diff --git a/2017/input/day_15.txt b/2017/input/day_15.txt
new file mode 100644
index 0000000..f05ed96
--- /dev/null
+++ b/2017/input/day_15.txt
@@ -0,0 +1,3 @@
+Generator A starts with 679
+Generator B starts with 771
+
diff --git a/2017/ruby/day_15.rb b/2017/ruby/day_15.rb
new file mode 100644
index 0000000..442a1f1
--- /dev/null
+++ b/2017/ruby/day_15.rb
@@ -0,0 +1,28 @@
+start_a, start_b = ARGF.read.strip.scan(/\d+/).map(&:to_i)
+
+class Generator
+  def initialize(start, factor)
+    @value = start
+    @factor = factor
+  end
+
+  def each
+    return enum_for(__method__) unless block_given?
+
+    loop do
+      @value *= @factor
+      @value %= 2147483647
+      yield @value
+    end
+  end
+end
+
+# start_a, start_b = 65, 8921
+a = Generator.new(start_a, 16807).each.lazy
+b = Generator.new(start_b, 48271).each.lazy
+
+p a.zip(b).take(40_000_000).with_index.count {|(a,b),i|
+  p i if i % 1_000_000 == 0
+  # a.to_s(2)[-16,16] == b.to_s(2)[-16,16]
+  a & 0b1111111111111111 == b & 0b1111111111111111
+}
diff --git a/2017/rust/src/main.rs b/2017/rust/src/main.rs
index bc88755..90c3624 100644
--- a/2017/rust/src/main.rs
+++ b/2017/rust/src/main.rs
@@ -14,12 +14,41 @@ mod day_03;
 mod day_04;
 mod day_05;
 
-fn main() {
-    if let Err(e) = run() {
-        eprintln!("{}", e);
+struct Generator {
+    value: usize,
+    factor: usize,
+}
+
+impl Iterator for Generator {
+    type Item = usize;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.value *= self.factor;
+        self.value %= 2147483647;
+        Some(self.value)
     }
 }
 
+fn main() {
+    let mut a = Generator{value: 679, factor: 16807};
+    let mut b = Generator{value: 771, factor: 48271};
+
+    let answer = a.filter(|a| a % 4 == 0).zip(b.filter(|b| b % 8 == 0))
+        .take(5_000_000)
+        .filter(|&(a,b)| a & 0b1111111111111111 == b & 0b1111111111111111)
+        .count();
+    println!("{}", answer);
+
+// p a.zip(b).take(40_000_000).with_index.count {|(a,b),i|
+  // p i if i % 1_000_000 == 0
+  // a.to_s(2)[-16,16] == b.to_s(2)[-16,16]
+// }
+
+    // if let Err(e) = run() {
+    //     eprintln!("{}", e);
+    // }
+}
+
 fn run() -> Result<(), Error> {
     let mut input = String::new();
     io::stdin().read_to_string(&mut input)?;