Add Y-Wing solver

src/layout/cells/cell.rs

@@ -90,6 +90,10 @@ impl Cell {
         PEERS[self.usize()]
     }
 
+    pub const fn sees(&self, other: Cell) -> bool {
+        PEERS[self.usize()].has(other)
+    }
+
     pub const fn label(&self) -> &'static str {
         label_from_index(self.0)
     }

src/puzzle/strategy.rs

@@ -52,4 +52,5 @@ pub enum Strategy {
     Jellyfish,
 
     SinglesChain,
+    YWing,
 }

src/solvers.rs

@@ -9,6 +9,7 @@ pub mod hidden_tuples;
 pub mod intersection_removals;
 pub mod naked_tuples;
 pub mod singles_chains;
+pub mod y_wings;
 
 use crate::layout::*;
 use crate::puzzle::*;

src/solvers/y_wings.rs

@@ -0,0 +1,53 @@
+use super::*;
+
+pub fn find_y_wings(board: &Board) -> Option<Effects> {
+    let mut effects = Effects::new();
+
+    let bi_values = board.cells_with_n_candidates(2);
+    let mut log = false;
+
+    println!("bi_values: {}", bi_values);
+
+    bi_values.iter().for_each(|cell| {
+        let (k1, k2) = board.candidates(cell).as_pair().unwrap();
+        let peers = cell.peers() & bi_values;
+        if peers.size() < 2 {
+            return;
+        }
+
+        log = cell.label() == "A2";
+
+        let k1_peers = peers & board.candidate_cells(k1);
+        let k2_peers = peers & board.candidate_cells(k2);
+
+        if log {
+            println!("{}: {}-{}: {}-{}", cell, k1, k2, k1_peers, k2_peers)
+        }
+
+        k1_peers.iter().for_each(|c1| {
+            let k1_other = board.candidates(c1) - k1;
+            k2_peers.iter().for_each(|c2| {
+                let k2_other = board.candidates(c2) - k2;
+                if k1_other != k2_other || c1.sees(c2) {
+                    return;
+                }
+
+                let k = k1_other.iter().next().unwrap();
+                let erase = c1.peers() & c2.peers() & board.candidate_cells(k);
+                if erase.is_empty() {
+                    return;
+                }
+
+                let mut action = Action::new(Strategy::YWing);
+                action.erase_cells(erase, k);
+                effects.add_action(action);
+            });
+        });
+    });
+
+    if effects.has_actions() {
+        Some(effects)
+    } else {
+        None
+    }
+}