Palindrome dates: Difference between revisions

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=={{header|Haskell}}==

<syntaxhighlight lang="haskell">import Data.~~Time.Calendar~~ (~~Day, fromGregorianValid~~)

<syntaxhighlight lang="haskell">import Data.List (unfoldr)

import Data.List.Split (chunksOf)

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import Data.~~List~~ (~~unfoldr~~)

import Data.Tuple (swap)

import Data.Bool (bool)

import Data.Maybe (mapMaybe)

import Data.Time.Calendar (Day, fromGregorianValid)

⚫

import Data.Tuple (swap)

-------------------- PALINDROMIC DATES -------------------

palinDates :: [Day]

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palinDay y = fromGregorianValid y m d

where

[m, d] = unDigits <$> chunksOf 2 ~~(reversedDecimalDigits (fromInteger y))~~

[m, d] = unDigits <$> chunksOf 2

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(reversedDecimalDigits (fromInteger y))

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reversedDecimalDigits :: Int -> [Int]

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reversedDecimalDigits =

unfoldr ((flip bool Nothing . Just . swap . flip quotRem 10) <*> (0 ==))

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unDigits :: [Int] -> Int

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unDigits = foldl ((+) . (10 *)) 0

--------------------------- TEST -------------------------

main :: IO ()

main = do

main =

let n = length palinDates

putStrLn $ "Count of palindromic dates [2021..9999]: " ~~++ show n~~

in putStrLn ("Count of palindromic dates [2021..9999]: "

<> show n) >>

putStrLn "\nFirst 15:"

putStrLn "\nFirst 15:" >>

mapM_ print $ take 15 palinDates

mapM_ print (take 15 palinDates) >>

putStrLn "\nLast 15:"

putStrLn "\nLast 15:" >>

mapM_ print $ take 15 (drop (n - 15) palinDates)~~</syntaxhighlight>~~

mapM_ print (take 15 (drop (n - 15) palinDates))

------------------------- GENERIC ------------------------

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reversedDecimalDigits :: Int -> [Int]

reversedDecimalDigits = unfoldr go

where

go n

| 0 < n = Just $ swap (quotRem n 10)

| otherwise = Nothing

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unDigits :: [Int] -> Int

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unDigits = foldl ((+) . (10 *)) 0</syntaxhighlight>

<pre>Count of palindromic dates [2021..9999]: 284