-- This are my solutions to the exercises from https://github.com/moocfi/haskell-mooc

import Data.List

---------
-- Set.01
-- https://github.com/moocfi/haskell-mooc/blob/master/exercises/Set1.hs

-- Ex. 1

one :: Integer -- function declaration: types of arguments and output
one = 1        -- function definition

two :: Integer
two = 2

-- Ex. 2

double :: Integer -> Integer
double x = x * 2

-- Ex. 3

quadruple :: Integer -> Integer
quadruple x = double (double x)

-- Ex. 4 

distance :: Double -> Double -> Double -> Double -> Double
-- exponentiation operator: ^ for integers, ^^ for fractionals and ** for floating point
distance x1 y1 x2 y2 = sqrt (abs(x1 - x2)**2 + abs(y1 - y2)**2)

-- Ex. 5

eeny :: Integer -> String
eeny x = if even x 
         then "eeny" 
         else "meeny"

-- Ex. 6

checkPassword :: String -> String
checkPassword password = if password == "swordfish" || password == "mellon"
                         then "You're in."
                         else "ACCESS DENIED!"

-- Ex. 7

postagePrice :: Int -> Int
postagePrice weight = if weight > 5000 then 6000
                      else if weight > 500 then 300 + weight * 1
                      else 250

-- Ex. 8

isZero :: Int -> Bool
isZero 0 = True -- this is pattern matching
isZero x = False

-- Ex. 9

sumTo :: Int -> Int
sumTo 1 = 1
sumTo n = n + sumTo (n - 1)

-- Ex. 10

power :: Int -> Int -> Int
power n 1 = n
power n k = n * power n (k - 1)

-- Ex. 11

ilog3 :: Int -> Int
ilog3 0 = 0
ilog3 n = 1 + ilog3 (div n 3)

----------
-- Set.02a
-- https://github.com/moocfi/haskell-mooc/blob/master/exercises/Set2a.hs

-- Functions you'll need: head, tail, take, drop, length, null

-- Ex. 1

years = [1982, 2004, 2020]

-- Ex. 2

takeFinal :: Int -> [a] -> [a] -- a is the Haskell equivalent of generics (T) in C#
takeFinal n xs
    | n > length xs = xs
    | otherwise = drop (length xs - n) xs

-- Ex. 3

updateAt :: Int -> a -> [a] -> [a]
-- elements before i + x + elemenets after i ; ++ concatenates arrays
updateAt i x xs = take i xs ++ [x] ++ drop (i + 1) xs 

-- Ex. 4

substring :: Int -> Int -> String -> String
substring i j s = drop i (take j s)

-- Ex. 5

isPalindrome :: String -> Bool
isPalindrome s = s == reverse s

-- Ex. 6

palindromify :: String -> String
palindromify s = if isPalindrome s then s -- can easily make it: if s == reverse s then s , but decided this way was more readable
                 else palindromify (take (length s - 2) (drop 1 s))

-- Ex. 7

safeDiv :: Int -> Int -> String
safeDiv x y
    | y == 0 = "Nothing"
    | otherwise = "Just " ++ show (div x y) -- show casts the value to string

-- Ex. 8

greet :: String -> Maybe String -> String -- maybe means the value can (String) or can not be given (Nothing) ; you need to use pattern matching
greet firstName Nothing = "Hello, " ++ firstName ++ "!"
greet firstName (Just lastName) = "Hello, " ++ firstName ++ " " ++ lastName ++ "!"

-- Ex. 9

safeIndex :: [a] -> Int -> Maybe a
safeIndex xs i = if 0 > i || i >= length xs then Nothing
                 else Just (xs !! i)

-- Ex. 10

eitherDiv :: Int -> Int -> Either String Int -- a value can be the left one or the right one (specified when given)
eitherDiv x 0 = Left (show x ++ "/0")
eitherDiv x y = Right (div x y)

-- Ex. 11

addEithers :: Either String Int -> Either String Int -> Either String Int
addEithers (Right x) (Right y) = Right (x + y)
addEithers (Right x) (Left y)  = Left y
addEithers (Left x)  (Right y) = Left x