From 5db6b005f990512544b5a81e01bd0b9e6037a482 Mon Sep 17 00:00:00 2001
From: kageru <kageru@encode.moe>
Date: Thu, 10 Dec 2020 10:56:13 +0100
Subject: [PATCH] Add 2020/10

---
 2020/inputs/day10     |  92 +++++++++++++++++++++++++++++++
 2020/src/bin/day10.rs | 125 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 217 insertions(+)
 create mode 100644 2020/inputs/day10
 create mode 100644 2020/src/bin/day10.rs

diff --git a/2020/inputs/day10 b/2020/inputs/day10
new file mode 100644
index 0000000..fc15d97
--- /dev/null
+++ b/2020/inputs/day10
@@ -0,0 +1,92 @@
+76
+51
+117
+97
+7
+77
+63
+18
+137
+10
+23
+14
+130
+131
+8
+91
+17
+29
+2
+36
+110
+35
+113
+30
+112
+61
+83
+122
+28
+75
+124
+82
+101
+135
+42
+44
+128
+32
+55
+85
+119
+114
+72
+111
+107
+123
+54
+3
+98
+96
+11
+62
+22
+49
+37
+1
+104
+43
+24
+31
+129
+69
+4
+21
+48
+39
+9
+38
+58
+125
+81
+89
+65
+90
+118
+64
+25
+138
+16
+78
+92
+102
+88
+95
+132
+47
+50
+15
+68
+84
+136
+103
diff --git a/2020/src/bin/day10.rs b/2020/src/bin/day10.rs
new file mode 100644
index 0000000..b2ebe98
--- /dev/null
+++ b/2020/src/bin/day10.rs
@@ -0,0 +1,125 @@
+#![feature(test)]
+extern crate test;
+use std::{env, iter};
+
+use itertools::Itertools;
+
+type Parsed = Vec<usize>;
+
+fn read_input() -> String {
+    std::fs::read_to_string(
+        env::args()
+            .nth(1)
+            .filter(|n| n != "--bench")
+            .unwrap_or(String::from("inputs/day10")),
+    )
+    .unwrap()
+}
+
+fn parse_input(raw: &str) -> Parsed {
+    let mut xs: Vec<usize> = raw.lines().map(|l| l.parse().unwrap()).collect();
+    // faster than using sorted() directly on the iterator
+    xs.sort_unstable();
+    xs
+}
+
+fn part1(input: &Parsed) -> (usize, usize) {
+    iter::once(&0)
+        .chain(input.iter())
+        .tuple_windows()
+        .fold((0, 1), |(one, three), (a, b)| match b - a {
+            1 => (one + 1, three),
+            2 => (one, three),
+            3 => (one, three + 1),
+            _ => unreachable!(),
+        })
+}
+
+fn part2(input: &Parsed) -> usize {
+    let max = *input.last().unwrap();
+    let mut paths = vec![0; max + 4];
+    paths[max] = 1;
+    for i in input.iter().rev().skip(1) {
+        let mut n = 0;
+        for j in 1..=3 {
+            n += paths[i + j];
+        }
+        paths[*i] = n;
+    }
+    paths[1] + paths[2] + paths[3]
+}
+
+fn main() {
+    let input = parse_input(&read_input());
+    let (ones, threes) = part1(&input);
+    println!("Part 1: {}", ones * threes);
+    println!("Part 2: {}", part2(&input));
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use test::black_box;
+
+    const TEST_INPUT: &str = "28
+33
+18
+42
+31
+14
+46
+20
+48
+47
+24
+23
+49
+45
+19
+38
+39
+11
+1
+32
+25
+35
+8
+17
+7
+9
+4
+2
+34
+10
+3";
+
+    #[test]
+    fn part1_test() {
+        let input = parse_input(TEST_INPUT);
+        assert_eq!(part1(&input), (22, 10));
+    }
+
+    #[test]
+    fn part2_test() {
+        let input = parse_input(TEST_INPUT);
+        assert_eq!(part2(&input), 19208);
+    }
+
+    #[bench]
+    fn bench_input_parsing(b: &mut test::Bencher) {
+        let raw = read_input();
+        b.iter(|| parse_input(black_box(&raw)))
+    }
+
+    #[bench]
+    fn bench_part1(b: &mut test::Bencher) {
+        let input = parse_input(&read_input());
+        b.iter(|| assert_eq!(part1(black_box(&input)), (69, 24)));
+    }
+
+    #[bench]
+    fn bench_part2(b: &mut test::Bencher) {
+        let input = parse_input(&read_input());
+        b.iter(|| assert_eq!(part2(black_box(&input)), 56693912375296));
+    }
+}