Implement day 21.

2018/src/day21.rs

@@ -1,9 +1,48 @@
+use std::collections::HashSet;
 use std::io::Read;
 
 use common::Solution;
 
 #[derive(Default)]
-pub struct Day21 {}
+pub struct Day21 {
+}
+
+struct ValidInputs {
+    f: i64
+}
+
+impl ValidInputs {
+    pub fn new(start: i64) -> Self {
+        ValidInputs {
+            f: start
+        }
+    }
+}
+
+impl Iterator for ValidInputs {
+    type Item = i64;
+
+    fn next(&mut self) -> Option<i64> {
+        let mut f = self.f;
+        let mut e = f | 0x10000;
+        f = 13284195;
+
+        loop {
+            let d = e & 0xff;
+            f += d;
+            f &= 0xffffff;
+            f *= 65899;
+            f &= 0xffffff;
+
+            if 0x100 > e {
+                self.f = f;
+                return Some(f);
+            }
+
+            e >>= 8;
+        }
+    }
+}
 
 impl Day21 {
     pub fn new() -> Self {
@@ -13,13 +52,22 @@ impl Day21 {
 
 impl Solution for Day21 {
     fn part1(&mut self, _input: &mut Read) -> String {
-        unimplemented!()
+        format!("{}", ValidInputs::new(0).next().unwrap())
     }
 
     fn part2(&mut self, _input: &mut Read) -> String {
-        unimplemented!()
+        let inputs = ValidInputs::new(0);
+        let mut seen = HashSet::new();
+        let mut last = None;
+
+        for input in inputs {
+            if seen.contains(&input) {
+                return format!("{}", last.unwrap());
+            } else {
+                last = Some(input);
+                seen.insert(input);
+            }
+        }
+        unreachable!();
     }
 }
-
-#[cfg(test)]
-mod tests {}