An answer for http://nabetani.sakura.ne.jp/hena/ord11arithseq/

arithseq.hs

module Main (main) where
import Data.Char
import Data.List
import Data.List.Split
import Test.HUnit

charToInt :: Char -> Int
charToInt c | isDigit c = fromEnum c - fromEnum '0'
            | otherwise = fromEnum c - fromEnum 'a' + 10

goWithDiff :: [Int] -> Int -> Int
goWithDiff [] _ = 0
goWithDiff _ 0 = 1
goWithDiff (x:xs) n = length withCommonDiff + 1
  where
    withCommonDiff = takeWhile (`elem` xs) (map (\i -> x + n * i) [1 ..])

solve :: String -> String
solve input = show . maximum $ [goWithDiff l n | l <- tails list,
                                                 n <- [0 .. maximum list]]
  where
    list = map charToInt input

doAssert :: [String] -> Assertion
doAssert (name:input:expected:_) = assertEqual name expected (solve input)
doAssert _ = error "Specify a list which contains more than 3 items."

main :: IO ()
main =
    readFile "patterns.tsv"
    >>= runTestTT . test . map (doAssert . splitOn "\t") . lines
    >>  return ()