-- setting the "warn-incomplete-patterns" flag asks GHC to warn you
-- about possible missing cases in pattern-matching definitions
{-# OPTIONS_GHC -fwarn-incomplete-patterns #-}

-- see https://wiki.haskell.org/Safe_Haskell
{-# LANGUAGE Safe #-}


module Homework3 (gasUsage , luhnDouble , luhn) where

import Types

gasUsage :: (Fractional a, Ord a) => a -> Classification
gasUsage g | g < 3 = Low
           | 3 <= g && g < 5 = Medium
           | 5 <= g && g < 7 = High
           | otherwise = SuperHigh

luhnDouble :: Int -> Int
luhnDouble x = let y = x * 2 in
                if y > 9 then y - 9 else y

luhn :: Int -> Int -> Int -> Int -> Bool
luhn a b c d = let x = [a, b, c, d] in
               sum (
                    map
                        (\(i, y) -> if i `mod` 2 == 0 then luhnDouble y else y)
                        (zip [1..4] (reverse x))
               ) `mod` 10 == 0