3 files changed, 51 insertions, 35 deletions

diff --git a/src/main.rs b/src/main.rs
index e77ce2b..5dde7cb 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -6,7 +6,7 @@ pub mod position;
 pub mod solve;
 
 use position::Position;
-use solve::solve;
+use solve::{solve, solve_weak};
 
 
 fn main() {
@@ -33,9 +33,13 @@ fn main() {
                 let begin = Instant::now();
                 let score = solve(pos);
                 let elapsed = begin.elapsed();
+                println!("{:03}: score: {:3}, time: {:?}\n", total_solve, score, elapsed);
+                // if score != expected_score {
+                //     eprintln!("{:03}: score: {:3} {:3}", total_solve, score, expected_score);
+                // }
+                // assert_eq!(score, expected_score);
                 total_time += elapsed;
                 total_solve += 1;
-                println!("score: {:3} {:3}, time: {:?}\n", score, expected_score, elapsed);
             }
             Err(msg) =>
                 eprintln!("wrong score format {:?}: {}", fields[1], msg),
diff --git a/src/position.rs b/src/position.rs
index 9ed617d..7fd65c2 100644
--- a/src/position.rs
+++ b/src/position.rs
@@ -1,6 +1,7 @@
 pub const HEIGHT:      u64 = 6;
 pub const WIDTH:       u64 = 7;
 pub const FULL_HEIGHT: u64 = HEIGHT + 1;
+pub const MIN_SCORE: i32 = -((WIDTH * HEIGHT) as i32) / 2 + 3;
 
 #[derive(Debug, Eq, PartialEq)]
 enum Cell {
@@ -157,27 +158,27 @@ impl fmt::Debug for Position {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         use Cell::*;
         writeln!(f, "play_count: {}", self.play_count)?;
-        writeln!(f, "{:10}{:10}{:10}", "position", "mask", "player")?;
+        writeln!(f, "{:16}{:16}{:16}", "position", "mask", "player")?;
         for y in (0..FULL_HEIGHT).rev() {
             for x in 0..WIDTH {
                 match self.at(y, x) {
-                    Empty         => write!(f, ".")?,
-                    CurrentPlayer => write!(f, "x")?,
-                    OtherPlayer   => write!(f, "o")?,
+                    Empty         => write!(f, ". ")?,
+                    CurrentPlayer => write!(f, "x ")?,
+                    OtherPlayer   => write!(f, "o ")?,
                 }
             }
             write!(f, "  ")?;
             for x in 0..WIDTH {
                 match self.at(y, x) {
-                    Empty                       => write!(f, ".")?,
-                    CurrentPlayer | OtherPlayer => write!(f, "#")?,
+                    Empty                       => write!(f, ". ")?,
+                    CurrentPlayer | OtherPlayer => write!(f, "# ")?,
                 }
             }
             write!(f, "  ")?;
             for x in 0..WIDTH {
                 match self.at(y, x) {
-                    Empty | OtherPlayer => write!(f, ".")?,
-                    CurrentPlayer       => write!(f, "#")?,
+                    Empty | OtherPlayer => write!(f, ". ")?,
+                    CurrentPlayer       => write!(f, "# ")?,
                 }
             }
             write!(f, "\n")?;
diff --git a/src/solve.rs b/src/solve.rs
index b950ab6..9561ff1 100644
--- a/src/solve.rs
+++ b/src/solve.rs
@@ -1,18 +1,22 @@
 use std::collections::HashMap;
 
-use crate::position::{Position, WIDTH, HEIGHT};
+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, -100000, 100000, &mut cache)
+    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 let Some(score) = cache.get(&p.key()) {
-        return *score;
-    }
     if p.is_draw() {
         return 0;
     }
@@ -25,32 +29,39 @@ fn solve_rec(p: Position, a: i32, b: i32, cache: &mut HashMap<u64, i32>) -> i32
     let mut alpha = a;
     let mut beta = b;
 
-    let max = (((WIDTH * HEIGHT + 1) as i32) - (p.play_count as i32)) / 2;
-
-    if beta > max {
-        beta = max;
-        if alpha >= beta {
-            return beta;
+    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;
+            }
         }
     }
 
-    for x_unordered in 0..(WIDTH as usize) {
-        let x = COLUMNS_ORDER[x_unordered];
-        if !p.is_valid_play(x) {
-            continue
-        }
-
+    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 >= beta {
-            return score;
+        if score > best {
+            best = score;
         }
-
-        if score > alpha {
-            alpha = 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(), alpha);
-    return alpha;
+    cache.insert(p.key(), best);
+    return best;
 }