2022 Day 20 Part 1

2022/src/bin/day20.rs

@@ -0,0 +1,86 @@
+use std::collections::VecDeque;
+
+const INPUT: &str = include_str!("../../input/day20.txt");
+
+fn main() {
+    let ciphertext = parse(INPUT);
+
+    let mixed = mix(ciphertext);
+    println!("{}", grove_hash(mixed));
+}
+
+fn parse(input: &str) -> Vec<i32> {
+    aoc::lines(input).map(|s| s.parse().unwrap()).collect()
+}
+
+fn mix(ciphertext: Vec<i32>) -> Vec<i32> {
+    let mut decrypter = Decrypter::new(&ciphertext);
+
+    for (i, &n) in ciphertext.iter().enumerate() {
+        decrypter.mv(i, n);
+    }
+
+    decrypter.read()
+}
+
+fn grove_hash(msg: Vec<i32>) -> i32 {
+    let zero = msg.iter().position(|&n| n == 0).unwrap();
+    let get = |i: usize| -> i32 { msg[(i + zero) % msg.len()] };
+
+    get(1_000) + get(2_000) + get(3_000)
+}
+
+#[derive(Debug, Clone)]
+struct Decrypter {
+    message: Vec<i32>,
+    len: usize,
+
+    ptr: VecDeque<usize>,
+}
+
+impl Decrypter {
+    fn new(ciphertext: &Vec<i32>) -> Self {
+        Self {
+            message: ciphertext.clone(),
+            ptr: (0..ciphertext.len()).collect(),
+            len: ciphertext.len(),
+        }
+    }
+
+    fn read(&self) -> Vec<i32> {
+        self.ptr.iter().map(|&p| self.message[p]).collect()
+    }
+
+    fn mv(&mut self, i: usize, n: i32) {
+        let mut p = self.ptr.iter().position(|&p| p == i).unwrap();
+        if n >= 0 {
+            for _ in 1..=n {
+                let q = (p + 1) % self.len;
+                self.ptr.swap(p, q);
+
+                if q == 0 {
+                    // Swapped off the end, fix.
+                    let extra = self.ptr.pop_back().unwrap();
+                    self.ptr.push_front(extra);
+                    p = 1;
+                } else {
+                    p = q;
+                }
+            }
+        } else {
+            for _ in 1..=(-n) {
+                let q = (p + self.len - 1) % self.len;
+                self.ptr.swap(p, q);
+
+                if q == 0 {
+                    // Swapped off the end, fix.
+                    let extra = self.ptr.pop_front().unwrap();
+                    self.ptr.push_back(extra);
+                    p = self.len - 1;
+                } else {
+                    p = q
+                }
+            }
+        }
+    }
+}