day4 part2

day04/src/main.rs

@@ -1,3 +1,5 @@
+#![feature(let_chains)]
+
 static MAS: [char; 3] = ['M', 'A', 'S'];
 static DIRS: [Dir; 8] = [
     Dir::N,
@@ -50,8 +52,48 @@ fn find_xmas(board: &Board, loc: (usize, usize), dir: Dir) -> bool {
     true
 }
 
-fn pt2(_input: &str) -> u32 {
-    0
+fn pt2(input: &str) -> u32 {
+    let mut total = 0;
+    let board = Board::new(input);
+
+    for (r, row) in board.rows.iter().enumerate() {
+        for (c, letter) in row.iter().enumerate() {
+            if letter == &'A' {
+                if find_x_mas(&board, (r, c)) {
+                    total += 1;
+                }
+            }
+        }
+    }
+
+    total
+}
+
+fn find_x_mas(board: &Board, loc: (usize, usize)) -> bool {
+    let one = if let Some(&nw) = (board.next(loc, Dir::NW))
+        && let Some(&se) = board.next(loc, Dir::SE)
+    {
+        if (nw == 'M' && se == 'S') || (nw == 'S' && se == 'M') {
+            true
+        } else {
+            false
+        }
+    } else {
+        false
+    };
+    let two = if let Some(&ne) = (board.next(loc, Dir::NE))
+        && let Some(&sw) = board.next(loc, Dir::SW)
+    {
+        if (ne == 'M' && sw == 'S') || (ne == 'S' && sw == 'M') {
+            true
+        } else {
+            false
+        }
+    } else {
+        false
+    };
+
+    one && two
 }
 
 struct Board {
@@ -108,7 +150,7 @@ impl Dir {
 mod test {
     use super::*;
 
-    static I1: &str = "MMMSXXMASM
+    static INPUT: &str = "MMMSXXMASM
 MSAMXMSMSA
 AMXSXMAAMM
 MSAMASMSMX
@@ -121,9 +163,11 @@ MXMXAXMASX";
 
     #[test]
     fn p1() {
-        assert_eq!(18, pt1(I1));
+        assert_eq!(18, pt1(INPUT));
     }
 
     #[test]
-    fn p2() {}
+    fn p2() {
+        assert_eq!(9, pt2(INPUT));
+    }
 }