add simulation to get the best move

battlesnake/src/logic.rs

@@ -10,11 +10,12 @@
 // To get you started we've included code to prevent your Battlesnake from moving backwards.
 // For more info see docs.battlesnake.com
 
+use std::{cmp::Ordering, time::Instant};
+
 use log::info;
-use rand::seq::SliceRandom;
 use serde_json::{json, Value};
 
-use crate::{Action, Battlesnake, Board, Direction, Game, MAX_HEALTH};
+use crate::{simulation, Action, Battlesnake, Board, Direction, Game, MAX_HEALTH};
 
 impl Battlesnake {
     fn possible_actions_without_heads<'a>(
@@ -109,17 +110,72 @@ pub fn end(_game: &Game, _turn: i32, _board: &Board, _you: &Battlesnake) {
 // Valid moves are "up", "down", "left", or "right"
 // See https://docs.battlesnake.com/api/example-move for available data
 pub fn get_move(game: &Game, turn: i32, board: &Board, you: &Battlesnake) -> Option<Action> {
-    let actions = you.possible_actions(game, board);
-    if actions.is_empty() {
-        return None;
-    }
+    let id_map = board
+        .snakes
+        .iter()
+        .enumerate()
+        .map(|(i, snake)| (snake.id.clone(), u8::try_from(i).unwrap()))
+        .collect();
+    let board = simulation::Board::from_game_board(board, &id_map, turn);
 
-    // Choose a random move from the safe ones
-    let chosen = actions.choose(&mut rand::thread_rng())?;
+    let my_id = id_map[&you.id];
+    let my_index = board.snake_index(my_id)?;
+
+    let possible_actions = board.possible_actions();
+
+    let my_actions = &possible_actions[my_index];
+
+    let actions = my_actions
+        .iter()
+        .map(|direction| {
+            let mut actions = vec![None; possible_actions.len()];
+            actions[my_index] = Some(*direction);
+            let mut wins = 0;
+            let mut total_turns = 0;
+            let start = Instant::now();
+            for _ in 0..100 {
+                let mut board = board.clone();
+                board.simulate_with_initial_until(&actions[..], |board| {
+                    !board.is_alive(my_id)
+                        || (game.ruleset.name != "solo" && board.alive_snakes() <= 1)
+                });
+                if board.is_alive(my_id) {
+                    // we survived
+                    wins += 2;
+                } else if board.alive_snakes() == 0 {
+                    // no snake is alive. This is a draw
+                    wins += 1;
+                } else {
+                    // we lost
+                    wins += 0;
+                }
+                total_turns += board.turn();
+            }
+            let end = Instant::now();
+            info!(
+                "Simulation for {direction:?} took {}s",
+                (end - start).as_secs_f32()
+            );
+
+            (direction, wins, total_turns)
+        })
+        .collect::<Vec<_>>();
+
+    info!("actions: {actions:?}");
+
+    let (&chosen, _, _) =
+        actions
+            .into_iter()
+            .max_by(
+                |(_, score1, turns1), (_, score2, turns2)| match score1.cmp(score2) {
+                    Ordering::Equal => turns1.cmp(turns2),
+                    order => order,
+                },
+            )?;
 
     info!("DIRECTION {}: {:?}", turn, chosen);
     Some(Action {
-        r#move: *chosen,
+        r#move: chosen,
         shout: None,
     })
 }

battlesnake/src/main.rs

@@ -11,6 +11,7 @@ use serde_json::Value;
 use std::env;
 
 mod logic;
+mod simulation;
 
 const MAX_HEALTH: i32 = 100;
 
@@ -119,7 +120,7 @@ pub struct RulesetSquad {
     shared_length: bool,
 }
 
-#[derive(Deserialize, Serialize, Debug)]
+#[derive(Deserialize, Serialize, Debug, Clone)]
 pub struct Board {
     /// The number of rows in the y-axis of the game board.
     height: i32,
@@ -134,7 +135,7 @@ pub struct Board {
     hazards: Vec<Coord>,
 }
 
-#[derive(Deserialize, Serialize, Debug)]
+#[derive(Deserialize, Serialize, Debug, Clone)]
 pub struct Battlesnake {
     /// Unique identifier for this Battlesnake in the context of the current Game
     id: String,

battlesnake/src/simulation.rs

@@ -0,0 +1,257 @@
+use std::collections::{BTreeSet, HashMap, VecDeque};
+
+use rand::seq::SliceRandom;
+
+use crate::{Coord, Direction};
+
+const MAX_HEALTH: u8 = crate::MAX_HEALTH as u8;
+
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub struct Board {
+    turn: i32,
+    /// Height of the board
+    height: i32,
+    /// Width of the board
+    width: i32,
+    /// Food on the board
+    food: BTreeSet<Coord>,
+    /// Alive snakes
+    snakes: Vec<Battlesnake>,
+}
+
+impl Board {
+    pub fn from_game_board(board: &crate::Board, id_map: &HashMap<String, u8>, turn: i32) -> Self {
+        let width = board.width;
+        debug_assert!(width > 0);
+        let height = board.height;
+        debug_assert!(height > 0);
+        let food = board.food.iter().copied().collect();
+        let snakes = board
+            .snakes
+            .iter()
+            .map(|snake| {
+                let id = id_map[&snake.id];
+                Battlesnake::from_game_snake(snake, id)
+            })
+            .collect();
+
+        Self {
+            turn,
+            height,
+            width,
+            food,
+            snakes,
+        }
+    }
+
+    pub const fn turn(&self) -> i32 {
+        self.turn
+    }
+
+    pub fn snake_index(&self, id: u8) -> Option<usize> {
+        self.snakes
+            .iter()
+            .enumerate()
+            .find(|(_, snake)| snake.id == id)
+            .map(|(i, _)| i)
+    }
+
+    pub fn is_alive(&self, id: u8) -> bool {
+        self.snakes.iter().any(|snake| snake.id == id)
+    }
+
+    pub fn alive_snakes(&self) -> usize {
+        self.snakes.len()
+    }
+
+    pub fn simulate_actions(&mut self, actions: &[Direction]) {
+        debug_assert_eq!(self.snakes.len(), actions.len());
+
+        // move snakes
+        for (snake, direction) in self.snakes.iter_mut().zip(actions.iter()) {
+            snake.perform_action(*direction);
+        }
+
+        // feed snakes
+        for snake in &mut self.snakes {
+            let head = snake.head();
+            if self.food.remove(head) {
+                snake.health = MAX_HEALTH;
+            }
+        }
+
+        // kill snakes
+        let alive_ids = self
+            .snakes
+            .iter()
+            .filter(|snake| {
+                // snake must have enough health
+                snake.health != 0
+            })
+            .map(|snake| (snake.id, snake.body.len(), *snake.head()))
+            .filter(|(_, _, head)| {
+                // head in bounds
+                (0..self.width).contains(&head.x) && (0..self.height).contains(&head.y)
+            })
+            .filter(|(_, _, head)| {
+                // body collision
+                !self
+                    .snakes
+                    .iter()
+                    .flat_map(|snake2| snake2.body.iter().skip(1))
+                    .any(|body| body == head)
+            })
+            .filter(|(id, len, head)| {
+                // head to head collision
+                !self
+                    .snakes
+                    .iter()
+                    .filter(|snake2| snake2.id != *id && snake2.body.len() >= *len)
+                    .any(|snake2| snake2.head() == head)
+            })
+            .map(|(id, _, _)| id)
+            .collect::<Vec<_>>();
+        self.snakes.retain(|snake| alive_ids.contains(&snake.id));
+
+        self.turn += 1;
+    }
+
+    pub fn simulate_with_initial_until(
+        &mut self,
+        actions: &[Option<Direction>],
+        exit: impl Fn(&Self) -> bool,
+    ) {
+        debug_assert_eq!(actions.len(), self.snakes.len());
+        let possible_actions = self.possible_actions();
+        let actions = actions
+            .iter()
+            .enumerate()
+            .map(|(i, direction)| {
+                direction.unwrap_or_else(|| {
+                    possible_actions[i]
+                        .choose(&mut rand::thread_rng())
+                        .copied()
+                        .unwrap_or(Direction::Up)
+                })
+            })
+            .collect::<Vec<_>>();
+
+        self.simulate_actions(&actions);
+        while !exit(self) {
+            let actions = self
+                .possible_actions()
+                .iter()
+                .map(|actions| {
+                    actions
+                        .choose(&mut rand::thread_rng())
+                        .copied()
+                        .unwrap_or(Direction::Up)
+                })
+                .collect::<Vec<_>>();
+            self.simulate_actions(&actions);
+        }
+    }
+
+    pub fn possible_actions(&self) -> Vec<Vec<Direction>> {
+        let possible_actions = self
+            .snakes
+            .iter()
+            .map(|snake| {
+                enum_iterator::all::<Direction>()
+                    .map(|direction| (direction, snake.head().move_to(direction)))
+                    .filter(|(_, target)| {
+                        // don't move out of bounds
+                        (0..self.width).contains(&target.x) && (0..self.height).contains(&target.y)
+                    })
+                    .filter(|(_, target)| {
+                        // don't collide with other snakes
+                        !self
+                            .snakes
+                            .iter()
+                            .flat_map(|snake| {
+                                let has_eaten = snake.health == MAX_HEALTH;
+                                snake
+                                    .body
+                                    .iter()
+                                    .take(snake.body.len() - usize::from(!has_eaten))
+                            })
+                            .any(|coord| coord == target)
+                    })
+                    .map(|(direction, _)| direction)
+                    .collect::<Vec<_>>()
+            })
+            .collect::<Vec<_>>();
+
+        // don't move into bigger snakes heads with only one movement option
+        possible_actions
+            .iter()
+            .enumerate()
+            .map(|(i, actions)| {
+                let snake = &self.snakes[i];
+                let length = snake.body.len();
+                let head = snake.head();
+                actions
+                    .iter()
+                    .copied()
+                    .filter(|direction| {
+                        let target = head.move_to(*direction);
+                        !self
+                            .snakes
+                            .iter()
+                            .enumerate()
+                            .filter(|(_, snake)| {
+                                // only snakes that are longer
+                                snake.body.len() > length
+                            })
+                            .filter_map(|(i, snake)| match &possible_actions[i][..] {
+                                // only snakes that have a single action option
+                                [direction] => Some(snake.head().move_to(*direction)),
+                                _ => None,
+                            })
+                            .any(|coord| coord == target)
+                    })
+                    .collect()
+            })
+            .collect()
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub struct Battlesnake {
+    /// Id of the snake. Unique inside a game
+    id: u8,
+    /// health points
+    health: u8,
+    /// Body of the snake. The head is the first element in the queue
+    body: VecDeque<Coord>,
+}
+
+impl Battlesnake {
+    pub fn from_game_snake(snake: &crate::Battlesnake, id: u8) -> Self {
+        let body: VecDeque<_> = snake.body.iter().copied().collect();
+        debug_assert_eq!(body.len(), usize::try_from(snake.length).unwrap());
+        debug_assert!(snake.health <= crate::MAX_HEALTH);
+        let health = u8::try_from(snake.health).expect("max health is 100");
+        Self { id, health, body }
+    }
+
+    pub fn perform_action(&mut self, direction: Direction) {
+        debug_assert!(!self.body.is_empty());
+        // move the head along
+        self.body.push_front(self.head().move_to(direction));
+
+        // move tail
+        if self.health != MAX_HEALTH {
+            // only move the tail if we didn't eat
+            self.body.pop_back();
+        }
+
+        // decrease helth
+        self.health = self.health.saturating_sub(1);
+    }
+
+    pub fn head(&self) -> &Coord {
+        debug_assert!(!self.body.is_empty());
+        self.body.front().expect("not empty")
+    }
+}