Day 19

2 files changed, 38 insertions, 0 deletions

diff --git a/input/day19.txt b/input/day19.txt
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index 0000000..9394d61
--- /dev/null
+++ b/input/day19.txt
@@ -0,0 +1 @@
+3005290
diff --git a/src/bin/day19.rs b/src/bin/day19.rs
new file mode 100644
index 0000000..89013b3
--- /dev/null
+++ b/src/bin/day19.rs
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+use std::io::{self, Read};
+
+#[derive(Clone, Copy)]
+struct Elf {
+    gifts: usize,
+    next: usize,
+}
+
+fn solve(count: usize) -> usize {
+    let mut circle: Vec<Elf> = (0..count).map(|i| Elf { gifts: 1, next: i + 1 }).collect();
+    circle.last_mut().unwrap().next = 0;
+
+    let mut index = 0;
+
+    while circle[index].gifts < count {
+        let next = circle[index].next;
+        circle[index].gifts += circle[next].gifts;
+        circle[next].gifts = 0;
+        circle[index].next = circle[next].next;
+
+        index = circle[index].next;
+    }
+
+    index + 1
+}
+
+fn main() {
+    let mut input = String::new();
+    io::stdin().read_to_string(&mut input).unwrap();
+
+    println!("Part 1: {}", solve(input.trim().parse().unwrap()));
+}
+
+#[test]
+fn part1() {
+    assert_eq!(3, solve(5));
+}