Add solutions to day 25

Author: maxdavidson

src/day25/mod.rs

@@ -0,0 +1,35 @@
+pub fn part1(door_public_key: u64, card_public_key: u64) -> u64 {
+    let mut loop_size = 0;
+    let mut public_key = 1;
+
+    while public_key != door_public_key && public_key != card_public_key {
+        public_key = (public_key * 7) % 20201227;
+        loop_size += 1;
+    }
+
+    let subject_number = if public_key == door_public_key {
+        card_public_key
+    } else {
+        door_public_key
+    };
+
+    let mut encryption_key = 1;
+
+    while loop_size != 0 {
+        encryption_key = (encryption_key * subject_number) % 20201227;
+        loop_size -= 1;
+    }
+
+    encryption_key
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn part1_works() {
+        assert_eq!(part1(5764801, 17807724), 14897079);
+        assert_eq!(part1(1614360, 7734663), 5414549);
+    }
+}

src/lib.rs

@@ -22,3 +22,4 @@ pub mod day21;
 pub mod day22;
 pub mod day23;
 pub mod day24;
+pub mod day25;