Finish part-2

2025/day-07/src/part2.rs

@@ -1,11 +1,55 @@
-use miette::Result;
+use std::collections::HashMap;
+use tracing::info;
+
+type Column = usize;
+type PathCount = usize;
+
+#[derive(Debug, Clone)]
+struct PathCounts {
+    positions: HashMap<Column, PathCount>,
+}
+
+impl PathCounts {
+    fn new(start_column: Column) -> Self {
+        let mut positions = HashMap::new();
+        positions.insert(start_column, 1);
+        Self { positions }
+    }
+    fn apply_row(&self, line: &str) -> Self {
+        let mut new_positions = HashMap::new();
+
+        for (&column, &count) in &self.positions {
+            if line.as_bytes()[column] == b'^' {
+                info!(column, "split at");
+                *new_positions.entry(column - 1).or_insert(0) += count;
+                *new_positions.entry(column + 1).or_insert(0) += count;
+            } else {
+                *new_positions.entry(column).or_insert(0) += count;
+            }
+        }
+
+        Self {
+            positions: new_positions,
+        }
+    }
+
+    fn total_paths(&self) -> usize {
+        self.positions.values().sum()
+    }
+}
 
 #[tracing::instrument]
 #[allow(clippy::missing_panics_doc)]
 #[allow(clippy::missing_errors_doc)]
-pub fn process(input: &str) -> Result<usize> {
-    todo!("day xx - part 2");
-    Ok(0)
+pub fn process(input: &str) -> miette::Result<usize> {
+    let mut lines = input.lines().enumerate();
+    let (_, first_line) = lines.next().unwrap();
+    let start_column = first_line.chars().position(|ch| ch == 'S').unwrap();
+
+    let final_state = lines.fold(PathCounts::new(start_column), |state, (_, line)| {
+        state.apply_row(line)
+    });
+    Ok(final_state.total_paths())
 }
 
 #[cfg(test)]
@@ -13,10 +57,25 @@ mod tests {
     use super::*;
 
     #[test]
-    fn test_process() -> Result<()> {
-        let input = "";
-        todo!("haven't built test yet");
-        let result = 0;
+    fn test_process() -> miette::Result<()> {
+        let input = ".......S.......
+...............
+.......^.......
+...............
+......^.^......
+...............
+.....^.^.^.....
+...............
+....^.^...^....
+...............
+...^.^...^.^...
+...............
+..^...^.....^..
+...............
+.^.^.^.^.^...^.
+...............
+";
+        let result = 40;
         assert_eq!(process(input)?, result);
         Ok(())
     }