From 3ab0f9711d613b156ac7635e94a1202bc7d87a69 Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Tue, 15 Dec 2020 09:06:17 +0100
Subject: [PATCH] Implementation day 15

---
 2020/inputs/15.txt |  1 +
 2020/src/day15.rs  | 53 ++++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 52 insertions(+), 2 deletions(-)
 create mode 100644 2020/inputs/15.txt

diff --git a/2020/inputs/15.txt b/2020/inputs/15.txt
new file mode 100644
index 0000000..86739eb
--- /dev/null
+++ b/2020/inputs/15.txt
@@ -0,0 +1 @@
+1,0,18,10,19,6
diff --git a/2020/src/day15.rs b/2020/src/day15.rs
index fbbf738..a90e653 100644
--- a/2020/src/day15.rs
+++ b/2020/src/day15.rs
@@ -1,12 +1,61 @@
 use std::io::Read;
 
+use crate::common::numbers_and_stuff;
 use crate::Solution;
 
+fn nth_number(start: &[usize], n: usize) -> usize {
+    // Don't want to special case this
+    assert!(n >= start.len());
+
+    let mut history = vec![0; n];
+
+    let mut prev = 0;
+
+    for (i, &n) in start.iter().enumerate() {
+        history[prev] = i;
+        prev = n;
+    }
+
+    for i in start.len()..n {
+        let last_seen = history[prev];
+        history[prev] = i;
+
+        prev = if last_seen == 0 { 0 } else { i - last_seen };
+    }
+
+    prev
+}
+
 #[derive(Default)]
 pub struct Day15;
 
 impl Solution for Day15 {
-    fn part1(&mut self, _input: &mut dyn Read) -> String {
-        todo!()
+    fn part1(&mut self, input: &mut dyn Read) -> String {
+        let numbers: Vec<usize> = numbers_and_stuff(input);
+
+        nth_number(&numbers, 2020).to_string()
+    }
+
+    fn part2(&mut self, input: &mut dyn Read) -> String {
+        let numbers: Vec<usize> = numbers_and_stuff(input);
+
+        nth_number(&numbers, 30000000).to_string()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_nth_number() {
+        assert_eq!(0, nth_number(&[0, 3, 6], 10));
+        assert_eq!(436, nth_number(&[0, 3, 6], 2020));
+        assert_eq!(1, nth_number(&[1, 3, 2], 2020));
+        assert_eq!(10, nth_number(&[2, 1, 3], 2020));
+        assert_eq!(27, nth_number(&[1, 2, 3], 2020));
+        assert_eq!(78, nth_number(&[2, 3, 1], 2020));
+        assert_eq!(438, nth_number(&[3, 2, 1], 2020));
+        assert_eq!(1836, nth_number(&[3, 1, 2], 2020));
     }
 }