2023-13

crates/core/src/year2023/day13.rs

@@ -1,18 +1,73 @@
+use crate::common::array_stack::ArrayStack;
 use crate::input::Input;
 
-pub const fn solve(_input: &Input) -> Result<u64, String> {
-    Ok(0)
+pub fn solve(input: &Input) -> Result<usize, String> {
+    let expected_smudges = input.part_values(0, 1);
+
+    input
+        .text
+        .split("\n\n")
+        .map(|part| {
+            let mut cols = ArrayStack::<24, u32>::new();
+            let mut rows = ArrayStack::<24, u32>::new();
+            let mut num_cols = 0;
+            for (row_idx, row) in part.lines().enumerate() {
+                let mut row_bits = 0;
+                for (col_idx, b) in row.bytes().enumerate() {
+                    if row_idx == 0 {
+                        cols.push(0)?;
+                    }
+                    let col_bits = &mut cols.elements[col_idx];
+                    num_cols = num_cols.max(col_idx);
+                    if b == b'#' {
+                        row_bits |= 1 << col_idx;
+                        *col_bits |= 1 << row_idx;
+                    }
+                }
+                rows.push(row_bits)?;
+            }
+
+            Ok((1..cols.len())
+                .filter(|&col| is_reflection(cols.slice(), col, expected_smudges))
+                .next().unwrap_or_default()
+                + 100
+                    * (1..rows.len())
+                        .filter(|&row| is_reflection(rows.slice(), row, expected_smudges))
+                .next().unwrap_or_default())
+        })
+        .sum()
+}
+
+fn is_reflection(bits: &[u32], cols_to_left: usize, expected_smudges: u32) -> bool {
+    let offset = cols_to_left.min(bits.len() - cols_to_left);
+    let num_smudges: u32 = (0..offset)
+        .map(|o| (bits[cols_to_left - o - 1] ^ bits[cols_to_left + o]).count_ones())
+        .sum();
+    num_smudges == expected_smudges
 }
 
 #[test]
 pub fn tests() {
     use crate::input::{test_part_one_no_allocations, test_part_two_no_allocations};
 
-    let test_input = "0";
-    test_part_one_no_allocations!(test_input => 0);
-    test_part_two_no_allocations!(test_input => 0);
+    let test_input = "#.##..##.
+..#.##.#.
+##......#
+##......#
+..#.##.#.
+..##..##.
+#.#.##.#.
+
+#...##..#
+#....#..#
+..##..###
+#####.##.
+#####.##.
+..##..###
+#....#..#";
+    test_part_one_no_allocations!(test_input => 405);
 
     let real_input = include_str!("day13_input.txt");
-    test_part_one_no_allocations!(real_input => 0);
-    test_part_two_no_allocations!(real_input => 0);
+    test_part_one_no_allocations!(real_input => 32_035);
+    test_part_two_no_allocations!(real_input => 24_847);
 }