From d3430aa43278d7219acc5a6b84cf3e3f1eeb35c3 Mon Sep 17 00:00:00 2001
From: Tobias Berger <toby@tobot.dev>
Date: Sun, 3 Dec 2023 11:25:49 +0100
Subject: [PATCH] Rust Day 3 Part 2

---
 rust/Cargo.lock          |  2 +-
 rust/Cargo.toml          |  2 +-
 rust/src/day03/part_2.rs | 78 ++++++++++++++++++++++++++++++++++------
 3 files changed, 70 insertions(+), 12 deletions(-)

diff --git a/rust/Cargo.lock b/rust/Cargo.lock
index 7fa7307..60d92cf 100644
--- a/rust/Cargo.lock
+++ b/rust/Cargo.lock
@@ -4,4 +4,4 @@ version = 3
 
 [[package]]
 name = "advent-of-code"
-version = "23.3.1"
+version = "23.3.2"
diff --git a/rust/Cargo.toml b/rust/Cargo.toml
index cb7b221..27a56d8 100644
--- a/rust/Cargo.toml
+++ b/rust/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "advent-of-code"
-version = "23.3.1"
+version = "23.3.2"
 edition = "2021"
 default-run = "day03"
 
diff --git a/rust/src/day03/part_2.rs b/rust/src/day03/part_2.rs
index 928ee24..ef2a914 100644
--- a/rust/src/day03/part_2.rs
+++ b/rust/src/day03/part_2.rs
@@ -1,5 +1,69 @@
-pub(crate) fn part_2(_input: &'static str) -> u64 {
-    todo!("Part 2")
+use std::collections::HashMap;
+
+#[derive(Debug)]
+struct PartNumber {
+    number: u64,
+    adjacent_gear: usize,
+}
+
+pub(crate) fn part_2(input: &'static str) -> u64 {
+    let mut part_numbers = vec![];
+    for (y, line) in input.lines().enumerate() {
+        let mut search_index = 0;
+        loop {
+            let search_line = &line[search_index..];
+            if search_line.is_empty() {
+                break;
+            }
+            let Some(number_start) = search_line.find(|c| char::is_ascii_digit(&c)) else {
+                break;
+            };
+            let number_end = search_line[number_start..]
+                .find(|c| !char::is_ascii_digit(&c))
+                .unwrap_or(search_line.len() - number_start)
+                + number_start;
+
+            let number = search_line[number_start..number_end]
+                .parse()
+                .expect("Should be a valid number");
+
+            let mut gears = vec![];
+            // This `line.len()` assumes the input is square
+            for y in y.saturating_sub(1)..=usize::min(y + 1, line.len() - 1) {
+                for x in (search_index + number_start).saturating_sub(1)
+                    ..=usize::min(search_index + number_end, line.len())
+                {
+                    let idx = y * (line.len() + 1) + x;
+                    let char = input.as_bytes()[idx] as char;
+                    if matches!(char, '*') {
+                        gears.push(idx)
+                    }
+                }
+            }
+
+            part_numbers.extend(gears.iter().map(|&gear| PartNumber {
+                number,
+                adjacent_gear: gear,
+            }));
+            search_index += number_end;
+        }
+    }
+
+    let mut groups = HashMap::new();
+
+    for part_number in part_numbers {
+        let Some(group) = groups.get_mut(&part_number.adjacent_gear) else {
+            groups.insert(part_number.adjacent_gear, vec![part_number]);
+            continue;
+        };
+        group.push(part_number);
+    }
+
+    groups
+        .values()
+        .filter(|group| group.len() == 2)
+        .map(|group| group[0].number * group[1].number)
+        .sum()
 }
 
 #[cfg(test)]
@@ -8,17 +72,11 @@ mod tests {
 
     #[test]
     fn test_with_sample_solution() {
-        assert_eq!(
-            super::part_2(SAMPLE_INPUT),
-            todo!("Add result from example part 2")
-        );
+        assert_eq!(super::part_2(SAMPLE_INPUT), 467835);
     }
 
     #[test]
     fn test_with_solution() {
-        assert_eq!(
-            super::part_2(crate::INPUT),
-            todo!("Add result for solved part 2")
-        );
+        assert_eq!(super::part_2(crate::INPUT), 80703636);
     }
 }