From 0c4430ad01b84a4f1e7d44c094830c279d651e61 Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Wed, 13 Dec 2023 18:33:09 +0100
Subject: [PATCH] Implement 2023 day 13 part 2

---
 2023/src/day13.rs | 34 +++++++++++++++++++++++-----------
 1 file changed, 23 insertions(+), 11 deletions(-)

diff --git a/2023/src/day13.rs b/2023/src/day13.rs
index 3fdd3a8..24aef9b 100644
--- a/2023/src/day13.rs
+++ b/2023/src/day13.rs
@@ -15,12 +15,16 @@ impl Symmetry {
     }
 }
 
-fn find_symmetry(grid: Grid<'_>) -> Option<Symmetry> {
+fn find_symmetry(grid: Grid<'_>, differences: usize) -> Option<Symmetry> {
     // Attempt to find a vertical line of reflection first
     for c in 1..grid.width() {
         if grid
             .rows()
-            .all(|row| row[..c].iter().rev().zip(&row[c..]).all(|(a, b)| a == b))
+            .flat_map(|row| row[..c].iter().rev().zip(&row[c..]))
+            .filter(|(a, b)| a != b)
+            .take(differences + 1)
+            .count()
+            == differences
         {
             return Some(Symmetry::Vertical(c as u32));
         }
@@ -30,7 +34,11 @@ fn find_symmetry(grid: Grid<'_>) -> Option<Symmetry> {
         if (0..r)
             .rev()
             .zip(r..grid.height())
-            .all(|(a, b)| grid[a] == grid[b])
+            .flat_map(|(a, b)| grid[a].iter().zip(&grid[b]))
+            .filter(|(a, b)| a != b)
+            .take(differences + 1)
+            .count()
+            == differences
         {
             return Some(Symmetry::Horizontal(r as u32));
         }
@@ -62,19 +70,23 @@ fn parse_grids(input: &[u8]) -> anyhow::Result<Vec<Grid<'_>>> {
     Ok(result)
 }
 
-pub fn part1(input: &[u8]) -> anyhow::Result<String> {
+fn parts_common(input: &[u8], differences: usize) -> anyhow::Result<String> {
     let grids = parse_grids(input)?;
 
     let sum: u32 = grids
         .into_iter()
-        .filter_map(find_symmetry)
+        .filter_map(|grid| find_symmetry(grid, differences))
         .map(Symmetry::value)
         .sum();
     Ok(sum.to_string())
 }
 
-pub fn part2(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+pub fn part1(input: &[u8]) -> anyhow::Result<String> {
+    parts_common(input, 0)
+}
+
+pub fn part2(input: &[u8]) -> anyhow::Result<String> {
+    parts_common(input, 1)
 }
 
 #[cfg(test)]
@@ -88,8 +100,8 @@ mod tests {
         assert_eq!("405", part1(SAMPLE).unwrap());
     }
 
-    // #[test]
-    // fn sample_part2() {
-    //     assert_eq!("525152", part2(SAMPLE).unwrap());
-    // }
+    #[test]
+    fn sample_part2() {
+        assert_eq!("400", part2(SAMPLE).unwrap());
+    }
 }