diff --git a/2023/src/bin/day05.rs b/2023/src/bin/day05.rs
index ca5e9b9..161f01e 100644
--- a/2023/src/bin/day05.rs
+++ b/2023/src/bin/day05.rs
@@ -1,4 +1,4 @@
-#![feature(test)]
+#![feature(test, iter_array_chunks)]
 extern crate test;
 use std::ops::Range;
 
@@ -7,7 +7,7 @@ use itertools::Itertools;
 
 const DAY: usize = 5;
 type I = i64;
-type Mapping = Vec<(Range<I>, I)>;
+type Mapping = Vec<(Range<I>, Range<I>, I)>;
 type Parsed = (Vec<I>, Vec<Mapping>);
 
 fn parse_input(raw: &str) -> Parsed {
@@ -18,7 +18,8 @@ fn parse_input(raw: &str) -> Parsed {
             g.lines()
                 .skip(1)
                 .map(|l| l.split(' ').map(parse_num).collect_tuple().unwrap())
-                .map(|(dst, src, len)| (src..src + len, dst - src))
+                // We swap the order of dst and src here because it just makes much more sense to me this way.
+                .map(|(dst, src, len)| (src..src + len, dst..dst + len, dst - src))
                 .collect()
         })
         .collect();
@@ -26,15 +27,40 @@ fn parse_input(raw: &str) -> Parsed {
 }
 
 fn resolve(start: I, mappings: &Vec<Mapping>) -> I {
-    mappings.iter().fold(start, |i, map| map.iter().find_map(|(range, offset)| range.contains(&i).then_some(i + offset)).unwrap_or(i))
+    mappings.iter().fold(start, |i, map| map.iter().find_map(|(range, _, offset)| range.contains(&i).then_some(i + offset)).unwrap_or(i))
 }
 
 fn part1((seeds, mappings): &Parsed) -> I {
     seeds.iter().map(|&s| resolve(s, mappings)).min().unwrap()
 }
 
-fn part2(parsed: &Parsed) -> I {
-    unimplemented!()
+fn resolve_backwards(start: I, mappings: &Vec<Mapping>) -> I {
+    mappings.iter().fold(start, |i, map| map.iter().find_map(|(_, range, offset)| range.contains(&i).then_some(i - offset)).unwrap_or(i))
+}
+
+fn part2((seeds, mappings): &Parsed) -> I {
+    let seed_ranges = seeds.iter().array_chunks().map(|[&a, &b]| a..a + b).collect_vec();
+    let mut mappings = mappings.clone();
+    // let smallest_range = mappings.iter().flatten().map(|(r, _, _)| r.try_len().unwrap()).min().unwrap();
+
+    let mut destinations = mappings.remove(mappings.len() - 1);
+    destinations.sort_by_key(|(_, range, _)| range.start);
+    mappings.reverse();
+
+    let mut ranges = Vec::<(Range<I>, I)>::new();
+    for (_, range, offset) in destinations {
+        ranges.push((ranges.last().map(|(r, _)| r.end).unwrap_or(1)..range.start, 0));
+        ranges.push((range, offset));
+    }
+    ranges.into_iter().find_map(|(range, offset)| starting_seed(range, offset, &seed_ranges, &mappings, 1)).unwrap()
+}
+
+fn starting_seed(range: Range<I>, offset: i64, seed_ranges: &Vec<Range<I>>, mappings: &Vec<Mapping>, step: usize) -> Option<i64> {
+    range.step_by(step).find(|&s| {
+        let seed = resolve_backwards(s - offset, &mappings);
+        // If seed == s, the entire resolution didn’t hit a single mapping, so we don’t need to check seeds.
+        seed != s && seed_ranges.iter().any(|r| r.contains(&seed))
+    })
 }
 
 boilerplate! {
@@ -73,9 +99,9 @@ humidity-to-location map:
 56 93 4",
     tests: {
         part1: { TEST_INPUT => 35 },
-        part2: { TEST_INPUT => 0 },
+        part2: { TEST_INPUT => 46 },
     },
     bench1 == 462648396,
-    bench2 == 0,
+    bench2 == 2520479,
     bench_parse: |(v1, v2): &Parsed| (v1.len(), v2.len()) => (20, 7),
 }