input/2020/day15.txt

@@ -0,0 +1 @@
+1,12,0,20,8,16

src/day15.rs

@@ -0,0 +1,61 @@
+use aoc_runner_derive::aoc;
+
+use std::collections::{hash_map::Entry, HashMap};
+
+fn spoken_word_n(mut spoken: HashMap<usize, usize>, mut last_spoken: usize, n: usize) -> usize {
+    for turn in spoken.len() + 1..=n {
+        //dbg!(turn, last_spoken);
+        last_spoken = match spoken.entry(last_spoken) {
+            Entry::Occupied(mut occupied) => {
+                let difference = turn - 1 - *occupied.get();
+                occupied.insert(turn - 1);
+                difference
+            }
+            Entry::Vacant(vacant) => {
+                vacant.insert(turn - 1);
+                0
+            }
+        }
+    }
+    last_spoken
+}
+
+#[aoc(day15, part1)]
+fn solve_d15_p1(input: &str) -> usize {
+    let mut last_spoken = None;
+    let spoken: HashMap<usize, usize> = input
+        .split(',')
+        .enumerate()
+        .map(|(idx, spoken)| {
+            let spoken = spoken.parse().unwrap();
+            last_spoken = Some(spoken);
+            (spoken, idx + 1)
+        })
+        .collect();
+    spoken_word_n(spoken, last_spoken.unwrap(), 2020)
+}
+
+#[aoc(day15, part2)]
+fn solve_d15_p2(input: &str) -> usize {
+    let mut last_spoken = None;
+    let spoken: HashMap<usize, usize> = input
+        .split(',')
+        .enumerate()
+        .map(|(idx, spoken)| {
+            let spoken = spoken.parse().unwrap();
+            last_spoken = Some(spoken);
+            (spoken, idx + 1)
+        })
+        .collect();
+    spoken_word_n(spoken, last_spoken.unwrap(), 30_000_000)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_part1() {
+        assert_eq!(solve_d15_p1("1,3,2"), 1);
+    }
+}

src/lib.rs

@@ -4,6 +4,7 @@ pub mod day11;
 pub mod day12;
 pub mod day13;
 pub mod day14;
+pub mod day15;
 pub mod day2;
 pub mod day3;
 pub mod day4;