Avoid more allocations in day 7 part 1

2021/src/day07.rs

@@ -4,29 +4,33 @@ 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();
+fn compute_cumulative<'a, I, It>(it: I) -> Vec<usize>
+where
+    I: IntoIterator<IntoIter = It>,
+    It: Iterator<Item = &'a (usize, usize)> + ExactSizeIterator,
+{
+    let mut it = it.into_iter().copied();
+    let mut costs = Vec::with_capacity(it.len());
+    costs.push(0);
 
     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.push(new_cost);
     }
 
     costs
 }
 
-fn read_input(input: &mut dyn Read) -> Vec<usize> {
+fn read_input(input: &mut dyn Read) -> Vec<(usize, usize)> {
     let mut buf = String::new();
     input.read_to_string(&mut buf).unwrap();
 
@@ -38,22 +42,19 @@ fn read_input(input: &mut dyn Read) -> Vec<usize> {
 
     crabs.sort_unstable();
 
-    crabs
+    crabs.into_iter().dedup_with_count().collect()
 }
 
 pub fn part1(input: &mut dyn Read) -> String {
     let crabs = read_input(input);
 
-    let forward_costs = compute_groups(&crabs);
-    let backwards_costs = compute_groups(crabs.iter().rev());
+    let forwards_costs = compute_cumulative(&crabs);
+    let backwards_costs = compute_cumulative(crabs.iter().rev());
 
-    backwards_costs
-        .into_iter()
-        .rev()
-        .zip(forward_costs)
-        .map(|((_, cost_b), (_, cost_f))| cost_f + cost_b)
-        .min()
-        .unwrap()
+    // Note: the optimal position can be proven to be one of the original positions.
+    ternary_search(0, forwards_costs.len() - 1, |idx| {
+        forwards_costs[idx] + backwards_costs[backwards_costs.len() - 1 - idx]
+    })
     .to_string()
 }
 
@@ -72,13 +73,13 @@ fn cost_at(pos: usize, groups: &[(usize, usize)]) -> usize {
         .sum()
 }
 
-fn ternary_search(groups: &[(usize, usize)], mut min: usize, mut max: usize) -> usize {
+fn ternary_search(mut min: usize, mut max: usize, callback: impl Fn(usize) -> usize) -> usize {
     while max - min > 6 {
         let mid1 = min + (max - min) / 3;
         let mid2 = max - (max - min) / 3;
 
-        let cost1 = cost_at(mid1, groups);
-        let cost2 = cost_at(mid2, groups);
+        let cost1 = callback(mid1);
+        let cost2 = callback(mid2);
 
         if cost1 < cost2 {
             max = mid2 - 1
@@ -88,17 +89,16 @@ fn ternary_search(groups: &[(usize, usize)], mut min: usize, mut max: usize) ->
     }
 
     // Ternary search isn't effective at such small intervals so we iterate the remaining part
-    (min..=max).map(|pos| cost_at(pos, groups)).min().unwrap()
+    (min..=max).map(callback).min().unwrap()
 }
 
 pub fn part2(input: &mut dyn Read) -> String {
-    let crabs = read_input(input);
-    let groups: Vec<_> = crabs.into_iter().dedup_with_count().collect();
+    let groups = read_input(input);
 
     let min = groups.first().unwrap().1;
     let max = groups.last().unwrap().1;
 
-    ternary_search(&groups, min, max).to_string()
+    ternary_search(min, max, |pos| cost_at(pos, &groups)).to_string()
 }
 
 #[cfg(test)]