solution: day 14

data/examples/14.txt

@@ -0,0 +1,12 @@
+p=0,4 v=3,-3
+p=6,3 v=-1,-3
+p=10,3 v=-1,2
+p=2,0 v=2,-1
+p=0,0 v=1,3
+p=3,0 v=-2,-2
+p=7,6 v=-1,-3
+p=3,0 v=-1,-2
+p=9,3 v=2,3
+p=7,3 v=-1,2
+p=2,4 v=2,-3
+p=9,5 v=-3,-3

src/bin/14.rs

@@ -0,0 +1,107 @@
+use hashbrown::HashMap;
+use num_integer::Integer;
+use regex::Regex;
+
+fn parse_input(input: &str) -> Vec<((isize, isize), (isize, isize))> {
+    let r = Regex::new(r"p=(\d+),(\d+) v=(-?\d+),(-?\d+)").unwrap();
+
+    let mut parsed = Vec::new();
+
+    for line in input.lines() {
+        let c = r.captures(line).unwrap();
+        let j: isize = c.get(1).unwrap().as_str().parse().unwrap();
+        let i: isize = c.get(2).unwrap().as_str().parse().unwrap();
+        let y: isize = c.get(3).unwrap().as_str().parse().unwrap();
+        let x: isize = c.get(4).unwrap().as_str().parse().unwrap();
+
+        parsed.push(((i, j), (x, y)));
+    }
+
+    parsed
+}
+
+pub fn part_one(input: &str) -> Option<usize> {
+    let parsed = parse_input(input);
+    let mut r = [0; 4];
+
+    let h = 103;
+    let w = 101;
+
+    for ((pi, pj), (vi, vj)) in parsed {
+        let ni = (pi + vi * 100).rem_euclid(h);
+        let nj = (pj + vj * 100).rem_euclid(w);
+
+        if ni < h / 2 && nj > w / 2 {
+            r[0] += 1;
+        }
+
+        if ni < h / 2 && nj < w / 2 {
+            r[1] += 1;
+        }
+
+        if ni > h / 2 && nj < w / 2 {
+            r[2] += 1;
+        }
+
+        if ni > h / 2 && nj > w / 2 {
+            r[3] += 1;
+        }
+    }
+
+    r.into_iter().product::<usize>().into()
+}
+
+pub fn part_two(input: &str) -> Option<usize> {
+    let pv = parse_input(input);
+
+    let h = 103;
+    let w = 101;
+
+    let mut n = 1;
+
+    // hardcoded side lengths of the "easter egg"
+    let th = 33;
+    let tw = 31;
+
+    for ((pi, pj), (vi, vj)) in pv.iter().copied() {
+        let mut m = 1;
+
+        loop {
+            let ni = (pi + vi * m).rem_euclid(h);
+            let nj = (pj + vj * m).rem_euclid(w);
+            if (ni, nj) == (pi, pj) {
+                break;
+            }
+            m += 1;
+        }
+
+        n = n.lcm(&m);
+    }
+
+    let mut sh = HashMap::new();
+    let mut sw = HashMap::new();
+
+    for m in 0..n {
+        sh.clear();
+        sw.clear();
+
+        for ((pi, pj), (vi, vj)) in pv.iter().copied() {
+            let ni = (pi + vi * m).rem_euclid(h);
+            let nj = (pj + vj * m).rem_euclid(w);
+
+            *sw.entry(ni).or_default() += 1_usize;
+            *sh.entry(nj).or_default() += 1_usize;
+        }
+
+        let ch = sh.values().filter(|&&x: &&usize| x >= th).count();
+        let cw = sw.values().filter(|&&x: &&usize| x >= tw).count();
+
+        if ch >= 2 && cw >= 2 {
+            return Some(n as usize);
+        }
+    }
+
+    None
+}
+
+aoc::solution!(14);