Apply naked single strategy to solve a NYT easy sudoku.

src/strategy/naked_single.rs

@@ -0,0 +1,33 @@
+use crate::{board::Board, square::Square, strategy::Strategy};
+
+pub struct NakedSingle {
+    index: usize,
+    value: u8,
+}
+
+impl Strategy for NakedSingle {
+    fn find_instance(board: &crate::board::Board) -> Option<Self>
+    where
+        Self: Sized,
+    {
+        board
+            .squares
+            .iter()
+            .enumerate()
+            .filter_map(|(index, square)| match square {
+                Square::Options(marks) => Some((index, marks)),
+                _ => None,
+            })
+            .find(|(_index, marks)| marks.len() == 1)
+            .map(|(index, marks)| NakedSingle {
+                index,
+                value: marks.first_index().unwrap() as u8 + 1,
+            })
+    }
+
+    fn apply(&self, mut board: Board) -> Board {
+        board.squares[self.index] = Square::Value(self.value);
+        board.restrict_marks();
+        board
+    }
+}