2024-03

crates/core/src/year2024/day03.rs

@@ -1,18 +1,68 @@
 use crate::input::Input;
 
-pub const fn solve(_input: &Input) -> Result<u32, String> {
-    Ok(0)
+pub fn solve(input: &Input) -> Result<u32, String> {
+    Ok(if input.is_part_one() {
+        add_multiplications(input.text)
+    } else {
+        input
+            .text
+            .split("do()")
+            .map(|x| x.find("don't()").map(|idx| &x[..idx]).unwrap_or(x))
+            .map(add_multiplications)
+            .sum()
+    })
+}
+
+fn find_subsequence(haystack: &[u8], needle: &[u8]) -> Option<usize> {
+    haystack
+        .windows(needle.len())
+        .position(|window| window == needle)
+}
+
+fn find_next_num(s: &[u8], followed_by: u8) -> Option<(usize, u32)> {
+    let mut num = 0;
+    for (i, &c) in s.iter().enumerate().take(4.min(s.len())) {
+        if i <= 3 && c.is_ascii_digit() {
+            num = num * 10 + (s[i] - b'0') as u32;
+        } else {
+            return (c == followed_by).then_some((i + 1, num));
+        }
+    }
+    None
+}
+
+fn add_multiplications(input: &str) -> u32 {
+    let mut s = input.as_bytes();
+    let mut sum = 0;
+    while let Some(idx) = find_subsequence(s, b"mul(") {
+        s = &s[(idx + 4)..];
+        if let Some((offset, first_num)) = find_next_num(s, b',') {
+            s = &s[offset..];
+            if let Some((offset, second_num)) = find_next_num(s, b')') {
+                s = &s[offset..];
+                sum += first_num * second_num;
+            }
+        }
+    }
+    sum
 }
 
 #[test]
 pub fn tests() {
     use crate::input::{test_part_one_no_allocations, test_part_two_no_allocations};
 
-    let test_input = "";
+    let test_input = "xmul(2,4)%&mul[3,7]!@^do_not_mul(5,5)+mul(32,64]then(mul(11,8)mul(8,5))";
+    test_part_one_no_allocations!(test_input => 161);
+    let test_input = "xmul(1000,4)";
     test_part_one_no_allocations!(test_input => 0);
-    test_part_two_no_allocations!(test_input => 0);
+    let test_input = "xmul(,4)mul(3,3)";
+    test_part_one_no_allocations!(test_input => 9);
+    let test_input = "xmul(2,4)&mul[3,7]!^don't()_mul(5,5)+mul(32,64](mul(11,8)undo()?mul(8,5))";
+    test_part_two_no_allocations!(test_input => 48);
+    let test_input = "xmul(2,4)";
+    test_part_two_no_allocations!(test_input => 8);
 
     let real_input = include_str!("day03_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 => 181_345_830);
+    test_part_two_no_allocations!(real_input => 98_729_041);
 }