path: root/src/strategy/mcts.rs

use crate::command::{Action, Command};
use crate::game::{GameBoard, SimulationOutcome};

use std::cmp;
use std::collections::HashMap;
use std::ops::*;
use time::{Duration, PreciseTime};

pub fn choose_move(
    state: &GameBoard,
    previous_root: Option<Node>,
    start_time: PreciseTime,
    max_time: Duration,
) -> (Command, Node) {
    let mut root_node = match previous_root {
        None => Node {
            state: state.clone(),
            score_sum: ScoreSum::new(),
            player_score_sums: [HashMap::new(), HashMap::new()],
            unexplored: mcts_move_combo(state),
            children: HashMap::new(),
        },
        Some(mut node) => node
            .children
            .drain()
            .map(|(_k, n)| n)
            .find(|n| n.state == *state)
            .unwrap_or_else(|| {
                eprintln!("Previous round did not appear in the cache");
                Node {
                    state: state.clone(),
                    score_sum: ScoreSum::new(),
                    player_score_sums: [HashMap::new(), HashMap::new()],
                    unexplored: mcts_move_combo(state),
                    children: HashMap::new(),
                }
            }),
    };

    while start_time.to(PreciseTime::now()) < max_time {
        let _ = mcts(&mut root_node);
    }

    eprintln!("Number of simulations: {}", root_node.score_sum.visit_count);
    for (command, score_sum) in &root_node.player_score_sums[0] {
        eprintln!(
            "{} = {} ({} visits)",
            command,
            score_sum.avg().val,
            score_sum.visit_count
        );
    }

    let chosen_command = best_player_move(&root_node);

    root_node
        .children
        .retain(|[c1, _], _| *c1 == chosen_command);

    (chosen_command, root_node)
}

pub struct Node {
    state: GameBoard,
    score_sum: ScoreSum,
    player_score_sums: [HashMap<Command, ScoreSum>; 2],
    unexplored: Vec<[Command; 2]>,
    children: HashMap<[Command; 2], Node>,
}

#[derive(Clone, Copy, Debug, PartialEq, PartialOrd)]
struct Score {
    val: f32,
}

impl AddAssign for Score {
    fn add_assign(&mut self, other: Self) {
        self.val = self.val + other.val;
    }
}

impl Div<u32> for Score {
    type Output = Self;
    fn div(self, other: u32) -> Self {
        Score {
            val: self.val / other as f32,
        }
    }
}

impl cmp::Eq for Score {}
impl cmp::Ord for Score {
    fn cmp(&self, other: &Score) -> cmp::Ordering {
        self.val
            .partial_cmp(&other.val)
            .unwrap_or(cmp::Ordering::Equal)
    }
}

struct ScoreSum {
    sum: Score,
    visit_count: u32,
}

impl ScoreSum {
    fn new() -> ScoreSum {
        ScoreSum {
            sum: Score { val: 0. },
            visit_count: 0,
        }
    }
    fn with_initial(score: Score) -> ScoreSum {
        ScoreSum {
            sum: score,
            visit_count: 1,
        }
    }
    fn avg(&self) -> Score {
        self.sum / self.visit_count
    }
}

impl AddAssign<Score> for ScoreSum {
    fn add_assign(&mut self, other: Score) {
        self.sum += other;
        self.visit_count = self.visit_count.saturating_add(1);
    }
}

fn mcts(node: &mut Node) -> Score {
    if node.state.outcome != SimulationOutcome::Continue {
        score(&node.state)
    } else if let Some(commands) = node.unexplored.pop() {
        let mut new_state = node.state.clone();
        new_state.simulate(commands);
        let score = rollout(&new_state);
        let unexplored = if new_state.outcome == SimulationOutcome::Continue {
            mcts_move_combo(&new_state)
        } else {
            Vec::new()
        };

        let new_node = Node {
            state: new_state,
            score_sum: ScoreSum::with_initial(score),
            player_score_sums: [HashMap::new(), HashMap::new()],
            unexplored,
            children: HashMap::new(),
        };
        node.children.insert(commands, new_node);

        update(node, commands, score);
        score
    } else {
        let commands = choose_existing(node);
        let score = mcts(
            node.children
                .get_mut(&commands)
                .expect("The existing node hasn't been tried yet"),
        );
        update(node, commands, score);
        score
    }
}

fn mcts_move_combo(state: &GameBoard) -> Vec<[Command; 2]> {
    let player_moves = state.valid_moves(0);
    let opponent_moves = state.valid_moves(1);
    debug_assert!(!player_moves.is_empty(), "No player moves");
    debug_assert!(!opponent_moves.is_empty(), "No opponent moves");

    let mut result = Vec::with_capacity(player_moves.len() * opponent_moves.len());
    for p in &player_moves {
        for o in &opponent_moves {
            result.push([*p, *o]);
        }
    }

    result
}

fn best_player_move(node: &Node) -> Command {
    node.player_score_sums[0]
        .iter()
        .max_by_key(|(_command, score_sum)| score_sum.avg())
        .map(|(command, _score_sum)| *command)
        .unwrap_or_else(|| Command::new(Action::DoNothing))
}

fn score(state: &GameBoard) -> Score {
    Score {
        val: match state.outcome {
            SimulationOutcome::PlayerWon(0) => 3000.,
            SimulationOutcome::PlayerWon(1) => -3000.,
            _ => (state.players[0].score() - state.players[1].score()) as f32,
        },
    }
}

fn rollout(state: &GameBoard) -> Score {
    score(state)
}

fn choose_existing(node: &Node) -> [Command; 2] {
    [choose_one_existing(node, 0), choose_one_existing(node, 1)]
}

fn choose_one_existing(node: &Node, player_index: usize) -> Command {
    let ln_n = (node.score_sum.visit_count as f32).ln();
    let c = 100.;
    let multiplier = if player_index == 0 { 1. } else { -1. };
    node.player_score_sums[player_index]
        .iter()
        .max_by_key(|(_command, score_sum)| {
            (multiplier * (score_sum.avg().val + c * (ln_n / score_sum.visit_count as f32).sqrt()))
                as i32
        })
        .map(|(command, _score_sum)| *command)
        .unwrap_or_else(|| Command::new(Action::DoNothing))
}

fn update(node: &mut Node, commands: [Command; 2], score: Score) {
    *node.player_score_sums[0]
        .entry(commands[0])
        .or_insert_with(ScoreSum::new) += score;
    *node.player_score_sums[1]
        .entry(commands[1])
        .or_insert_with(ScoreSum::new) += score;
    node.score_sum += score;
}