diff --git a/src/bin/predprey/main.rs b/src/bin/predprey/main.rs
index ed83642..2ba9fbe 100644
--- a/src/bin/predprey/main.rs
+++ b/src/bin/predprey/main.rs
@@ -11,6 +11,7 @@ use rand::Rng;
 use pixelfoo::color::Color;
 use pixelfoo::point2d::Point2d;
 use pixelfoo::rect2d::Rect2d;
+use pixelfoo::v2;
 use pixelfoo::vec2d::Vec2d;
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
@@ -21,10 +22,39 @@ enum Square {
     Fox,
 }
 
-type Board = Vec<Vec<Square>>;
+struct Board(Vec<Vec<Square>>);
+
+impl Board {
+    pub fn new(size: Vec2d, mut f: impl FnMut() -> Square) -> Board {
+        Board(
+            repeat_with(|| {
+                repeat_with(&mut f)
+                    .take(size.x as usize)
+                    .collect::<Vec<_>>()
+            })
+            .take(size.y as usize)
+            .collect::<Vec<_>>(),
+        )
+    }
+    pub fn size(&self) -> Vec2d {
+        let x_size = self.0[0].len() as i32;
+        let y_size = self.0.len() as i32;
+        v2!(x_size, y_size)
+    }
+    pub fn rect(&self) -> Rect2d {
+        Rect2d::new(0, self.size().x, 0, self.size().y)
+    }
+    pub fn get(&self, pos: Point2d) -> Square {
+        self.0[pos.y as usize][pos.x as usize]
+    }
+    pub fn set(&mut self, pos: Point2d, sq: Square) {
+        self.0[pos.y as usize][pos.x as usize] = sq;
+    }
+}
 
 fn send<T: Write>(w: &mut T, board: &Board) -> std::io::Result<()> {
-    for line in board {
+    let Board(lines) = board;
+    for line in lines {
         for square in line {
             let c = match square {
                 Square::Empty => Color::blue(),
@@ -41,12 +71,10 @@ fn send<T: Write>(w: &mut T, board: &Board) -> std::io::Result<()> {
 const DEFAULT_ARG: usize = 10;
 
 fn grow(board: &Board, pos: Point2d, grow: Square, die: Square) -> Square {
-    let x_size = board[0].len() as i32;
-    let y_size = board.len() as i32;
     for dir in Vec2d::directions() {
         let pos1 = pos + dir;
-        if 0 <= pos1.x && pos1.x < x_size && 0 <= pos1.y && pos1.y < y_size {
-            let neigh_sq = board[pos1.y as usize][pos1.x as usize];
+        if board.rect().contains(pos1) {
+            let neigh_sq = board.get(pos);
             if neigh_sq == grow {
                 return grow;
             }
@@ -65,35 +93,28 @@ fn main() -> std::io::Result<()> {
     eprintln!("screen size {}x{}, arg {}", x_size, y_size, arg);
 
     let mut rng = thread_rng();
+    let size = v2!(x_size as i32, y_size as i32);
 
     let p_empty = 0.25;
     let p_grass = 0.25;
     let p_rabbit = 0.25;
     // p_fox = 0.05
-    let mut board = repeat_with(|| {
-        repeat_with(|| {
-            let p = rng.gen::<f64>();
-            if p < p_empty {
-                Square::Empty
-            } else if p < p_empty + p_grass {
-                Square::Grass
-            } else if p < p_empty + p_grass + p_rabbit {
-                Square::Rabbit
-            } else {
-                Square::Fox
-            }
-        })
-        .take(x_size)
-        .collect::<Vec<_>>()
-    })
-    .take(y_size)
-    .collect::<Vec<_>>();
+    let mut board = Board::new(size, || {
+        let p = rng.gen::<f64>();
+        if p < p_empty {
+            Square::Empty
+        } else if p < p_empty + p_grass {
+            Square::Grass
+        } else if p < p_empty + p_grass + p_rabbit {
+            Square::Rabbit
+        } else {
+            Square::Fox
+        }
+    });
 
     let t_frame = 0.040; // s
     let delay = Duration::new(0, (1_000_000_000.0 * t_frame) as u32);
 
-    let board_rect = Rect2d::new(0, x_size as i32, 0, y_size as i32);
-
     // mid point of the board
     let x_mid = (x_size - 1) as f64 / 2.0;
     let y_mid = (y_size - 1) as f64 / 2.0;
@@ -103,8 +124,8 @@ fn main() -> std::io::Result<()> {
 
     loop {
         for _ in 0..arg {
-            let pos = board_rect.random_point(&mut rng);
-            let sq = board[pos.y as usize][pos.x as usize];
+            let pos = board.rect().random_point(&mut rng);
+            let sq = board.get(pos);
             let dx = (pos.x as f64 - x_mid as f64) / r as f64;
             let dy = (pos.y as f64 - y_mid as f64) / r as f64;
             let p0 = (dx * dx + dy * dy).min(1.0);
@@ -119,12 +140,15 @@ fn main() -> std::io::Result<()> {
             } else {
                 Square::Empty
             };
-            board[pos.y as usize][pos.x as usize] = match sq {
-                Square::Empty => grow(&board, pos, Square::Grass, new_sq),
-                Square::Grass => grow(&board, pos, Square::Rabbit, new_sq),
-                Square::Rabbit => grow(&board, pos, Square::Fox, new_sq),
-                Square::Fox => new_sq,
-            };
+            board.set(
+                pos,
+                match sq {
+                    Square::Empty => grow(&board, pos, Square::Grass, new_sq),
+                    Square::Grass => grow(&board, pos, Square::Rabbit, new_sq),
+                    Square::Rabbit => grow(&board, pos, Square::Fox, new_sq),
+                    Square::Fox => new_sq,
+                },
+            );
         }
 
         let mut buf = Vec::with_capacity(x_size * y_size * 3);