From 97f5266304a9d380907bdde21cd0c880d93e4144 Mon Sep 17 00:00:00 2001
From: Reiner Herrmann <reiner@reiner-h.de>
Date: Thu, 12 Dec 2024 16:24:41 +0100
Subject: day10 solution 2

---
 src/bin/day10.rs | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

(limited to 'src')

diff --git a/src/bin/day10.rs b/src/bin/day10.rs
index d7c80b3..3a1547e 100644
--- a/src/bin/day10.rs
+++ b/src/bin/day10.rs
@@ -5,7 +5,7 @@ static DAY: u8 = 10;
 fn main() {
     let input = advent::read_lines(DAY);
     println!("{DAY}a: {}", Map::new(&input).trailhead_score());
-    println!("{DAY}b: {}", 0);
+    println!("{DAY}b: {}", Map::new(&input).distinct_trails());
 }
 
 #[derive(Eq, PartialEq, Hash, Copy, Clone)]
@@ -58,11 +58,34 @@ impl Map {
         peaks
     }
 
+    fn count_trails(&self, pos: &Position) -> usize {
+        let height = *self.map.get(pos).unwrap();
+        if height == 9 {
+            return 1;
+        }
+        let mut trails = 0;
+        for (x, y) in [(-1, 0), (1, 0), (0, -1), (0, 1)] {
+            let neighbor = Position { x: pos.x + x, y: pos.y + y };
+            if let Some(height_neighbor) = self.map.get(&neighbor) {
+                if *height_neighbor == height + 1 {
+                    trails += self.count_trails(&neighbor);
+                }
+            }
+        }
+        trails
+    }
+
     fn trailhead_score(&self) -> usize {
         self.trailheads().iter()
                          .map(|trailhead| self.reachable_peaks(trailhead).len())
                          .sum()
     }
+
+    fn distinct_trails(&self) -> usize {
+        self.trailheads().iter()
+                         .map(|trailhead| self.count_trails(trailhead))
+                         .sum()
+    }
 }
 
 #[cfg(test)]
@@ -82,5 +105,6 @@ mod tests {
           "10456732",
         ].iter().map(|&x| String::from(x)).collect::<Vec<_>>();
         assert_eq!(Map::new(&input).trailhead_score(), 36);
+        assert_eq!(Map::new(&input).distinct_trails(), 81);
     }
 }
-- 
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