solution: day 3

data/examples/03.txt

@@ -0,0 +1 @@
+xmul(2,4)&mul[3,7]!^don't()_mul(5,5)+mul(32,64](mul(11,8)undo()?mul(8,5))

src/bin/03.rs

@@ -0,0 +1,93 @@
+use std::iter::once;
+
+use regex::Regex;
+
+fn get_mul_a_b_start(input: &str) -> Vec<(usize, usize, usize)> {
+    let r = Regex::new(r"mul\((\d+),(\d+)\)").unwrap();
+
+    r.captures_iter(input)
+        .map(|x| {
+            let a = x.get(1).unwrap().as_str().parse().unwrap();
+            let b = x.get(2).unwrap().as_str().parse().unwrap();
+            let start = x.get(0).unwrap().start();
+
+            (a, b, start)
+        })
+        .collect()
+}
+
+fn get_do_start(input: &str) -> Vec<usize> {
+    let d = Regex::new(r"do\(\)").unwrap();
+    // instructions are enabled at the beginning
+    once(0)
+        .chain(d.captures_iter(input).map(|x| x.get(0).unwrap().start()))
+        .collect()
+}
+
+fn get_dont_start(input: &str) -> Vec<usize> {
+    let n = Regex::new(r"don't\(\)").unwrap();
+    n.captures_iter(input)
+        .map(|x| x.get(0).unwrap().start())
+        .collect()
+}
+
+pub fn part_one(input: &str) -> Option<usize> {
+    get_mul_a_b_start(input)
+        .into_iter()
+        .map(|(a, b, _)| a * b)
+        .sum::<usize>()
+        .into()
+}
+
+pub fn part_two(input: &str) -> Option<usize> {
+    let ds = get_do_start(input);
+    let ns = get_dont_start(input);
+
+    let mut c = 0;
+    let mut d = None;
+    let mut n = None;
+
+    // increment pointer p while ps[p] < s, ps needs to be sorted
+    let mv_ptr = |ps: &[usize], p: Option<usize>, s| {
+        ps.iter()
+            .enumerate()
+            .skip(p.unwrap_or_default())
+            .take_while(|(_, &x)| x < s)
+            .last()
+            .map(|x| x.0)
+    };
+
+    for (a, b, s) in get_mul_a_b_start(input) {
+        d = mv_ptr(&ds, d, s);
+        n = mv_ptr(&ns, n, s);
+
+        // d is always some becaues we push a `0` to the start of array in `get_do_start` so we
+        // only need to check the (Some(_), Some(_)) case
+        if let (Some(dd), Some(nn)) = (d, n) {
+            if ds[dd] < ns[nn] {
+                continue;
+            }
+        }
+
+        c += a * b;
+    }
+    c.into()
+}
+
+aoc::solution!(3);
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn test_part_one() {
+        assert_eq!(
+            part_one(&aoc::template::read_file("examples", 3)),
+            Some(161)
+        );
+    }
+    #[test]
+    fn test_part_two() {
+        assert_eq!(part_two(&aoc::template::read_file("examples", 3)), Some(48));
+    }
+}