module Db where

import Data.Time

data DatabaseItem = DbString String
                  | DbNumber Integer
                  | DbDate UTCTime
                  deriving (Eq, Ord, Show)

theDatabase :: [DatabaseItem]
theDatabase =
    [ DbDate (UTCTime (fromGregorian 1911 5 1) (secondsToDiffTime 34123))
    , DbNumber 9001
    , DbString "Hello, world!"
    , DbDate (UTCTime (fromGregorian 1921 5 1) (secondsToDiffTime 34123))
    ]

filterDbDate :: [DatabaseItem] -> [UTCTime]
filterDbDate xs = foldr f1 [] (filter f2 xs)
    where f1 (DbDate d) acc = d : acc
          f2 (DbDate d) = True
          f2 _          = False

filterDbNumber :: [DatabaseItem] -> [Integer]
filterDbNumber xs = foldr f1 [] (filter f2 xs)
    where f1 (DbNumber d) acc = d : acc
          f2 (DbNumber d) = True
          f2 _            = False

mostRecent :: [DatabaseItem] -> UTCTime
mostRecent xs = maximum (filterDbDate xs)

sumDb :: [DatabaseItem] -> Integer
sumDb xs = foldr (+) 0 (filterDbNumber xs)

avgDb :: [DatabaseItem] -> Double
avgDb xs = (fromIntegral (sumDb xs)) / (fromIntegral (length (filterDbNumber xs)))

fibs :: [Integer]
fibs = 1 : scanl (+) 1 fibs

fibs20 :: [Integer]
fibs20 = take 20 (1 : scanl (+) 1 fibs)

fibs100 :: [Integer]
fibs100 = takeWhile (\x -> x < 100) fibs

fibsN :: Int -> Integer
fibsN x = fibs !! x

factorial :: [Integer]
factorial = scanl (*) 1 [1..]