Day 22 part 2.

Cargo.lock

@@ -16,6 +16,7 @@ name = "aoc_2019"
 version = "0.1.0"
 dependencies = [
  "itertools",
+ "modinverse",
  "num-integer",
  "regex",
 ]
@@ -47,6 +48,15 @@ version = "2.4.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "308cc39be01b73d0d18f82a0e7b2a3df85245f84af96fdddc5d202d27e47b86a"
 
+[[package]]
+name = "modinverse"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2f62f577f148cc1a9466e7065a22e59466a7d537cceba5e77e57181d0f706633"
+dependencies = [
+ "num-integer",
+]
+
 [[package]]
 name = "num-integer"
 version = "0.1.44"

Cargo.toml

@@ -10,3 +10,4 @@ edition = "2021"
 itertools = "0.10.3"
 regex = "1.5.5"
 num-integer = "0.1.44"
+modinverse = "0.1.1"

src/tasks/day22.rs

@@ -1,14 +1,16 @@
+use modinverse::modinverse;
+use std::collections::HashMap;
 use std::collections::VecDeque;
 
-#[allow(dead_code)]
 pub fn run() {
     let input = std::fs::read_to_string("input/day22.txt").unwrap();
-    task1(input);
+    println!("{}", task2(input, 119315717514047, 2020, 101741582076661));
 }
 
 type Deck = VecDeque<u16>;
+type Size = i128;
 
-fn task1(input: String) {
+fn task1(input: String) -> usize {
     let deck_size = 10_007;
     let mut deck: Deck = (0..deck_size).collect();
     for line in input.lines() {
@@ -31,6 +33,7 @@ fn task1(input: String) {
         .find_map(|(i, card)| if *card == 2019 { Some(i) } else { None });
 
     println!("Result: {:?}", pos_2019);
+    pos_2019.unwrap()
 }
 
 fn into_new_stack(deck: Deck) -> Deck {
@@ -55,3 +58,80 @@ fn deal_with_increment(deck: Deck, n: usize) -> Deck {
     }
     new.into_iter().collect()
 }
+
+fn task2(input: String, deck_size: Size, field_after: Size, num_iterations: usize) -> Size {
+    let shuffles: Vec<String> = input.trim().lines().rev().map(|it| it.into()).collect();
+    let mut results: HashMap<Size, usize> = HashMap::new();
+
+    let mut result = field_after;
+    results.insert(result, 0);
+
+    let mut iteration = 1;
+    while iteration <= num_iterations {
+        result = revert_iteration_find_start(&shuffles, deck_size, result);
+        if let Some(last_iteration) = results.get(&result) {
+            println!("check after {} iterations", iteration);
+            let iterations = iteration - last_iteration;
+            let full_cycles_remaining = (num_iterations - iteration) / iterations;
+            iteration += full_cycles_remaining * iterations;
+        } else {
+            results.insert(result, iteration);
+        }
+        iteration += 1;
+        if iteration % 1000 == 0 {
+            println!("iteration {}", iteration);
+        }
+    }
+
+    result
+}
+
+fn revert_iteration_find_start(shuffles: &Vec<String>, deck_size: Size, field_after: Size) -> Size {
+    let mut current: Size = field_after;
+    for line in shuffles {
+        if line.starts_with("deal into new stack") {
+            current = deck_size - 1 - current;
+        } else if line.starts_with("cut") {
+            let s = line.split("cut ").collect::<Vec<_>>();
+            let n = s[1].parse::<Size>().unwrap();
+            current += n;
+            current += deck_size;
+            current %= deck_size;
+        } else if line.starts_with("deal with increment") {
+            let s = line.split("deal with increment ").collect::<Vec<_>>();
+            let n = s[1].parse::<Size>().unwrap();
+            current = revert_finite_field_multiplikation(deck_size, n, current);
+        }
+    }
+
+    current
+}
+
+fn revert_finite_field_multiplikation(
+    field_length: Size,
+    multiplicator: Size,
+    result: Size,
+) -> Size {
+    let multiplicator_inverse =
+        modinverse(multiplicator, field_length).expect("no modular inverse found");
+    (result * multiplicator_inverse) % field_length
+}
+
+#[cfg(test)]
+mod test {
+    use crate::tasks::day22::task1;
+    use crate::tasks::day22::task2;
+
+    #[test]
+
+    fn part1() {
+        let input = std::fs::read_to_string("input/day22.txt").unwrap();
+        assert_eq!(7545, task1(input));
+    }
+
+    #[test]
+    fn part2() {
+        let input = std::fs::read_to_string("input/day22.txt").unwrap();
+        assert_eq!(2019, task2(input, 10007, 7545, 1));
+    }
+}

src/tasks/mod.rs

@@ -16,5 +16,6 @@ pub mod day16;
 pub mod day17;
 pub mod day18;
 pub mod day21;
+#[allow(dead_code)]
 pub mod day22;
 pub mod day24;