Implement day 7 part 1

2021/src/day07.rs

@@ -1,9 +1,74 @@
 use std::io::Read;
 
-pub fn part1(_input: &mut dyn Read) -> String {
-    todo!()
+use itertools::Itertools;
+
+use crate::common::ordered;
+
+fn compute_groups<'a>(it: impl IntoIterator<Item = &'a usize>) -> Vec<(usize, usize)> {
+    let mut it = it.into_iter().copied().dedup_with_count();
+
+    let (mut population, mut last_pos) = it.next().unwrap();
+    let mut last_cost = 0;
+
+    let mut costs = vec![(last_pos, 0)];
+
+    for (number, pos) in it {
+        let (first, last) = ordered(last_pos, pos);
+        let new_cost = last_cost + population * (last - first);
+
+        costs.push((pos, new_cost));
+
+        population += number;
+        last_pos = pos;
+        last_cost = new_cost;
+    }
+
+    costs
+}
+
+pub fn part1(input: &mut dyn Read) -> String {
+    let mut buf = String::new();
+    input.read_to_string(&mut buf).unwrap();
+
+    let mut crabs: Vec<usize> = buf
+        .trim_end()
+        .split(',')
+        .map(|s| s.parse().unwrap())
+        .collect();
+
+    crabs.sort_unstable();
+
+    let forward_costs = compute_groups(&crabs);
+    let backwards_costs = compute_groups(crabs.iter().rev());
+
+    backwards_costs
+        .into_iter()
+        .rev()
+        .zip(forward_costs)
+        .map(|((pos_b, cost_b), (pos_f, cost_f))| {
+            debug_assert_eq!(pos_f, pos_b);
+
+            cost_f + cost_b
+        })
+        .min()
+        .unwrap()
+        .to_string()
 }
 
 pub fn part2(_input: &mut dyn Read) -> String {
     todo!()
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::test_implementation;
+
+    use super::*;
+
+    const SAMPLE: &[u8] = &*b"16,1,2,0,4,2,7,1,2,14";
+
+    #[test]
+    fn sample_part1() {
+        test_implementation(part1, SAMPLE, 37);
+    }
+}

2021/src/lib.rs

@@ -91,7 +91,7 @@ pub fn get_implementation(day: usize, part2: bool) -> Solution {
 }
 
 #[cfg(test)]
-fn test_implementation(solution: Solution, data: impl AsRef<[u8]>, answer: impl ToString) {
+fn test_implementation(solution: Solution, data: &[u8], answer: impl ToString) {
     let result = solution(&mut data.as_ref());
 
     assert_eq!(answer.to_string(), result);