Day 15

Generators are cool. Please compile this, otherwise it will burn
your CPU.

2 files changed, 31 insertions, 0 deletions

diff --git a/2017/15.hs b/2017/15.hs
new file mode 100644
index 0000000..855b179
--- /dev/null
+++ b/2017/15.hs
@@ -0,0 +1,29 @@
+import Control.Monad
+import Data.Bits
+
+
+modv :: Int
+modv = 2147483647
+
+gen :: Int -> Int -> [Int]
+gen mulv startv = lowbits r : gen mulv r
+    where r = mulv * startv `rem` modv
+
+gen2 :: Int -> Int -> Int -> [Int]
+gen2 checkv mulv startv = if l .&. checkv == 0 then l : rest else rest
+    where r = mulv * startv `rem` modv
+          l = lowbits r
+          rest = gen2 checkv mulv r
+
+lowbits :: Int -> Int
+lowbits n = n .&. 0xffff
+
+main :: IO ()
+main = do
+    [sA, sB] <- liftM (map (read . last . words) . lines) (readFile "15.in")
+
+    print $ sum $ map (fromEnum . uncurry (==)) $
+        zip (take 40000000 $ gen 16807 sA) (take 40000000 $ gen 48271 sB)
+
+    print $ sum $ map (fromEnum . uncurry (==)) $
+        zip (take 5000000 $ gen2 3 16807 sA) (take 5000000 $ gen2 7 48271 sB)
diff --git a/2017/15.in b/2017/15.in
new file mode 100644
index 0000000..99a8029
--- /dev/null
+++ b/2017/15.in
@@ -0,0 +1,2 @@
+Generator A starts with 116
+Generator B starts with 299