day-04 part2

2024/day-04/src/part2.rs

@@ -1,9 +1,108 @@
-use miette::Result;
+use itertools::Itertools;
+use miette::{Diagnostic, Result};
+use std::str::FromStr;
+use thiserror::Error;
+
+#[derive(Debug, Clone, Copy)]
+enum Direction {
+    UpRight,   // (1, -1)
+    UpLeft,    // (-1, -1)
+    DownRight, // (1, 1)
+    DownLeft,  // (-1, 1)
+}
+
+impl Direction {
+    const fn get_vector(&self) -> (i32, i32) {
+        match self {
+            Direction::UpRight => (1, -1),
+            Direction::UpLeft => (-1, -1),
+            Direction::DownRight => (1, 1),
+            Direction::DownLeft => (-1, 1),
+        }
+    }
+
+    fn pairs() -> impl Iterator<Item = (Self, Self)> {
+        [Self::UpRight, Self::UpLeft, Self::DownRight, Self::DownLeft]
+            .into_iter()
+            .combinations(2)
+            .map(|pair| (pair[0], pair[1]))
+    }
+}
+
+#[derive(Debug, Error, Diagnostic)]
+enum GridError {
+    #[error("Error parsing")]
+    ParseError,
+}
+
+#[derive(Debug)]
+struct Grid {
+    data: Vec<Vec<char>>,
+    rows: usize,
+    cols: usize,
+}
+
+impl Grid {
+    fn check_direction(&self, row: i32, col: i32, direction: &Direction, word: &str) -> bool {
+        let (dx, dy) = direction.get_vector();
+        let start_row = row - dy;
+        let start_col = col - dx;
+        word.chars().enumerate().all(|(idx, char)| {
+            let new_row = start_row + dy * idx as i32;
+            let new_col = start_col + dx * idx as i32;
+            self.is_valid_position(new_row, new_col)
+                && self.data[new_row as usize][new_col as usize] == char
+        })
+    }
+
+    fn is_valid_position(&self, row: i32, col: i32) -> bool {
+        row >= 0 && row < self.rows as i32 && col >= 0 && col < self.cols as i32
+    }
+
+    fn count_word(&self, word: &str) -> usize {
+        (0..self.rows)
+            .flat_map(|row| {
+                (0..self.cols).filter(move |&col| {
+                    if self.data[row][col] != 'A' {
+                        return false;
+                    }
+                    Direction::pairs().any(|(dir1, dir2)| {
+                        self.check_direction(row as i32, col as i32, &dir1, word)
+                            && self.check_direction(row as i32, col as i32, &dir2, word)
+                    })
+                })
+            })
+            .count()
+    }
+}
+
+impl FromStr for Grid {
+    type Err = GridError;
+    fn from_str(s: &str) -> std::result::Result<Self, Self::Err> {
+        let data = s
+            .lines()
+            .filter(|line| !line.is_empty())
+            .map(|line| line.chars().collect::<Vec<_>>())
+            .collect::<Vec<_>>();
+
+        let rows = data.len();
+        if rows == 0 {
+            return Err(GridError::ParseError);
+        }
+        let cols = data[0].len();
+
+        if data.iter().any(|row| row.len() != cols) {
+            return Err(GridError::ParseError);
+        }
+
+        Ok(Grid { data, rows, cols })
+    }
+}
 
 #[tracing::instrument]
 pub fn process(input: &str) -> Result<usize> {
-    todo!("day xx - part 2");
-    Ok(0)
+    let grid = Grid::from_str(input)?;
+    Ok(grid.count_word("MAS"))
 }
 
 #[cfg(test)]
@@ -12,9 +111,18 @@ mod tests {
 
     #[test]
     fn test_process() -> Result<()> {
-        let input = "";
-        todo!("haven't built test yet");
-        let result = 0;
+        let input = "
+MMMSXXMASM
+MSAMXMSMSA
+AMXSXMAAMM
+MSAMASMSMX
+XMASAMXAMM
+XXAMMXXAMA
+SMSMSASXSS
+SAXAMASAAA
+MAMMMXMMMM
+MXMXAXMASX";
+        let result = 9;
         assert_eq!(process(input)?, result);
         Ok(())
     }