day10

src/day10.rs

@@ -0,0 +1,72 @@
+use aoc_runner_derive::aoc;
+
+#[derive(Debug,Copy,Clone,PartialEq,Eq,PartialOrd,Ord)]
+struct Jolts(usize);
+impl std::ops::Add for Jolts {
+    type Output = Jolts;
+    fn add(self, b: Jolts) -> Jolts {
+        Jolts(self.0 + b.0)
+    }
+}
+impl std::ops::Sub for Jolts {
+    type Output = Jolts;
+    fn sub(self, b: Jolts) -> Jolts {
+        Jolts(self.0 - b.0)
+    }
+}
+
+#[aoc(day10, part1)]
+pub fn solve_d10_p1(input: &str) -> usize {
+    let jolts = {
+        let mut jolts: Vec<Jolts> = std::iter::once(Jolts(0))
+            .chain(input.split('\n').map(|x| Jolts(x.parse().unwrap())))
+            .collect();
+        jolts.sort();
+        jolts.push(*jolts.last().unwrap() + Jolts(3)); // The devices built-in adapter is always 3 more than the highest.
+        jolts
+    };
+
+    // Count the number of 1-jolt differences and 3-jolt differences in the
+    // chain.
+    let (j1_diff, j3_diff) =
+        jolts
+            .windows(2)
+            .fold((0, 0), |(j1_diff, j3_diff), entry| match entry {
+                &[a, b] if b - a == Jolts(1) => (j1_diff + 1, j3_diff),
+                &[a, b] if b - a == Jolts(3) => (j1_diff, j3_diff + 1),
+                _ => panic!("fail"),
+            });
+    j1_diff * j3_diff
+}
+
+#[aoc(day10, part2)]
+pub fn solve_d10_p2(input: &str) -> usize {
+    let jolts = {
+        let mut jolts: Vec<Jolts> = std::iter::once(Jolts(0))
+            .chain(input.split('\n').map(|x| Jolts(x.parse().unwrap())))
+            .collect();
+        jolts.sort();
+        jolts.push(*jolts.last().unwrap() + Jolts(3)); // The devices built-in adapter is always 3 more than the highest.
+        jolts
+    };
+
+    // Iterate over the list of jolt value for the adapters. For each entry
+    // store the number of combinations this adapter has that lead back to zero
+    // in `combinations_count`
+    // To calculate the number of combinations the current adapter has you first
+    // determine which are the potential upstream adapters and then sum their
+    // combinations.
+    let mut combinations_count = Vec::with_capacity(jolts.len());
+    combinations_count.push(1);
+    for (idx, current_jolts) in jolts.iter().copied().enumerate().skip(1) {
+        let mut adapter_paths_to_zero = 0;
+
+        for upstream_idx in idx.saturating_sub(3) .. idx {
+            if current_jolts - jolts[upstream_idx] <= Jolts(3) {
+                adapter_paths_to_zero += combinations_count[upstream_idx];
+            }
+        }
+        combinations_count.push(adapter_paths_to_zero);
+    }
+    *combinations_count.last().unwrap()
+}

src/lib.rs

@@ -7,5 +7,6 @@ pub mod day6;
 pub mod day7;
 pub mod day8;
 pub mod day9;
+pub mod day10;
 
 aoc_runner_derive::aoc_lib! { year = 2020 }