Add Capture the Flag game

docs/games.md

@@ -22,6 +22,7 @@ Status                                                                 | Game
 🟢                                                                      | [Breakthrough](https://en.wikipedia.org/wiki/Breakthrough_\(board_game\))                      | 2       | ✅              | ✅            | Simplified chess using only pawns.
 🟢                                                                      | [Bridge](https://en.wikipedia.org/wiki/Contract_bridge)                                        | 4       | ❌              | ❌            | A card game where players compete in pairs.
 🟢                                                                      | [(Uncontested) Bridge bidding](https://en.wikipedia.org/wiki/Contract_bridge)                  | 2       | ❌              | ❌            | Players score points by forming specific sets with the cards in their hands.
+🔶                                                                      | [Capture the Flag](https://en.wikipedia.org/wiki/Capture_the_flag)                             | 2       | ❌              | ✅            | Simultaneous-move adversarial grid game. Each player guards a flag and tries to bring the opponent's flag back to their own base. A carrier in the defender's home territory is vulnerable to being tagged, which returns the flag to its base. References: [Agapiou et al. '22, Melting Pot 2.0](https://arxiv.org/abs/2211.13746).
 🔶                                                                      | Catch                                                                                          | 1       | ❌              | ✅            | Agent must move horizontally to 'catch' a descending ball. Designed to test basic learning. References: [Mnih et al. 2014, Recurrent Models of Visual Attention](https://papers.nips.cc/paper/5542-recurrent-models-of-visual-attention.pdf). [Osband et al '19, Behaviour Suite for Reinforcement Learning, Appendix A](https://arxiv.org/abs/1908.03568).
 🔶                                                                      | [Checkers](https://en.wikipedia.org/wiki/Checkers)                                             | 2       | ✅              | ✅            | Players move pieces around the board with the goal of eliminating the opposing pieces.
 🔶                                                                      | [Chinese Checkers](https://en.wikipedia.org/wiki/Chinese_checkers)                             | 2-6     | ✅              | ✅            | Star-shaped board game where players race to move all 10 pieces to the opposite triangle via steps and chain hops. Supports 2, 3, 4, or 6 players.

open_spiel/games/CMakeLists.txt

@@ -26,6 +26,8 @@ set(GAME_SOURCES
   bridge/bridge_scoring.h
   bridge/bridge_uncontested_bidding.cc
   bridge/bridge_uncontested_bidding.h
+  capture_the_flag/capture_the_flag.cc
+  capture_the_flag/capture_the_flag.h
   catch/catch.cc
   catch/catch.h
   checkers/checkers.cc
@@ -380,6 +382,10 @@ add_executable(bridge_test bridge/bridge_test.cc ${OPEN_SPIEL_OBJECTS}
                $<TARGET_OBJECTS:tests>)
 add_test(bridge_test bridge_test)
 
+add_executable(capture_the_flag_test capture_the_flag/capture_the_flag_test.cc
+               ${OPEN_SPIEL_OBJECTS} $<TARGET_OBJECTS:tests>)
+add_test(capture_the_flag_test capture_the_flag_test)
+
 add_executable(catch_test catch/catch_test.cc ${OPEN_SPIEL_OBJECTS}
                $<TARGET_OBJECTS:tests>)
 add_test(catch_test catch_test)