From e9e7478fe2951c1b709f097efcd1b5381c9f61ce Mon Sep 17 00:00:00 2001
From: Daniel Seiller <earthnuker@gmail.com>
Date: Wed, 16 Dec 2020 20:10:02 +0100
Subject: [PATCH] Day 15 (finally!)

---
 src/day15.rs | 69 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)
 create mode 100644 src/day15.rs

diff --git a/src/day15.rs b/src/day15.rs
new file mode 100644
index 0000000..61a114b
--- /dev/null
+++ b/src/day15.rs
@@ -0,0 +1,69 @@
+use aoc_runner_derive::{aoc, aoc_generator};
+use std::collections::BTreeMap;
+use std::{collections::VecDeque, io::Write};
+type Data = Vec<usize>;
+
+#[derive(Debug)]
+enum Seen {
+    Never,
+    Once(usize),
+    Twice(usize, usize),
+}
+
+impl Default for Seen {
+    fn default() -> Self {
+        Seen::Never
+    }
+}
+
+impl Seen {
+    fn add(&mut self, n: usize) -> usize {
+        *self = match self {
+            Self::Never => Self::Once(n),
+            Self::Once(i) => Self::Twice(*i, n),
+            Self::Twice(_, i) => Self::Twice(*i, n),
+        };
+        return self.diff();
+    }
+
+    fn diff(&self) -> usize {
+        match self {
+            Seen::Never => unreachable!(),
+            Seen::Once(_) => 0,
+            Seen::Twice(a, b) => b - a,
+        }
+    }
+}
+
+#[aoc_generator(day15)]
+pub fn input_generator(input: &str) -> Data {
+    input
+        .split(",")
+        .map(|num| num.parse())
+        .collect::<Result<Vec<usize>, _>>()
+        .unwrap()
+}
+
+fn get_number(start: &[usize], num_turns: usize) -> usize {
+    let mut cache: BTreeMap<usize, Seen> = BTreeMap::new();
+    let mut nums = start.to_vec();
+    let mut num=std::usize::MAX;
+    for i in 1..num_turns {
+        let e = cache.entry(nums[i - 1]).or_default();
+        num=e.add(i);
+        if i >= nums.len() {
+            nums.push(num);
+        }
+    }
+    return num;
+}
+
+#[aoc(day15, part1)]
+pub fn solve_part1(input: &Data) -> usize {
+    return get_number(input, 2020);
+}
+
+#[aoc(day15, part2)]
+pub fn solve_part2(input: &Data) -> usize {
+    return get_number(input, 30_000_000);
+}