Solutions for 2022, as well as 2015-2018 and 2019 up to day 11

2015/day20-infinite_elves_and_infinite_houses/Cargo.toml

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+[package]
+name = "day20-infinite_elves_and_infinite_houses"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

2015/day20-infinite_elves_and_infinite_houses/challenge.txt

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+\--- Day 20: Infinite Elves and Infinite Houses ---
+----------
+
+To keep the Elves busy, Santa has them deliver some presents by hand, door-to-door. He sends them down a street with infinite houses numbered sequentially: `1`, `2`, `3`, `4`, `5`, and so on.
+
+Each Elf is assigned a number, too, and delivers presents to houses based on that number:
+
+* The first Elf (number `1`) delivers presents to every house: `1`, `2`, `3`, `4`, `5`, ....
+* The second Elf (number `2`) delivers presents to every second house: `2`, `4`, `6`, `8`, `10`, ....
+* Elf number `3` delivers presents to every third house: `3`, `6`, `9`, `12`, `15`, ....
+
+There are infinitely many Elves, numbered starting with `1`. Each Elf delivers presents equal to *ten times* his or her number at each house.
+
+So, the first nine houses on the street end up like this:
+
+```
+House 1 got 10 presents.
+House 2 got 30 presents.
+House 3 got 40 presents.
+House 4 got 70 presents.
+House 5 got 60 presents.
+House 6 got 120 presents.
+House 7 got 80 presents.
+House 8 got 150 presents.
+House 9 got 130 presents.
+
+```
+
+The first house gets `10` presents: it is visited only by Elf `1`, which delivers `1 * 10 = 10` presents. The fourth house gets `70` presents, because it is visited by Elves `1`, `2`, and `4`, for a total of `10 + 20 + 40 = 70` presents.
+
+What is the *lowest house number* of the house to get at least as many presents as the number in your puzzle input?
+
+Your puzzle answer was `665280`.
+
+\--- Part Two ---
+----------
+
+The Elves decide they don't want to visit an infinite number of houses. Instead, each Elf will stop after delivering presents to `50` houses. To make up for it, they decide to deliver presents equal to *eleven times* their number at each house.
+
+With these changes, what is the new *lowest house number* of the house to get at least as many presents as the number in your puzzle input?
+
+Your puzzle answer was `705600`.
+
+Both parts of this puzzle are complete! They provide two gold stars: \*\*
+
+At this point, all that is left is for you to [admire your Advent calendar](/2015).
+
+Your puzzle input was `29000000`.

2015/day20-infinite_elves_and_infinite_houses/src/lib.rs

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+pub fn run(input: usize) -> (usize, usize) {
+    let first = presents_count_for_house(input, 10, None);
+    let second = presents_count_for_house(input, 11, Some(50));
+    (first, second)
+}
+
+fn presents_count_for_house(number: usize, multiplier: usize, max: Option<usize> ) -> usize {
+    let max = max.unwrap_or(number/multiplier);
+    let size = number/multiplier+1;
+    let mut counts = vec![0; size];
+    (1..=size).for_each(|n| {
+        (n..=size.min(max*n)).step_by(n).for_each(|i| {
+            counts[i-1] += multiplier*n;
+        });
+    });
+    counts.iter().position(|i| *i >= number).unwrap_or(0) + 1
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_sample() {
+        let exptected = [
+            (9, (1, 1)),
+            (10, (1, 1)),
+            (11, (2, 1)),
+            (30, (2, 2)),
+            (40, (3, 3)),
+            (70, (4, 4)),
+            (60, (4, 4)),
+            (120, (6, 6)),
+            (80, (6, 6)),
+            (150, (8, 8)),
+            (130, (8, 6)),
+        ];
+        for (input, output) in exptected {
+            assert_eq!(run(input), output);
+        }
+    }
+
+    #[test]
+    fn test_challenge() {
+        let challenge_input = 29_000_000;
+        assert_eq!(run(challenge_input), (665280, 705600));
+    }
+}