--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/7-Haskell/day2.hs	Sun Jun 16 21:09:33 2019 +0100
@@ -0,0 +1,57 @@
+module Day2 where
+    -- We could just "import Data.List" and then use "sort", but let's do it by hand with an ugly O(n^2) approach
+    my_sort :: Ord a => [a] -> [a]
+    my_sort lst = my_sort' (<) lst []
+
+--    my_sort' :: Ord a => [a] -> [a] -> [a]
+--    my_sort' [] res = res
+--    my_sort' (h:t) [] = my_sort' t [h]
+--    my_sort' (h:t) (h_res:t_res)
+--        | h < h_res = my_sort' t (h:h_res:t_res)
+--        | otherwise = my_sort' t (h_res:my_sort' (h:[]) t_res)
+
+    my_sort' :: Ord a => (a -> a -> Bool) -> [a] -> [a] -> [a]
+    my_sort' cmp [] res = res
+    my_sort' cmp (h:t) [] = my_sort' cmp t [h]
+    my_sort' cmp (h:t) (h_res:t_res)
+        | cmp h h_res = my_sort' cmp t (h:h_res:t_res)
+        | otherwise = my_sort' cmp t (h_res:my_sort' cmp (h:[]) t_res)
+    
+    parse_int :: String -> Int
+    parse_int str = parse_int' str 0
+
+    parse_int' :: String -> Int -> Int
+    parse_int' "" val = val
+    parse_int' (h:t) val
+        | fromEnum h >= 48 && fromEnum h <= 57 = parse_int' t (val * 10 + ((fromEnum h) - 48))
+        | otherwise = parse_int' t val
+    
+    every_three :: Integer -> [Integer]
+    every_three = every_n 3
+
+    every_five :: Integer -> [Integer]
+    every_five = every_n 5
+
+    every_n :: Integer -> Integer -> [Integer]
+    every_n n x = [x, x + n ..]
+
+    -- Usage: every_m_n (every_five) 5 (every_three) 3
+    every_m_n :: (Integer -> [Integer]) -> Integer -> (Integer -> [Integer]) -> Integer -> [Integer]
+    every_m_n _m x _n y = zipWith (+) (_m x) (_n y)
+
+    halve :: Double -> Double
+    halve x = (/ 2) x -- It's ugly, but it's the "partially applied" version of "x / 2" - "(/ 2)" becomes an anonymous function that gets applied to x
+
+    new_line :: String -> String
+    new_line x = (++ "\n") x
+
+    -- Let's go Euclidean: https://en.wikipedia.org/wiki/Greatest_common_divisor#Euclid's_algorithm
+    my_gcd :: Integer -> Integer -> Integer
+    my_gcd a b
+        | a == b = a
+        | a > b = my_gcd (a - b) b
+        | otherwise = my_gcd a (b - a)
+    
+    -- Wilson's theorem seems easiest: https://en.wikipedia.org/wiki/Wilson%27s_theorem
+    primes :: [Integer]
+    primes = [x | x <- [2 ..], (gcd ((product [1 .. x-1]) + 1) x) == x]
\ No newline at end of file