simple monte carlo simulation

battlesnake/src/main.rs

@@ -9,10 +9,13 @@ use battlesnake::types::{
     wire::{Request, Response},
     Direction,
 };
-use log::{debug, info, warn};
+use log::{debug, error, info, warn};
 use rand::prelude::*;
 use serde::Serialize;
-use tokio::{net::TcpListener, time::Instant};
+use tokio::{
+    net::TcpListener,
+    time::{Duration, Instant},
+};
 
 #[tokio::main]
 async fn main() {
@@ -59,20 +62,59 @@ async fn start(request: Json<Request>) {
 }
 
 async fn get_move(request: Json<Request>) -> response::Json<Response> {
-    let board = Board::from(&*request);
-    info!("got move request: {board}");
-    let actions = board.valid_actions(0).collect::<Vec<_>>();
-    info!("valid actions: {:?}", actions);
     let start = Instant::now();
-    for _ in 0..100 {
-        let mut board = board.clone();
-        let score = board.simulate_random(|board| (board.num_snakes() <= 1).then_some(1));
-        std::hint::black_box(score);
+    let board = Board::from(&*request);
+    let id = board.get_id(&request.you.id).unwrap_or_else(|| {
+        error!("My id is not in the simulation board");
+        0
+    });
+    info!("got move request: {board}");
+    let actions = board.valid_actions(id).collect::<Vec<_>>();
+    if actions.len() <= 1 {
+        info!(
+            "only one possible action. Fast forwarding {:?}",
+            actions.first()
+        );
+        return response::Json(Response {
+            direction: actions.first().copied().unwrap_or(Direction::Up),
+            shout: None,
+        });
     }
-    let elapsed = start.elapsed();
-    debug!("simulated 100 random games in {elapsed:?}");
-    let action = actions.choose(&mut thread_rng()).copied();
-    if action.is_none() {
+    info!("valid actions: {:?}", actions);
+
+    let mut action_data = [(0, 0); 4];
+    let mut total_simulations = 0;
+    let base_turns = board.turn();
+    while start.elapsed() < Duration::from_millis(250) {
+        let mut board = board.clone();
+        let action = *actions.choose(&mut thread_rng()).unwrap_or(&Direction::Up);
+        board.next_turn(&[(id, action)]);
+        let turns = board.simulate_random(|board| {
+            if board.num_snakes() == 0
+                || board.turn() > base_turns + u32::from(request.you.length) * 3
+            {
+                Some(board.turn())
+            } else {
+                None
+            }
+        });
+        let action_data = &mut action_data[usize::from(action)];
+        action_data.0 += turns - base_turns;
+        action_data.1 += 1;
+        total_simulations += 1;
+    }
+    debug!("action data: {action_data:?}");
+
+    let action = actions.into_iter().max_by_key(|action| {
+        let action_data = action_data[usize::from(*action)];
+        action_data.0 / action_data.1
+    });
+
+    if let Some(action) = action {
+        let action_data = action_data[usize::from(action)];
+        let avg_turns = action_data.0 / action_data.1;
+        info!("found action {action:?} after {total_simulations} simulations with an average of {avg_turns} turns.");
+    } else {
         warn!("unable to find a valid action");
     }
     info!("chose {action:?}");

battlesnake/src/types/mod.rs

@@ -45,3 +45,14 @@ pub enum Direction {
     /// Move in positive x direction
     Right,
 }
+
+impl From<Direction> for usize {
+    fn from(value: Direction) -> Self {
+        match value {
+            Direction::Up => 0,
+            Direction::Down => 1,
+            Direction::Left => 2,
+            Direction::Right => 3,
+        }
+    }
+}

battlesnake/src/types/simulation.rs

@@ -187,6 +187,14 @@ impl Display for Board {
 }
 
 impl Board {
+    #[must_use]
+    pub fn get_id(&self, str_id: &str) -> Option<u8> {
+        self.id_map
+            .iter()
+            .find(|(_, str_id2)| *str_id == **str_id2)
+            .map(|(id, _)| *id)
+    }
+
     #[must_use]
     pub const fn turn(&self) -> u32 {
         self.turn
@@ -197,6 +205,11 @@ impl Board {
         self.snakes.len()
     }
 
+    #[must_use]
+    pub fn alive(&self, id: u8) -> bool {
+        self.id_to_index(id).is_some()
+    }
+
     #[must_use]
     pub fn is_food(&self, tile: Coord) -> bool {
         let index = self.coord_to_linear(tile);
@@ -219,7 +232,7 @@ impl Board {
     }
 
     pub fn valid_actions(&self, id: u8) -> impl Iterator<Item = Direction> + use<'_> {
-        let index = self.snakes.binary_search_by_key(&id, |snake| snake.id).ok();
+        let index = self.id_to_index(id);
         if index.is_none() {
             warn!("Asked for a snake that doesn't exist");
         }
@@ -228,10 +241,25 @@ impl Board {
             .flat_map(|index| self.valid_actions_index(index))
     }
 
-    pub fn random_actions(
-        &self,
-    ) -> impl Iterator<Item = (u8, impl Iterator<Item = Direction> + use<'_>)> {
-        (0..self.snakes.len()).map(|index| (self.snakes[index].id, self.valid_actions_index(index)))
+    #[must_use]
+    pub fn random_action(&self, id: u8) -> Direction {
+        let Some(index) = self.id_to_index(id) else {
+            return Direction::Up;
+        };
+        self.valid_actions_index(index)
+            .choose(&mut thread_rng())
+            .unwrap_or(Direction::Up)
+    }
+
+    pub fn random_actions(&self) -> impl Iterator<Item = (u8, Direction)> + use<'_> {
+        (0..self.snakes.len()).map(|index| {
+            (
+                self.snakes[index].id,
+                self.valid_actions_index(index)
+                    .choose(&mut thread_rng())
+                    .unwrap_or(Direction::Up),
+            )
+        })
     }
 
     pub fn simulate_random<T>(&mut self, stop: impl Fn(&Self) -> Option<T>) -> T {
@@ -254,6 +282,10 @@ impl Board {
         self.turn += 1;
     }
 
+    fn id_to_index(&self, id: u8) -> Option<usize> {
+        self.snakes.binary_search_by_key(&id, |snake| snake.id).ok()
+    }
+
     fn valid_actions_index(&self, index: usize) -> impl Iterator<Item = Direction> + use<'_> {
         let head = self.snakes[index].head();
         enum_iterator::all::<Direction>()
@@ -338,9 +370,9 @@ impl Board {
             let snake = &self.snakes[i];
             if snake.health == 0 || !self.is_in_bounds(snake.head()) {
                 let snake = self.snakes.remove(i);
-                for tile in snake.body {
-                    if self.is_in_bounds(tile) {
-                        let index = self.coord_to_linear(tile);
+                for tile in snake.body.iter().skip(1) {
+                    if self.is_in_bounds(*tile) {
+                        let index = self.coord_to_linear(*tile);
                         self.free.set(index, true);
                     }
                 }
@@ -413,11 +445,21 @@ impl Board {
             )
         };
 
+        if needed_food == 0 {
+            return;
+        }
+
         let food_spots = self
             .free
             .iter()
             .enumerate()
             .filter_map(|(i, free)| free.then_some(i))
+            .filter(|i| {
+                self.snakes
+                    .iter()
+                    .all(|snake| self.coord_to_linear(snake.tail()) != *i)
+            })
+            .filter(|i| !self.food[*i])
             .choose_multiple(&mut thread_rng(), needed_food);
         for index in food_spots {
             self.food.set(index, true);