The final day!!

src/bin/combo_breaker.rs

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+use advent2020::errors::TopLevelError;
+
+fn round(x: usize, subject_number: usize) -> usize {
+    (x * subject_number) % 20201227
+}
+
+fn find_rounds(subject_number: usize, target: usize) -> usize {
+    let mut round_no = 1;
+    let mut value = subject_number;
+
+    while value != target {
+       value = round(value, subject_number);
+       round_no += 1; 
+    }
+
+    round_no
+}
+
+fn compute_key(rounds: usize, subject_number: usize) -> usize {
+    let mut value = 1;
+
+    for _ in 0..rounds {
+        value = round(value, subject_number);
+    }
+
+    value
+}
+
+fn compute_encryption_key(card_public: usize, door_public: usize) -> usize {
+    println!("Computing encryption key for CARD {} / DOOR {}", card_public, door_public);
+    let card_rounds = find_rounds(7, card_public);
+    let door_rounds = find_rounds(7, door_public);
+    println!("  card rounds: {}", card_rounds);
+    println!("  door rounds: {}", door_rounds);
+    let card_key = compute_key(card_rounds, door_public);
+    let door_key = compute_key(door_rounds, card_public);
+    println!("  card key: {}", card_key);
+    println!("  door key: {}", door_key);
+    assert_eq!(card_key, door_key);
+    card_key
+}
+
+fn main() -> Result<(), TopLevelError> {
+    println!("---- TEST CASE ----");
+    let test_key = compute_encryption_key(5764801, 17807724);
+    println!();
+    println!("---- REAL INPUT ----");
+    let real_key = compute_encryption_key(12090988, 240583);
+    println!();
+    Ok(())
+}