2025 day 5 part 2

Author: timocp

src/y2025/day5.rs

@@ -8,12 +8,10 @@ pub fn parse_input(input: &str) -> Input {
     let mut fresh_ranges = vec![];
     let mut ingredients = vec![];
 
-    for line in input.lines().filter(|&line| line.len() > 0) {
-        if let Some(hyphen) = line.find(|c| c == '-') {
-            fresh_ranges.push((
-                line[0..hyphen].parse().unwrap(),
-                line[(hyphen + 1)..].parse().unwrap(),
-            ));
+    for line in input.lines().filter(|&line| !line.is_empty()) {
+        if let Some(hyphen) = line.find('-') {
+            let range = line.split_at(hyphen);
+            fresh_ranges.push((range.0.parse().unwrap(), (range.1[1..]).parse().unwrap()));
         } else {
             ingredients.push(line.parse().unwrap());
         }
@@ -38,8 +36,45 @@ pub fn part1(input: &Input) -> usize {
         .count()
 }
 
-pub fn part2(input: &Input) -> &str {
-    "unimplemented"
+pub fn part2(input: &Input) -> u64 {
+    let ranges = &input.fresh_ranges;
+    // track an array of unfresh regions an ranges starting with all integers and split as fresh ranges known
+    // i don't know why but i thought this would be easier than tracking fresh explicitly?
+    let mut unfresh: Vec<(u64, u64)> = vec![(0, u64::MAX)];
+    for &(low, high) in ranges {
+        unfresh = unfresh
+            .into_iter()
+            .flat_map(|(start, end)| {
+                if low < start {
+                    if high > end {
+                        vec![]
+                    } else if high >= start {
+                        vec![(high + 1, end)]
+                    } else {
+                        vec![(start, end)]
+                    }
+                } else if low == start {
+                    if high < end {
+                        vec![(high + 1, end)]
+                    } else {
+                        vec![]
+                    }
+                } else if low < end {
+                    if high < end {
+                        vec![(start, low - 1), (high + 1, end)]
+                    } else {
+                        vec![(start, low - 1)]
+                    }
+                } else if low == end {
+                    vec![(start, low - 1)]
+                } else {
+                    vec![(start, end)]
+                }
+            })
+            .collect();
+    }
+
+    u64::MAX - unfresh.iter().map(|r| r.1 - r.0 + 1).sum::<u64>() + 1
 }
 
 #[test]
@@ -59,4 +94,5 @@ fn test() {
 ";
     let input = parse_input(test_input);
     assert_eq!(3, part1(&input));
+    assert_eq!(14, part2(&input));
 }