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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];
pub fn solve(p: Position) -> i32 {
let mut cache = HashMap::with_capacity(30000);
solve_rec(p, -1000, 1000, &mut cache)
}
// the weak solver only tells if the position is a win/lose/draw
// it's faster but less precise
pub fn solve_weak(p: Position) -> i32 {
let mut cache = HashMap::with_capacity(30000);
solve_rec(p, -1, 1, &mut cache)
}
fn solve_rec(p: Position, a: i32, b: i32, cache: &mut HashMap<u64, i32>) -> i32 {
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;
}
}
let mut alpha = a;
let mut beta = b;
if let Some(max_score) = 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 = MIN_SCORE;
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 = -solve_rec(p.play(x), -beta, -alpha, cache);
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;
}
}
cache.insert(p.key(), best);
return best;
}
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