diff --git a/src/board.rs b/src/board.rs
index 35ab234..bb3e661 100644
--- a/src/board.rs
+++ b/src/board.rs
@@ -5,7 +5,7 @@ use crate::square::Square;
 
 #[derive(Clone)]
 pub struct Board {
-    squares: [Square; 81],
+    pub squares: [Square; 81],
 }
 
 impl Board {
@@ -63,16 +63,28 @@ impl Board {
         SquareIter::from_row(&self.squares, index / 9)
     }
 
+    pub fn row(&self, row: usize) -> SquareIter {
+        SquareIter::from_row(&self.squares, row)
+    }
+
     pub fn col_from_index(&self, index: usize) -> SquareIter {
         SquareIter::from_col(&self.squares, index % 9)
     }
 
+    pub fn col(&self, col: usize) -> SquareIter {
+        SquareIter::from_col(&self.squares, col)
+    }
+
     pub fn box_from_index(&self, index: usize) -> SquareIter {
         let row = index / 27;
         let col = (index % 9) / 3;
         SquareIter::from_box(&self.squares, (row * 3) + col)
     }
 
+    pub fn boxn(&self, boxn: usize) -> SquareIter {
+        SquareIter::from_box(&self.squares, boxn)
+    }
+
     fn url_puzzle_str(&self) -> String {
         self.squares
             .iter()
diff --git a/src/iterators.rs b/src/iterators.rs
index df6e06a..234c5a3 100644
--- a/src/iterators.rs
+++ b/src/iterators.rs
@@ -28,7 +28,7 @@ fn to_col(squares: &[Square; 81], col: usize) -> [Square; 9] {
     ]
 }
 
-static BOX_ROOTS: [usize; 9] = [0, 3, 6, 27, 30, 36, 54, 57, 60];
+static BOX_ROOTS: [usize; 9] = [0, 3, 6, 27, 30, 33, 54, 57, 60];
 
 fn to_box(squares: &[Square; 81], box_num: usize) -> [Square; 9] {
     let root = BOX_ROOTS[box_num];
diff --git a/src/main.rs b/src/main.rs
index 9669041..d8a3adb 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,8 +1,13 @@
 mod board;
 mod iterators;
 mod square;
+mod strategy;
+mod validator;
+
+use strategy::Strategy;
 
 fn main() {
+    /*
     let nyt_hard = "3...184.9\
                     8.47.....\
                     .....61..\
@@ -12,7 +17,40 @@ fn main() {
                     ...6.1...\
                     17.2.....\
                     5........";
-    let sudoku_board = board::Board::from_string(nyt_hard);
+    */
+    let nyt_easy = "154..627.\
+                    ..817..45\
+                    ..649..1.\
+                    ..9683...\
+                    321...86.\
+                    6....193.\
+                    5..9....3\
+                    .97...1.2\
+                    8...54...";
+    let mut sudoku_board = board::Board::from_string(nyt_easy);
+
+    if let validator::BoardState::Broken(ind) = validator::validate_board(&sudoku_board) {
+        println!("{}", sudoku_board.to_url());
+        panic!(
+            "Error invalid number at row {} col {}",
+            (ind / 9) + 1,
+            (ind % 9) + 1
+        );
+    }
+
+    while let Some(ns) = strategy::naked_single::NakedSingle::find_instance(&sudoku_board) {
+        println!("Applying naked single.");
+        sudoku_board = ns.apply(sudoku_board);
+
+        if let validator::BoardState::Broken(ind) = validator::validate_board(&sudoku_board) {
+            println!("{}", sudoku_board.to_url());
+            panic!(
+                "Error invalid number at row {} col {}",
+                (ind / 9) + 1,
+                (ind % 9) + 1
+            );
+        }
+    }
     println!("{:#?}", sudoku_board);
     println!("{}", sudoku_board.to_url());
 }
diff --git a/src/strategy/base.rs b/src/strategy/base.rs
new file mode 100644
index 0000000..c642395
--- /dev/null
+++ b/src/strategy/base.rs
@@ -0,0 +1,9 @@
+use crate::board::Board;
+
+pub trait Strategy {
+    fn find_instance(board: &Board) -> Option<Self>
+    where
+        Self: Sized;
+
+    fn apply(&self, board: Board) -> Board;
+}
diff --git a/src/strategy/mod.rs b/src/strategy/mod.rs
new file mode 100644
index 0000000..3f03a6f
--- /dev/null
+++ b/src/strategy/mod.rs
@@ -0,0 +1,5 @@
+pub mod base;
+
+pub mod naked_single;
+
+pub use base::Strategy;
diff --git a/src/strategy/naked_single.rs b/src/strategy/naked_single.rs
new file mode 100644
index 0000000..43c960a
--- /dev/null
+++ b/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
+    }
+}
diff --git a/src/validator.rs b/src/validator.rs
new file mode 100644
index 0000000..42ef7f4
--- /dev/null
+++ b/src/validator.rs
@@ -0,0 +1,63 @@
+use crate::{board::Board, square::Square};
+
+pub enum BoardState {
+    Finished,
+    Broken(u8),
+    InProgress,
+}
+
+fn are_unique<T>(iter: T) -> bool
+where
+    T: Iterator<Item = u8>,
+{
+    let mut bitmap: bitmaps::Bitmap<10> = bitmaps::Bitmap::new();
+
+    for item in iter {
+        if bitmap.get(item as usize) {
+            return false;
+        }
+        bitmap.set(item as usize, true);
+    }
+    true
+}
+
+pub fn validate_board(board: &Board) -> BoardState {
+    for r in 0..9 {
+        if !are_unique(board.row(r).filter_map(|sq| match sq {
+            Square::Value(x) => Some(x),
+            _ => None,
+        })) {
+            // TODO: Add the index.
+            return BoardState::Broken(0);
+        }
+    }
+
+    for c in 0..9 {
+        if !are_unique(board.col(c).filter_map(|sq| match sq {
+            Square::Value(x) => Some(x),
+            _ => None,
+        })) {
+            // TODO: Add the index.
+            return BoardState::Broken(0);
+        }
+    }
+
+    for b in 0..9 {
+        if !are_unique(board.boxn(b).filter_map(|sq| match sq {
+            Square::Value(x) => Some(x),
+            _ => None,
+        })) {
+            // TODO: Add the index.
+            return BoardState::Broken(0);
+        }
+    }
+
+    if board.squares.iter().any(|sq| match sq {
+        Square::Options(_) => true,
+        _ => false,
+    }) {
+        BoardState::InProgress
+    } else {
+        BoardState::Finished
+    }
+}