day 5 part 2

2023/day05/src/p2basic.rs

@@ -0,0 +1,61 @@
+use std::{cmp, ops::Range};
+
+pub fn part2(input: &str) -> u64 {
+    let mut lines = input.lines();
+    let seeds = lines
+        .next()
+        .unwrap()
+        .strip_prefix("seeds: ")
+        .unwrap()
+        .split_ascii_whitespace()
+        .map(|n| n.parse::<u64>().unwrap())
+        .collect::<Vec<_>>();
+    let seeds = seeds.chunks(2);
+
+    let maps = super::parse_maps(lines);
+
+    let mut min_loc = u64::MAX;
+
+    let mut current_seeds: Vec<(usize, usize, Range<u64>)> = seeds
+        .map(|seed| (0, 0, seed[0]..(seed[0] + seed[1])))
+        .collect();
+
+    'queue: while let Some((place, range_idx, numbers)) = current_seeds.pop() {
+        if place == maps.len() {
+            // Range is done.
+            min_loc = cmp::min(min_loc, numbers.start);
+            continue;
+        }
+        let ranges = &maps[place];
+        for range in ranges.iter().skip(range_idx) {
+            if range.source_end() <= numbers.start || range.source_start >= numbers.end {
+                // Completely out of range.
+                continue;
+            }
+
+            // Part that falls before this redirection, needs to be queued again.
+            let pre = numbers.start..range.source_start;
+            let in_ = cmp::max(numbers.start, range.source_start)
+                ..cmp::min(numbers.end, range.source_end());
+            let post = range.source_end()..(numbers.end);
+
+            if !pre.is_empty() {
+                current_seeds.push((place, range_idx + 1, pre));
+            }
+            if !post.is_empty() {
+                current_seeds.push((place, range_idx + 1, post));
+            }
+            let offset = in_.start - range.source_start;
+            let new = (range.dest_start + offset)..(range.dest_start + offset + (in_.end - in_.start));
+
+            if !new.is_empty() {
+                current_seeds.push((place + 1, 0, new));
+            }
+            continue 'queue;
+        }
+        // No change, pass unaffected
+        current_seeds.push((place + 1, 0, numbers));
+    }
+
+    min_loc
+}