1 files changed, 109 insertions, 0 deletions
diff --git a/src/bin/day4.rs b/src/bin/day4.rs
new file mode 100644
index 0000000..9a256a7
--- /dev/null
+++ b/src/bin/day4.rs
@@ -0,0 +1,109 @@
+use std::collections::HashMap;
+
+static DAY: u8 = 4;
+
+fn main() {
+    let input = advent::read_lines(DAY);
+    println!("{DAY}a: {}", count_xmas(&input));
+    println!("{DAY}b: {}", 0);
+}
+
+fn get_letter_grid(input: &[String]) -> HashMap<(isize,isize), char> {
+    let mut grid = HashMap::new();
+    for (y, line) in input.iter().enumerate() {
+        for (x, c) in line.chars().enumerate() {
+            grid.insert((x as isize, y as isize), c);
+        }
+    }
+    grid
+}
+
+fn word_search(grid: &HashMap<(isize,isize), char>, x: isize, y: isize, diffx: isize, diffy: isize, word: &str) -> bool {
+    for (i, c) in word.chars().enumerate() {
+        let i = i as isize;
+        if let Some(letter) = grid.get(&(x + i*diffx, y + i*diffy)) {
+            if *letter != c {
+                return false;
+            }
+        } else {
+            return false;
+        }
+    }
+    true
+}
+
+fn word_up(grid: &HashMap<(isize,isize), char>, x: isize, y: isize, word: &str) -> bool {
+    word_search(grid, x, y, 0, -1, word)
+}
+
+fn word_down(grid: &HashMap<(isize,isize), char>, x: isize, y: isize, word: &str) -> bool {
+    word_search(grid, x, y, 0, 1, word)
+}
+
+fn word_left(grid: &HashMap<(isize,isize), char>, x: isize, y: isize, word: &str) -> bool {
+    word_search(grid, x, y, -1, 0, word)
+}
+
+fn word_right(grid: &HashMap<(isize,isize), char>, x: isize, y: isize, word: &str) -> bool {
+    word_search(grid, x, y, 1, 0, word)
+}
+
+fn word_ne(grid: &HashMap<(isize,isize), char>, x: isize, y: isize, word: &str) -> bool {
+    word_search(grid, x, y, 1, -1, word)
+}
+
+fn word_se(grid: &HashMap<(isize,isize), char>, x: isize, y: isize, word: &str) -> bool {
+    word_search(grid, x, y, 1, 1, word)
+}
+
+fn word_sw(grid: &HashMap<(isize,isize), char>, x: isize, y: isize, word: &str) -> bool {
+    word_search(grid, x, y, -1, 1, word)
+}
+
+fn word_nw(grid: &HashMap<(isize,isize), char>, x: isize, y: isize, word: &str) -> bool {
+    word_search(grid, x, y, -1, -1, word)
+}
+
+fn count_xmas(input: &[String]) -> usize {
+    let grid = get_letter_grid(input);
+    let max_y = input.len() as isize;
+    let max_x = input[0].len() as isize;
+    let mut xmas_count = 0;
+
+    for x in 0..max_x {
+        for y in 0..max_y {
+            if word_up(&grid, x, y, "XMAS") { xmas_count += 1; }
+            if word_down(&grid, x, y, "XMAS") { xmas_count += 1; }
+            if word_left(&grid, x, y, "XMAS") { xmas_count += 1; }
+            if word_right(&grid, x, y, "XMAS") { xmas_count += 1; }
+            if word_ne(&grid, x, y, "XMAS") { xmas_count += 1; }
+            if word_se(&grid, x, y, "XMAS") { xmas_count += 1; }
+            if word_sw(&grid, x, y, "XMAS") { xmas_count += 1; }
+            if word_nw(&grid, x, y, "XMAS") { xmas_count += 1; }
+        }
+    }
+
+    xmas_count
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test() {
+        let input = [
+            "MMMSXXMASM",
+            "MSAMXMSMSA",
+            "AMXSXMAAMM",
+            "MSAMASMSMX",
+            "XMASAMXAMM",
+            "XXAMMXXAMA",
+            "SMSMSASXSS",
+            "SAXAMASAAA",
+            "MAMMMXMMMM",
+            "MXMXAXMASX",
+        ].iter().map(|&x| String::from(x)).collect::<Vec<_>>();
+        assert_eq!(count_xmas(&input), 18);
+    }
+}