Year 2020, Day 25, Part 1+2

Author: Quantumplation

advent-2020/input/day25.txt

@@ -0,0 +1,2 @@
+8252394
+6269621

advent-2020/src/main.rs

@@ -56,6 +56,8 @@ fn main() -> Result<()> {
     println!("23.2) {:?}", run(2020, "day23.txt", raw,              day23::part2::solve));
     println!("24.1) {:?}", run(2020, "day24.txt", vec_of,           day24::part1::solve));
     println!("24.2) {:?}", run(2020, "day24.txt", vec_of,           day24::part2::solve));
+    println!("25.1) {:?}", run(2020, "day25.txt", vec_of,           day25::part1::solve));
+    println!("25.2) {:?}", run(2020, "day25.txt", vec_of,           day25::part2::solve));
 
     Ok(())
 }

advent-2020/src/solutions/day24.rs

@@ -212,10 +212,8 @@ impl HexagonalGrid {
     }
 
     pub fn step_n(&mut self, n: u32) {
-        println!("Count: {}", self.count_black());
         for _ in 0..n {
             self.step();
-            println!("Count: {}", self.count_black());
         }
     }
 }

advent-2020/src/solutions/day25.rs

@@ -0,0 +1,52 @@
+pub mod part1 {
+    use anyhow::Result;
+
+    use super::*;
+    pub fn solve(numbers: Vec<u64>) -> Result<u64> {
+        Ok(find_encryption_key(numbers[0], numbers[1]))
+    }
+}
+
+pub mod part2 {
+    use anyhow::Result;
+
+    use super::*;
+    pub fn solve(numbers: Vec<u64>) -> Result<String> {
+        Ok("Merry Christmas!".to_string())
+    }
+}
+
+pub fn find_loop_number(subject: u64, target: u64) -> u64 {
+    let mut val = 1;
+    let mut idx = 0;
+    while val != target {
+        idx += 1;
+        val = (val * subject) % 20201227;
+    }
+    return idx;
+}
+
+pub fn find_encryption_key(door: u64, card: u64) -> u64 {
+    let card_loop = find_loop_number(7, card);
+    let mut val = 1;
+    for _ in 0..card_loop {
+        val = (val * door) % 20201227;
+    }
+    return val;
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn example() {
+        assert_eq!(8, find_loop_number(7, 5764801));
+        assert_eq!(11, find_loop_number(7, 17807724));
+        assert_eq!(14897079, find_encryption_key(5764801, 17807724));
+    }
+
+    #[test]
+    fn test() {
+        assert_eq!(181800, find_encryption_key(8252394, 6269621));
+    }
+}

advent-2020/src/solutions/mod.rs

@@ -21,4 +21,5 @@ pub mod day20;
 pub mod day21;
 pub mod day22;
 pub mod day23;
-pub mod day24;
+pub mod day24;
+pub mod day25;