1 files changed, 50 insertions, 8 deletions

diff --git a/src/bin/day13.rs b/src/bin/day13.rs
index 6f6ac97..02045ea 100644
--- a/src/bin/day13.rs
+++ b/src/bin/day13.rs
@@ -1,12 +1,14 @@
+use std::collections::HashSet;
+
 static DAY: u8 = 13;
 
 fn main() {
     let input = advent::read_lines(DAY);
     println!("{DAY}a: {}", summarize_patterns(&input));
-    println!("{DAY}b: {}", 0);
+    println!("{DAY}b: {}", summarize_patterns_with_smudge(&input));
 }
 
-#[derive(Clone,Copy)]
+#[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
 enum Reflection {
     Vertical(usize),
     Horizontal(usize),
@@ -15,12 +17,13 @@ enum Reflection {
 impl Reflection {
     fn summary(&self) -> usize {
         match *self {
-            Reflection::Vertical(column) => column,
-            Reflection::Horizontal(row) => row * 100,
+            Reflection::Vertical(column) => column + 1,
+            Reflection::Horizontal(row) => (row + 1) * 100,
         }
     }
 }
 
+#[derive(Clone)]
 struct Map {
     pattern: Vec<Vec<bool>>,
 }
@@ -38,6 +41,20 @@ impl Map {
         Map { pattern }
     }
 
+    fn _print_map(&self) {
+        for y in 0 .. self.pattern.len() {
+            for x in 0 .. self.pattern[0].len() {
+                if self.pattern[y][x] {
+                    print!("#");
+                } else {
+                    print!(".");
+                }
+            }
+            println!();
+        }
+        println!();
+    }
+
     fn is_horizontal_reflection(&self, y: usize) -> bool {
         for pair in (y+1 .. self.pattern.len()).zip((0 ..= y).rev()) {
             if self.pattern[pair.0] != self.pattern[pair.1] {
@@ -58,17 +75,34 @@ impl Map {
         true
     }
 
-    fn find_reflection(&self) -> Reflection {
+    fn find_reflections(&self) -> HashSet<Reflection> {
+        let mut reflections = HashSet::new();
         for y in 0 .. self.pattern.len() - 1 {
             if self.is_horizontal_reflection(y) {
-                return Reflection::Horizontal(y+1)
+                reflections.insert(Reflection::Horizontal(y));
             }
         }
         for x in 0 .. self.pattern[0].len() - 1 {
             if self.is_vertical_reflection(x) {
-                return Reflection::Vertical(x+1)
+                reflections.insert(Reflection::Vertical(x));
             }
         }
+        reflections
+    }
+
+    fn find_reflection_with_smudge(&self) -> Reflection {
+        let orig_reflection = self.find_reflections();
+        for y in 0 .. self.pattern.len() {
+            for x in 0 .. self.pattern[0].len() {
+                let mut map = self.clone();
+                map.pattern[y][x] = !map.pattern[y][x];
+                let reflections = map.find_reflections();
+                let new_reflections = reflections.difference(&orig_reflection).collect::<Vec<_>>();
+                if !new_reflections.is_empty() {
+                    return *new_reflections[0];
+                }
+           }
+        }
         panic!("no reflection found");
     }
 }
@@ -76,7 +110,14 @@ impl Map {
 fn summarize_patterns(input: &[String]) -> usize {
     input.split(|line| line.is_empty())
          .map(Map::new)
-         .map(|m| m.find_reflection().summary())
+         .map(|m| m.find_reflections().iter().next().unwrap().summary())
+         .sum()
+}
+
+fn summarize_patterns_with_smudge(input: &[String]) -> usize {
+    input.split(|line| line.is_empty())
+         .map(Map::new)
+         .map(|m| m.find_reflection_with_smudge().summary())
          .sum()
 }
 
@@ -104,5 +145,6 @@ mod tests {
             "#....#..#",
         ].iter().map(|&x| String::from(x)).collect::<Vec<_>>();
         assert_eq!(summarize_patterns(&input), 405);
+        assert_eq!(summarize_patterns_with_smudge(&input), 400);
     }
 }