Add 2020/13

2020/inputs/day13

@@ -0,0 +1,2 @@
+1000053
+19,x,x,x,x,x,x,x,x,x,x,x,x,37,x,x,x,x,x,523,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,13,x,x,x,x,23,x,x,x,x,x,29,x,547,x,x,x,x,x,x,x,x,x,41,x,x,x,x,x,x,17

2020/src/bin/day13.rs

@@ -0,0 +1,82 @@
+#![feature(test, destructuring_assignment)]
+extern crate test;
+use aoc2020::common::*;
+use itertools::Itertools;
+
+// Port of the rosetta code Python implementation
+fn chinese_remainder(divs: Vec<i64>, rems: Vec<i64>) -> i64 {
+    fn mul_inv(mut a: i64, mut b: i64) -> i64 {
+        if b == 1 {
+            return 1;
+        }
+        let (mut x0, mut x1, b0) = (0, 1, b);
+        while a > 1 {
+            let q = a / b;
+            (a, b) = (b, a % b);
+            (x0, x1) = (x1 - q * x0, x0);
+        }
+        if x1 < 0 {
+            x1 += b0;
+        }
+        return x1;
+    }
+    let mut sum = 0;
+    let prod: i64 = divs.iter().product();
+    for (div, rem) in divs.iter().zip(rems.iter()) {
+        let p = prod / div;
+        sum += rem * mul_inv(p, *div) * p;
+    }
+    return sum % prod;
+}
+
+type Parsed = (i64, Vec<Option<i64>>);
+
+fn read_input() -> String {
+    read_file(13)
+}
+
+fn parse_input(raw: &str) -> Parsed {
+    let (first, second) = raw.lines().next_tuple().unwrap();
+    (first.parse().unwrap(), second.split(',').map(|n| n.parse().ok()).collect())
+}
+
+fn part1((start, nums): &Parsed) -> i64 {
+    let nums = nums.iter().filter_map(|&n| n).collect_vec();
+    let (eta, line) = (*start..).find_map(|t| nums.iter().find(|&n| t % n == 0).map(|n| (t, n))).unwrap();
+    (eta - start) * line
+}
+
+fn part2((_, lines): &Parsed) -> i64 {
+    let rems = lines.iter().enumerate().filter_map(|(n, l)| l.map(|l| l - n as i64)).collect_vec();
+    let lines = lines.iter().filter_map(|&l| l).collect_vec();
+    chinese_remainder(lines, rems)
+}
+
+fn main() {
+    let input = parse_input(&read_input());
+    println!("Part 1: {}", part1(&input));
+    println!("Part 2: {}", part2(&input));
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use aoc2020::*;
+    use paste::paste;
+    use test::black_box;
+
+    const TEST_INPUT: &str = "939
+7,13,x,x,59,x,31,19";
+
+    #[test]
+    fn chinese_remainder_test() {
+        let divs = vec![3, 5, 7];
+        let rems = vec![2, 3, 2];
+        assert_eq!(chinese_remainder(divs, rems), 23);
+    }
+
+    test!(part1() == 295);
+    test!(part2() == 1068781);
+    bench!(part1() == 102);
+    bench!(part2() == 327300950120029);
+}