day 15

src/bin/day15.rs

@@ -0,0 +1,53 @@
+use std::collections::HashMap;
+
+const INPUT: &str = "5,2,8,16,18,0,1";
+
+struct Memory {
+    turn: usize,
+    mem: HashMap<usize, usize>,
+    previous: usize,
+}
+
+impl Memory {
+    fn new() -> Self {
+        Self {
+            turn: 0,
+            mem: HashMap::new(),
+            previous: 0,
+        }
+    }
+
+    fn remember(&mut self, value: usize) {
+        if self.turn > 0 {
+            self.mem.insert(self.previous, self.turn);
+        }
+        self.turn += 1;
+        self.previous = value;
+    }
+}
+
+fn main() {
+    let input: Vec<usize> = INPUT.split(",").map(|s| s.parse().unwrap()).collect();
+    let mut mem = Memory::new();
+    for &value in &input {
+        mem.remember(value);
+    }
+    while mem.turn < 2020 {
+        let value = mem.previous;
+        if let Some(&turn) = mem.mem.get(&value) {
+            mem.remember(mem.turn - turn);
+        } else {
+            mem.remember(0);
+        }
+    }
+    println!("{}", mem.previous);
+    while mem.turn < 30000000 {
+        let value = mem.previous;
+        if let Some(&turn) = mem.mem.get(&value) {
+            mem.remember(mem.turn - turn);
+        } else {
+            mem.remember(0);
+        }
+    }
+    println!("{}", mem.previous);
+}