day 7 solution

src/day07.rs

@@ -0,0 +1,79 @@
+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 compute_fuel(positions: &[isize], target: isize, cheap_fuel: bool) -> usize {
+    positions
+        .iter()
+        .map(|p| {
+            let length = (p - target).abs() as usize;
+            if cheap_fuel {
+                length
+            } else {
+                length * (length + 1) / 2
+            }
+        })
+        .sum()
+}
+
+fn find_cheapest_location(positions: &[isize], cheap_fuel: bool) -> usize {
+    // find start point
+    let mut position = get_median(positions);
+    let mut fuel = compute_fuel(positions, position, cheap_fuel);
+
+    // search up
+    loop {
+        let new_fuel = compute_fuel(positions, position + 1, cheap_fuel);
+        if new_fuel > fuel {
+            break;
+        }
+        position += 1;
+        fuel = new_fuel;
+    }
+
+    // search down
+    loop {
+        let new_fuel = compute_fuel(positions, position - 1, cheap_fuel);
+        if new_fuel > fuel {
+            break;
+        }
+        position -= 1;
+        fuel = new_fuel;
+    }
+
+    // done
+    fuel
+}
+
+pub(crate) fn run() -> Result<(usize, usize)> {
+    let data = parse_by_char::<isize>(include_str!("../data/day07.txt"), ',').unwrap();
+    Ok((
+        find_cheapest_location(&data, true),
+        find_cheapest_location(&data, false),
+    ))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[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);
+    }
+
+    #[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);
+    }
+}