day25

input/2020/day25.txt

@@ -0,0 +1,2 @@
+17773298
+15530095

src/day25.rs

@@ -0,0 +1,33 @@
+use crate::split_once;
+use aoc_runner_derive::aoc;
+
+const MOD: usize = 20201227;
+
+#[aoc(day25, part1)]
+fn solve_d25_p1(input: &str) -> usize {
+    let (pubkey1, pubkey2) = split_once(input, "\n").unwrap();
+    let pubkey1: usize = pubkey1.parse().unwrap();
+    let pubkey2: usize = pubkey2.parse().unwrap();
+    let loop_size = loop_size_from_pubkey(pubkey1);
+    encryption_key_from_pubkey(pubkey2, loop_size)
+}
+
+fn encryption_key_from_pubkey(pubkey: usize, loop_size: usize) -> usize {
+    let mut value = 1;
+    for _ in 0..loop_size {
+        value *= pubkey;
+        value %= MOD;
+    }
+    value
+}
+
+fn loop_size_from_pubkey(pubkey: usize) -> usize {
+    let mut loop_size = 0;
+    let mut value = 1;
+    while value != pubkey {
+        loop_size += 1;
+        value *= 7;
+        value %= MOD;
+    }
+    loop_size
+}

src/lib.rs

@@ -15,6 +15,7 @@ pub mod day21;
 pub mod day22;
 pub mod day23;
 pub mod day24;
+pub mod day25;
 pub mod day3;
 pub mod day4;
 pub mod day5;