From 1338681cf48e11f34fb73cba25b20ab74dffc713 Mon Sep 17 00:00:00 2001
From: kageru <kageru@encode.moe>
Date: Wed, 13 Dec 2023 12:30:51 +0100
Subject: [PATCH] make p2 linear time

---
 2023/src/bin/day13.rs | 77 ++++++++++++++++++++++++-------------------
 1 file changed, 43 insertions(+), 34 deletions(-)

diff --git a/2023/src/bin/day13.rs b/2023/src/bin/day13.rs
index 90e4420..a74cac9 100644
--- a/2023/src/bin/day13.rs
+++ b/2023/src/bin/day13.rs
@@ -12,13 +12,23 @@ fn parse_input(raw: &str) -> Parsed {
         .collect()
 }
 
-fn find_reflection(block: &[I], blacklist: Option<usize>) -> Option<usize> {
+fn find_reflection<const DEVIATIONS: u32>(block: &[I]) -> Option<usize> {
     'outer: for i in 0..block.len() - 1 {
         let mut offset = 0;
+        let mut deviations = DEVIATIONS;
         loop {
-            match try { (block.get(i.checked_sub(offset)?)?, block.get(i + 1 + offset)?) } {
-                None if Some(i) != blacklist => return Some(i + 1),
-                Some((a, b)) if a == b => offset += 1,
+            match try { block.get(i.checked_sub(offset)?)? ^ block.get(i + 1 + offset)? } {
+                None if deviations == 0 => return Some(i + 1),
+                Some(0) => offset += 1,
+                Some(diff) => {
+                    let diff = diff.count_ones();
+                    if diff > deviations {
+                        continue 'outer;
+                    } else {
+                        offset += 1;
+                        deviations -= diff;
+                    }
+                }
                 _ => continue 'outer,
             }
         }
@@ -26,13 +36,6 @@ fn find_reflection(block: &[I], blacklist: Option<usize>) -> Option<usize> {
     None
 }
 
-fn part1(blocks: &Parsed) -> usize {
-    blocks
-        .iter()
-        .map(|block| find_reflection(block, None).map(|n| n * 100).or_else(|| find_reflection(&transpose(block), None)).unwrap())
-        .sum()
-}
-
 fn transpose(orig: &[I]) -> Vec<I> {
     let max = orig.iter().max().unwrap();
     let len = (I::BITS - max.leading_zeros()) as usize;
@@ -47,31 +50,17 @@ fn transpose(orig: &[I]) -> Vec<I> {
     out
 }
 
+fn part1(blocks: &Parsed) -> usize {
+    blocks
+        .iter()
+        .map(|block| find_reflection::<0>(block).map(|n| n * 100).or_else(|| find_reflection::<0>(&transpose(block))).unwrap())
+        .sum()
+}
+
 fn part2(blocks: &Parsed) -> usize {
     blocks
         .iter()
-        .map(|block| {
-            let mut block = block.to_owned();
-            let mut transposed = transpose(&block);
-            let p1 = find_reflection(&block, None).map(|n| n - 1);
-            let p1_transposed = find_reflection(&transposed, None).map(|n| n - 1);
-            let max = block.iter().max().unwrap();
-            let len = (I::BITS - max.leading_zeros()) as usize;
-            for i in 0..block.len() {
-                for j in 0..len {
-                    block[i] ^= 1 << j;
-                    transposed[j] ^= 1 << i;
-                    if let Some(reflection) =
-                        find_reflection(&block, p1).map(|n| n * 100).or_else(|| find_reflection(&transposed, p1_transposed))
-                    {
-                        return reflection;
-                    }
-                    block[i] ^= 1 << j;
-                    transposed[j] ^= 1 << i;
-                }
-            }
-            unreachable!()
-        })
+        .map(|block| find_reflection::<1>(&block).map(|n| n * 100).or_else(|| find_reflection::<1>(&transpose(&block))).unwrap())
         .sum()
 }
 
@@ -91,15 +80,35 @@ boilerplate! {
 ##......#
 ..#.##.#.
 ..##..##.
-#.#.##.#."
+#.#.##.#.",
+    TEST_INPUT_475 == "\
+#..##....
+.#.##....
+##....##.
+#.#.#.##.
+.####.##.
+###..#..#
+#.#.#.##.
+.#.#.####
+..#.#.##.
+.##..####
+#####.##.
+.#.#.####
+.#...####
+.#.#.####
+.#.#.####
+#####.##.
+.##..####"
     for tests: {
         part1: {
             TEST_INPUT_HORIZONTAL => 400,
             TEST_INPUT_VERTICAL => 5,
+            TEST_INPUT_475 => 7,
         },
         part2: {
             TEST_INPUT_HORIZONTAL => 100,
             TEST_INPUT_VERTICAL => 300,
+            TEST_INPUT_475 => 1300,
         },
     },
     unittests: {