From 924c76c3b302c9f7018018dca7429c5202796075 Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Sat, 16 Dec 2017 23:38:17 +0100
Subject: [PATCH] Implement day 15, in Haskell.

---
 2017/README.md          |  4 ++--
 2017/day-15/.gitignore  |  3 +++
 2017/day-15/solution.hs | 22 ++++++++++++++++++++++
 3 files changed, 27 insertions(+), 2 deletions(-)
 create mode 100644 2017/day-15/.gitignore
 create mode 100644 2017/day-15/solution.hs

diff --git a/2017/README.md b/2017/README.md
index a6ec398..87e6583 100644
--- a/2017/README.md
+++ b/2017/README.md
@@ -14,7 +14,7 @@ The current plan, in no particular order:
 - [x] Clojure - [Day 03](./day-03/solution.clj)
 - [ ] Coffeescript
 - [x] Go - [Day 13](./day-13/solution.go)
-- [ ] Haskell
+- [x] Haskell - [Day 15](./day-15/solution.hs)
 - [ ] Java
 - [x] Kotlin - [Day 10](./day-10/solution.kt)
 - [x] Lex - [Day 09](./day-09/solution.l)
@@ -26,7 +26,7 @@ The current plan, in no particular order:
 - [ ] Prolog
 - [ ] Python
 - [x] Ruby - [Day 08](./day-08/solution.rb)
-- [ ] Rust
+- [x] Rust - [Day 14](./day-14/solution.rs)
 - [x] Scala - [Day 11](./day-11/solution.scala)
 - [ ] Scheme
 - [ ] SQL
diff --git a/2017/day-15/.gitignore b/2017/day-15/.gitignore
new file mode 100644
index 0000000..103996e
--- /dev/null
+++ b/2017/day-15/.gitignore
@@ -0,0 +1,3 @@
+solution
+solution.hi
+solution.o
diff --git a/2017/day-15/solution.hs b/2017/day-15/solution.hs
new file mode 100644
index 0000000..bcc8cde
--- /dev/null
+++ b/2017/day-15/solution.hs
@@ -0,0 +1,22 @@
+import Data.Bits
+
+genA n = n : genA ((n * 16807) `rem` 2147483647)
+
+genB n = n : genB ((n * 48271) `rem` 2147483647)
+
+judge :: (Integer, Integer) -> Bool
+judge (a, b) = (a .&. 0xffff) == (b .&. 0xffff)
+
+divisible :: Integer -> Integer -> Bool
+divisible b = (== 0) . (`rem` b)
+
+genA' n = filter (divisible 4) $ genA n
+genB' n = filter (divisible 8) $ genB n
+
+main :: IO ()
+main = do
+    let aStart = 618
+    let bStart = 814
+
+    print $ length $ filter judge $ take 40000000 $ drop 1 $ zip (genA aStart) (genB bStart)
+    print $ length $ filter judge $ take 5000000 $ zip (genA' aStart) (genB' bStart)