y2015d20

y2015/src/bin/d20.rs

@@ -0,0 +1,14 @@
+use y2015::days::d20;
+
+fn main() {
+    part1();
+    part2();
+}
+
+fn part1() {
+    println!("{}", d20::process_part1(36000000));
+}
+
+fn part2() {
+    println!("{}", d20::process_part2(36000000));
+}

y2015/src/days/d20.rs

@@ -0,0 +1,70 @@
+pub fn process_part1(input: u32) -> u32 {
+    // slow
+    //let mut house = 1;
+    //loop {
+    //    let mut presents = 0;
+    //    for elf in 1..=house {
+    //        if house % elf == 0 {
+    //            presents += elf * 10;
+    //            println!("{house} {presents}");
+    //        }
+    //    }
+    //    if presents >= input {
+    //        break;
+    //    }
+    //    house += 1;
+    //}
+    //house
+    let mut house = 0;
+    loop {
+        house += 1;
+        let mut divisors = get_divisors(house);
+        divisors.push(house);
+        if divisors.iter().sum::<usize>() * 10 >= input as usize {
+            break;
+        }
+    }
+    house as u32
+}
+
+fn get_divisors(n: usize) -> Vec<usize> {
+    let mut divisors = vec![1];
+    let mut potential_divisor = 2;
+    while (potential_divisor * potential_divisor) < n {
+        if n % potential_divisor == 0 {
+            divisors.push(potential_divisor);
+            divisors.push(n / potential_divisor);
+        }
+        potential_divisor += 1;
+    }
+    // This almost made me go mad
+    if potential_divisor * potential_divisor == n {
+        divisors.push(potential_divisor)
+    }
+    divisors
+}
+
+pub fn process_part2(input: u32) -> u32 {
+    let mut house = 0;
+    loop {
+        house += 1;
+        let mut divisors = get_divisors(house);
+        divisors.push(house);
+        let sum = divisors.iter().filter(|&&d| house / d <= 50).sum::<usize>();
+        if sum * 11 >= input as usize {
+            break;
+        }
+    }
+    house as u32
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn part1() {
+        let result = process_part1(100);
+        assert_eq!(result, 6);
+    }
+}

y2015/src/days/mod.rs

@@ -18,3 +18,5 @@ pub mod d8;
 pub mod d9;
 
 pub mod d19;
+
+pub mod d20;