1 files changed, 60 insertions, 0 deletions
diff --git a/src/bin/day7.rs b/src/bin/day7.rs
new file mode 100644
index 0000000..b6b8942
--- /dev/null
+++ b/src/bin/day7.rs
@@ -0,0 +1,60 @@
+fn main() {
+    let input : Vec<i32> = advent::read_file(7).trim_end()
+                                               .split_terminator(',')
+                                               .map(|n| n.parse::<i32>().unwrap())
+                                               .collect();
+    println!("7a: {}", least_fuel(&input));
+    println!("7b: {}", least_crab_fuel(&input));
+}
+
+fn least_fuel(positions: &[i32]) -> i32 {
+    let min_pos = *positions.iter().min().unwrap();
+    let max_pos = *positions.iter().max().unwrap();
+
+    (min_pos ..= max_pos).map(|pos| fuel_at_pos(positions, pos))
+                         .min()
+                         .unwrap()
+}
+
+fn fuel_at_pos(positions: &[i32], dest: i32) -> i32 {
+    positions.iter()
+             .map(|pos| (pos - dest).abs())
+             .sum()
+}
+
+fn least_crab_fuel(positions: &[i32]) -> i32 {
+    let min_pos = *positions.iter().min().unwrap();
+    let max_pos = *positions.iter().max().unwrap();
+
+    (min_pos ..= max_pos).map(|pos| crab_fuel_at_pos(positions, pos))
+                         .min()
+                         .unwrap()
+}
+
+fn gauss_sum(n: i32) -> i32 {
+    n * (n + 1) / 2
+}
+
+fn crab_fuel_at_pos(positions: &[i32], dest: i32) -> i32 {
+    positions.iter()
+             .map(|pos| gauss_sum((pos - dest).abs()))
+             .sum()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test() {
+        let positions = [16,1,2,0,4,2,7,1,2,14];
+        assert_eq!(fuel_at_pos(&positions, 2), 37);
+        assert_eq!(fuel_at_pos(&positions, 1), 41);
+        assert_eq!(fuel_at_pos(&positions, 3), 39);
+        assert_eq!(fuel_at_pos(&positions, 10), 71);
+        assert_eq!(least_fuel(&positions), 37);
+
+        assert_eq!(crab_fuel_at_pos(&positions, 2), 206);
+        assert_eq!(least_crab_fuel(&positions), 168);
+    }
+}