day 7 optimizations

- no need to sort thanks to `select_nth_unstable()`
- don't copy the array around

src/day07.rs

@@ -2,10 +2,10 @@ use anyhow::Result;
 
 use crate::utils::parse_by_char;
 
-fn get_median(positions: &[isize]) -> isize {
-    let mut sorted_pos = positions.to_owned();
-    sorted_pos.sort_unstable();
-    sorted_pos[sorted_pos.len() / 2]
+fn get_median(positions: &mut [isize]) -> isize {
+    let median_index = positions.len() / 2;
+    positions.select_nth_unstable(median_index); // quickselect in the standard library!
+    positions[median_index]
 }
 
 fn compute_fuel(positions: &[isize], target: isize, cheap_fuel: bool) -> usize {
@@ -22,7 +22,7 @@ fn compute_fuel(positions: &[isize], target: isize, cheap_fuel: bool) -> usize {
         .sum()
 }
 
-fn find_cheapest_location(positions: &[isize], cheap_fuel: bool) -> usize {
+fn find_cheapest_location(positions: &mut [isize], cheap_fuel: bool) -> usize {
     // find start point
     let start = get_median(positions);
     let mut position = start;
@@ -56,10 +56,10 @@ fn find_cheapest_location(positions: &[isize], cheap_fuel: bool) -> usize {
 }
 
 pub(crate) fn run() -> Result<(usize, usize)> {
-    let data = parse_by_char::<isize>(include_str!("../data/day07.txt"), ',').unwrap();
+    let mut data = parse_by_char::<isize>(include_str!("../data/day07.txt"), ',').unwrap();
     Ok((
-        find_cheapest_location(&data, true),
-        find_cheapest_location(&data, false),
+        find_cheapest_location(&mut data, true),
+        find_cheapest_location(&mut data, false),
     ))
 }
 
@@ -69,15 +69,15 @@ mod tests {
 
     #[test]
     fn test_example() {
-        let data = parse_by_char::<isize>(include_str!("../data/day07-test.txt"), ',').unwrap();
-        assert_eq!(find_cheapest_location(&data, true), 37);
-        assert_eq!(find_cheapest_location(&data, false), 168);
+        let mut data = parse_by_char::<isize>(include_str!("../data/day07-test.txt"), ',').unwrap();
+        assert_eq!(find_cheapest_location(&mut data, true), 37);
+        assert_eq!(find_cheapest_location(&mut data, false), 168);
     }
 
     #[test]
     fn test_my_data() {
-        let data = parse_by_char::<isize>(include_str!("../data/day07.txt"), ',').unwrap();
-        assert_eq!(find_cheapest_location(&data, true), 344138);
-        assert_eq!(find_cheapest_location(&data, false), 94862124);
+        let mut data = parse_by_char::<isize>(include_str!("../data/day07.txt"), ',').unwrap();
+        assert_eq!(find_cheapest_location(&mut data, true), 344138);
+        assert_eq!(find_cheapest_location(&mut data, false), 94862124);
     }
 }