Implementation day 23 part 2.

2020/src/day23.rs

@@ -1,5 +1,7 @@
 use std::io::Read;
 
+use num_integer::Integer;
+
 use crate::Solution;
 
 fn read_input(input: &mut dyn Read) -> Vec<u8> {
@@ -23,9 +25,16 @@ fn read_input(input: &mut dyn Read) -> Vec<u8> {
 }
 
 #[inline]
-fn target_num(mut current: u8, picked_up: &[u8]) -> u8 {
+fn target_num<T>(mut current: T, maximum: T, picked_up: &[T]) -> T
+where
+    T: Integer + Copy,
+{
     loop {
-        let next = if current == 1 { 9 } else { current - 1 };
+        let next = if current.is_one() {
+            maximum
+        } else {
+            current - T::one()
+        };
 
         if !picked_up.contains(&next) {
             return next;
@@ -43,7 +52,7 @@ fn play_game(numbers: &mut [u8], times: usize) -> String {
     assert!(numbers.len() >= 5); // Helps the compiler
 
     for _ in 0..times {
-        let target = target_num(numbers[0], &numbers[1..4]);
+        let target = target_num(numbers[0], 9, &numbers[1..4]);
         let target_pos = numbers.iter().position(|&c| c == target).unwrap();
 
         numbers[1..=target_pos].rotate_left(3);
@@ -59,6 +68,51 @@ fn play_game(numbers: &mut [u8], times: usize) -> String {
     stringify_state(&numbers[1..])
 }
 
+fn play_game_large(numbers: &[u8], to_play: usize, maximum: u32) -> Vec<u32> {
+    let mut next = vec![0; maximum as usize + 1];
+
+    // Initialize the specified numbers
+    for pair in numbers.windows(2) {
+        next[pair[0] as usize] = pair[1] as u32;
+    }
+
+    if numbers.len() < maximum as usize {
+        // Add the remaining numbers to the loop
+        next[*numbers.last().unwrap() as usize] = 10;
+
+        for (i, pos) in next.iter_mut().enumerate().skip(10) {
+            *pos = i as u32 + 1;
+        }
+
+        // Loop it around until the first spot
+        next[maximum as usize] = numbers[0] as u32;
+    } else {
+        next[*numbers.last().unwrap() as usize] = numbers[0] as u32;
+    }
+
+    let mut active = numbers[0] as u32;
+
+    for _ in 0..to_play {
+        //print_state(active, &next);
+
+        let picked = [
+            next[active as usize],
+            next[next[active as usize] as usize],
+            next[next[next[active as usize] as usize] as usize],
+        ];
+
+        let target = target_num(active, maximum as u32, &picked);
+
+        next[active as usize] = next[picked[2] as usize];
+        next[picked[2] as usize] = next[target as usize];
+        next[target as usize] = picked[0];
+
+        active = next[active as usize];
+    }
+
+    next
+}
+
 #[derive(Default)]
 pub struct Day23;
 
@@ -67,18 +121,34 @@ impl Solution for Day23 {
         let mut numbers = read_input(input);
         play_game(&mut numbers, 100)
     }
+
+    fn part2(&mut self, input: &mut dyn Read) -> String {
+        let numbers = read_input(input);
+
+        let result = play_game_large(&numbers, 10_000_000, 1_000_000);
+
+        let total = result[1] as u64 * result[result[1] as usize] as u64;
+        total.to_string()
+    }
 }
 
 #[cfg(test)]
 mod tests {
+    use crate::test_implementation;
+
     use super::*;
 
     const SAMPLE: &[u8] = b"389125467";
 
     #[test]
     fn sample_part1() {
-        let sample = read_input(&mut SAMPLE.as_ref());
+        let sample = read_input(&mut SAMPLE.clone());
         assert_eq!("92658374", &play_game(&mut sample.clone(), 10));
         assert_eq!("67384529", &play_game(&mut sample.clone(), 100));
     }
+
+    #[test]
+    fn sample_part2() {
+        test_implementation!(Day23, 2, SAMPLE, 149245887792u64);
+    }
 }

2020/src/lib.rs

@@ -72,14 +72,14 @@ macro_rules! test_implementation {
     ($impl:ident, 1, $source:ident, $output:expr) => {
         let mut implementation = $impl::default();
 
-        let result = implementation.part1(&mut $source.as_ref());
+        let result = implementation.part1(&mut $source.clone());
         assert_eq!($output.to_string(), result);
     };
 
     ($impl:ident, 2, $source:ident, $output:expr) => {
         let mut implementation = $impl::default();
 
-        let result = implementation.part2(&mut $source.as_ref());
+        let result = implementation.part2(&mut $source.clone());
         assert_eq!($output.to_string(), result);
     };
 }