day3 part 2

2023/day3/src/main.rs

@@ -1,3 +1,5 @@
+use std::collections::HashMap;
+
 fn main() {
     let kind = std::env::args().nth(1).unwrap_or("naive".into());
 
@@ -57,27 +59,81 @@ fn part1(input: &str) -> u64 {
 }
 
 fn part2(input: &str) -> u64 {
-    0
+    fn contains_gear(s: &str) -> Option<usize> {
+        s.chars().position(|c| !c.is_ascii_digit() && c != '.')
+    }
+
+    let len = input.lines().next().unwrap().len();
+    let empty_border = std::iter::repeat('.').take(len).collect::<String>();
+
+    let mut prev2 = empty_border.as_str();
+    let mut prev1 = input.lines().next().unwrap();
+
+    let mut gears: HashMap<(usize, usize), Vec<u64>> = HashMap::new();
+
+    for (line_number, next) in input
+        .lines()
+        .skip(1)
+        .chain(Some(empty_border.as_str()))
+        .enumerate()
+    {
+        let mut numbers = prev1.char_indices().peekable();
+        while let Some((start, c)) = numbers.next() {
+            if c.is_ascii_digit() {
+                let mut end = start;
+                while let Some((idx, '0'..='9')) = numbers.next() {
+                    end = idx;
+                }
+
+                let box_bounds = (start.saturating_sub(1))..std::cmp::min(end + 2, len);
+                let number = prev1[start..=end].parse::<u64>().unwrap();
+
+                if let Some(position) = contains_gear(&prev2[box_bounds.clone()]) {
+                    let key = (line_number - 1, box_bounds.start + position);
+                    gears.entry(key).or_default().push(number);
+                }
+
+                if let Some(position) = contains_gear(&prev1[box_bounds.clone()]) {
+                    let key = (line_number, box_bounds.start + position);
+                    gears.entry(key).or_default().push(number);
+                }
+
+                if let Some(position) = contains_gear(&next[box_bounds.clone()]) {
+                    let key = (line_number + 1, box_bounds.start + position);
+                    gears.entry(key).or_default().push(number);
+                }
+            }
+        }
+
+        prev2 = prev1;
+        prev1 = next;
+    }
+
+    gears
+        .values()
+        .filter(|v| v.len() == 2)
+        .map(|v| v[0] * v[1])
+        .sum()
 }
 
 #[cfg(test)]
 mod tests {
     #[test]
     fn part1_small() {
-        assert_eq!(super::part1(include_str!("../input_small.txt")), 8);
+        assert_eq!(super::part1(include_str!("../input_small.txt")), 4361);
     }
 
     #[test]
     fn part1() {
-        assert_eq!(super::part1(include_str!("../input.txt")), 1931);
+        assert_eq!(super::part1(include_str!("../input.txt")), 537832);
     }
 
     #[test]
     fn part2_small() {
-        assert_eq!(super::part2(include_str!("../input_small.txt")), 2286);
+        assert_eq!(super::part2(include_str!("../input_small.txt")), 467835);
     }
     #[test]
     fn part2() {
-        assert_eq!(super::part2(include_str!("../input.txt")), 83105);
+        assert_eq!(super::part2(include_str!("../input.txt")), 81939900);
     }
 }