     """A version of makes_use_of that works on functions or player classes."""
     try:

axelrod/strategies/_strategies.py

@@ -193,6 +193,7 @@
 from .memorytwo import AON2, MEM2, DelayedAON1
 from .memorytwo import MemoryTwoPlayer  # pylint: disable=unused-import
 
+from .momentum import Momentum
 from .mutual import Desperate, Hopeless, Willing
 from .negation import Negation
 from .oncebitten import FoolMeOnce, ForgetfulFoolMeOnce, OnceBitten
@@ -402,6 +403,7 @@
     MEM2,
     MathConstantHunter,
     Michaelos,
+    Momentum,
     NTitsForMTats,
     NaiveProber,
     Negation,

axelrod/strategies/frequency_analyzer.py

@@ -62,7 +62,7 @@ def __init__(self) -> None:
         """
         super().__init__()
         self.minimum_cooperation_ratio = 0.25
-        self.frequency_table = dict()
+        self.frequency_table: dict = dict()
         self.last_sequence = ""
         self.current_sequence = ""
 

axelrod/strategies/memorytwo.py

@@ -98,7 +98,7 @@ def set_sixteen_vector(self, sixteen_vector: Tuple[float, ...]):
 
     @staticmethod
     def compute_memory_depth(
-        sixteen_vector: Dict[Tuple[Action, Action], float]
+        sixteen_vector: Dict[Tuple[Action, Action], float],
     ) -> int:
         values = set(list(sixteen_vector.values()))
 

axelrod/strategies/momentum.py

@@ -0,0 +1,63 @@
+from axelrod.action import Action
+from axelrod.player import Player
+
+C, D = Action.C, Action.D
+
+
+class Momentum(Player):
+    """
+    This strategy is inspired by the concept of Gradual and the mathematical foundation of
+    the Momentum optimizer used in deep learning.
+
+    The idea is that trust (or cooperation) evolves dynamically. A shift in trust can
+    create significant and rapid changes in the player's behavior, much like how momentum
+    responds to gradients in optimization.
+
+    Parameters:
+     - alpha: Momentum decay factor that determines the rate of trust reduction. A higher value leads to slower decay, and the opponent's Defect acts as a trigger. (Optimized by Genetic Algorithm)
+     - threshold: The minimum momentum required to continue cooperation. If momentum falls below this value, the strategy switches to Defect as punishment. (Optimized by Genetic Algorithm)
+     - momentum: Represents the inertia of trust, dynamically changing based on past cooperation.
+
+    Names:
+     - Momentum: Original name by Dong Won Moon
+
+    """
+
+    name = "Momentum"
+    classifier = {
+        "memory_depth": float("inf"),
+        "stochastic": False,
+        "long_run_time": False,
+        "inspects_source": False,
+        "manipulates_source": False,
+        "manipulates_state": False,
+    }
+
+    def __init__(
+        self,
+        alpha=0.9914655399877477,
+        threshold=0.9676595613724907,
+    ) -> None:
+        super().__init__()
+        self.alpha = alpha
+        self.threshold = threshold
+        self.momentum = 1.0
+
+    def __repr__(self):
+        return f"Momentum: {self.momentum}, Alpha: {self.alpha}, Threshold: {self.threshold}"
+
+    def update_momentum(self, opponent_action):
+        # If the opponent defects, the momentum decreases, reflecting a loss of trust.
+        action_value = 1 if opponent_action == C else 0
+        self.momentum = (
+            self.alpha * self.momentum + (1 - self.alpha) * action_value
+        )
+
+    def strategy(self, opponent: Player) -> Action:
+        if len(self.history) == 0:
+            self.momentum = 1.0
+            return C
+
+        else:
+            self.update_momentum(opponent.history[-1])
+            return C if self.momentum >= self.threshold else D

axelrod/tests/strategies/test_gambler.py

@@ -1,5 +1,4 @@
-"""Test for the Gambler strategy. Most tests come from the LookerUp test suite.
-"""
+"""Test for the Gambler strategy. Most tests come from the LookerUp test suite."""
 
 import copy
 import unittest

axelrod/tests/strategies/test_momentum.py

@@ -0,0 +1,90 @@
+import axelrod as axl
+from axelrod import Action
+from axelrod.strategies.momentum import Momentum
+from axelrod.tests.strategies.test_player import TestPlayer
+
+C, D = Action.C, Action.D
+
+
+class TestMomentum(TestPlayer):
+    name = "Momentum"
+    player = Momentum
+    expected_classifier = {
+        "memory_depth": float("inf"),
+        "stochastic": False,
+        "long_run_time": False,
+        "inspects_source": False,
+        "manipulates_source": False,
+        "manipulates_state": False,
+    }
+
+    def test_initialisation(self):
+        player = self.player(alpha=0.9, threshold=0.8)
+        self.assertEqual(player.alpha, 0.9)
+        self.assertEqual(player.threshold, 0.8)
+        self.assertEqual(player.momentum, 1.0)
+
+    def test_repr(self):
+        player = self.player(alpha=0.9, threshold=0.8)
+        self.assertEqual(
+            repr(player), "Momentum: 1.0, Alpha: 0.9, Threshold: 0.8"
+        )
+
+    def test_strategy(self):
+        actions = [(C, C)]
+        self.versus_test(
+            axl.MockPlayer(actions=[C]),
+            expected_actions=actions,
+            init_kwargs={"alpha": 0.5, "threshold": 0.5},
+            attrs={"momentum": 1.0},
+        )
+
+        actions = [(C, D), (C, D), (D, D)]
+        self.versus_test(
+            axl.MockPlayer(actions=[D]),
+            expected_actions=actions,
+            init_kwargs={"alpha": 0.5, "threshold": 0.5},
+            attrs={"momentum": 0.25},
+        )
+
+    def test_vs_alternator(self):
+        actions = [(C, C), (C, D), (C, C), (C, D), (D, C)]
+        self.versus_test(
+            axl.Alternator(),
+            expected_actions=actions,
+            init_kwargs={"alpha": 0.5, "threshold": 0.5},
+        )
+
+    def test_vs_cooperator(self):
+        actions = [(C, C), (C, C), (C, C), (C, C), (C, C)]
+        self.versus_test(
+            axl.Cooperator(),
+            expected_actions=actions,
+            init_kwargs={"alpha": 0.5, "threshold": 0.5},
+        )
+
+    def test_vs_defector(self):
+        actions = [(C, D), (C, D), (D, D), (D, D), (D, D)]
+        self.versus_test(
+            axl.Defector(),
+            expected_actions=actions,
+            init_kwargs={"alpha": 0.5, "threshold": 0.5},
+        )
+
+    def test_vs_random(self):
+        actions = [(C, D), (C, C), (C, C), (C, D), (D, D)]
+        self.versus_test(
+            axl.Random(),
+            expected_actions=actions,
+            seed=17,
+            init_kwargs={"alpha": 0.5, "threshold": 0.5},
+        )
+
+    def test_vs_random2(self):
+        actions = [(C, C), (C, C), (C, C), (C, C)]
+        self.versus_test(
+            axl.Random(),
+            expected_actions=actions,
+            seed=3,
+            init_kwargs={"alpha": 0.5, "threshold": 0.5},
+        )

axelrod/tests/unit/test_resultset.py

@@ -4,7 +4,6 @@
 from collections import Counter
 
 import pandas as pd
-from dask.dataframe.core import DataFrame
 from hypothesis import given, settings
 from numpy import mean, nanmedian, std
 

docs/index.rst

@@ -53,7 +53,7 @@ Count the number of available players::
 
     >>> import axelrod as axl
     >>> len(axl.strategies)
-    241
+    242
 
 Create matches between two players::
 

docs/reference/strategy_index.rst

@@ -76,6 +76,8 @@ Here are the docstrings of all the strategies in the library.
    :members:
 .. automodule:: axelrod.strategies.memoryone
    :members:
+.. automodule:: axelrod.strategies.momentum
+   :members:
 .. automodule:: axelrod.strategies.meta
    :members:
 .. automodule:: axelrod.strategies.mutual

run_mypy.py

@@ -41,6 +41,7 @@
     "axelrod/strategies/mathematicalconstants.py",
     "axelrod/strategies/memoryone.py",
     "axelrod/strategies/memorytwo.py",
+    "axelrod/strategies/momentum.py",
     "axelrod/strategies/mutual.py",
     "axelrod/strategies/negation.py",
     "axelrod/strategies/oncebitten.py",