2021: day07: part 2

aoc2021/src/day07.rs

@@ -8,6 +8,7 @@ pub fn run() -> Result<String> {
     let mut res = String::with_capacity(128);
 
     writeln!(res, "part 1: {}", part1(INPUT)?)?;
+    writeln!(res, "part 2: {}", part2(INPUT)?)?;
 
     Ok(res)
 }
@@ -26,10 +27,44 @@ fn part1(input: &str) -> Result<u64> {
     Ok(horizontal_positions
         .iter()
         // TODO: use abs_diff when stabilized
-        .map(|n| n.max(&median) - n.min(&median))
+        .map(|n| abs_diff(*n, median))
         .sum())
 }
 
+fn part2(input: &str) -> Result<u64> {
+    let horizontal_positions = input
+        .trim()
+        .split(',')
+        .map(|n| n.parse::<u64>().context("couldn't parse position"))
+        .collect::<Result<Vec<_>>>()?;
+
+    let min = *horizontal_positions.iter().min().unwrap();
+    let max = *horizontal_positions.iter().max().unwrap();
+
+    let minimum_fuel = (min..=max)
+        .map(|pos| {
+            horizontal_positions
+                .iter()
+                .map(|n| part2_distance(*n, pos))
+                .sum::<f64>()
+                .floor() as u64
+        })
+        .min()
+        .unwrap();
+
+    Ok(minimum_fuel)
+}
+
+fn abs_diff(a: u64, b: u64) -> u64 {
+    a.max(b) - a.min(b)
+}
+
+fn part2_distance(a: u64, b: u64) -> f64 {
+    let distance = abs_diff(a, b) as f64;
+
+    distance * (distance + 1.0) / 2.0
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -45,4 +80,14 @@ mod tests {
     fn part1_real() {
         assert_eq!(part1(INPUT).unwrap(), 340056);
     }
+
+    #[test]
+    fn part2_provided() {
+        assert_eq!(part2(PROVIDED).unwrap(), 168);
+    }
+
+    #[test]
+    fn part2_real() {
+        assert_eq!(part2(INPUT).unwrap(), 96592275);
+    }
 }