Implementation day 23 part 1

2020/inputs/23.txt

@@ -0,0 +1 @@
+394618527

2020/src/day23.rs

@@ -2,11 +2,81 @@ use std::io::Read;
 
 use crate::Solution;
 
+fn read_input(input: &mut dyn Read) -> Vec<u8> {
+    let mut buffer = Vec::with_capacity(10);
+
+    input.read_to_end(&mut buffer).unwrap();
+
+    while let Some(&c) = buffer.last() {
+        if c.is_ascii_whitespace() {
+            buffer.pop();
+        } else {
+            break;
+        }
+    }
+
+    for c in buffer.iter_mut() {
+        *c -= b'0';
+    }
+
+    buffer
+}
+
+#[inline]
+fn target_num(mut current: u8, picked_up: &[u8]) -> u8 {
+    loop {
+        let next = if current == 1 { 9 } else { current - 1 };
+
+        if !picked_up.contains(&next) {
+            return next;
+        } else {
+            current = next;
+        }
+    }
+}
+
+fn stringify_state(numbers: &[u8]) -> String {
+    numbers.iter().map(|&c| (c + b'0') as char).collect()
+}
+
+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_pos = numbers.iter().position(|&c| c == target).unwrap();
+
+        numbers[1..=target_pos].rotate_left(3);
+
+        // Move to the new active position
+        numbers.rotate_left(1);
+    }
+
+    let one_pos = numbers.iter().position(|&c| c == 1).unwrap();
+
+    numbers.rotate_left(one_pos);
+
+    stringify_state(&numbers[1..])
+}
+
 #[derive(Default)]
 pub struct Day23;
 
 impl Solution for Day23 {
-    fn part1(&mut self, _input: &mut dyn Read) -> String {
-        todo!()
+    fn part1(&mut self, input: &mut dyn Read) -> String {
+        let mut numbers = read_input(input);
+        play_game(&mut numbers, 100)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn sample_part1() {
+        let sample = [3, 8, 9, 1, 2, 5, 4, 6, 7];
+        assert_eq!("92658374", &play_game(&mut sample.clone(), 10));
+        assert_eq!("67384529", &play_game(&mut sample.clone(), 100));
     }
 }