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use std::collections::HashMap;
use crate::position::{Position, WIDTH, HEIGHT, MIN_SCORE};
const COLUMNS_ORDER: [u64; 7] = [3, 2, 4, 1, 5, 0, 6];
type Cache = HashMap<u64, i32>;
pub struct Solver {
pub visited: usize,
pub cache: Cache,
}
const CACHE_SIZE: usize = 1 << 20 + 1;
impl Solver {
pub fn new() -> Solver {
Solver {
visited: 0,
cache: Cache::with_capacity(CACHE_SIZE),
}
}
pub fn solve(&mut self, p: Position) -> i32 {
self.solve_rec(p, -1000, 1000)
}
// the weak solver only tells if the position is a win/lose/draw
// it's faster but less precise
pub fn solve_weak(&mut self, p: Position) -> i32 {
self.solve_rec(p, -1, 1)
}
fn solve_rec(&mut self, p: Position, mut alpha: i32, mut beta: i32) -> i32 {
self.visited += 1;
if p.is_draw() {
return 0;
}
for x in 0..WIDTH {
if p.is_valid_play(x) && p.is_winning_play(x) {
return (((WIDTH * HEIGHT + 1) as i32) - (p.play_count as i32)) / 2;
}
}
if let Some(max_score) = self.cache.get(&p.key()) {
// can't return max_score directly
// because the alpha-beta context in the cache may be
// different than the current alpha-beta
if beta > *max_score {
beta = *max_score;
if alpha >= beta {
return beta;
}
}
}
let mut best = alpha;
for x in (0..(WIDTH as usize))
.map(|x| COLUMNS_ORDER[x])
.filter(|x| p.is_valid_play(*x))
{
// using negamax, variante of minimax where:
// max(player1, player2) == -min(-player1, -player2)
let score = -self.solve_rec(p.play(x), -beta, -alpha);
if score > best {
best = score;
// reduce alpha-beta range if found better score
if best > alpha {
alpha = best;
}
// impossible alpha-beta range reached (alpha is supposed to be < to beta)
if alpha >= beta {
return score;
}
}
}
self.cache.insert(p.key(), best);
return best;
}
pub fn reset(&mut self) {
self.visited = 0;
self.cache.clear();
}
}
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