Longest.hs

{-# LANGUAGE BangPatterns #-}
{-# OPTIONS_GHC -funbox-strict-fields  #-}
module Longest where

import qualified Data.ByteString as B
import qualified  Data.ByteString.Char8 as BC
import Data.Word (Word8)

-- using custom data type (only works for 2 elements)
data TwoQueue = Empty
              | OneIn !Word8
              | TwoIn !Word8 !Word8

data PairS = PS !B.ByteString !Int

-- using custom data type (only works for 2 elements)
findSequenceCustom :: BC.ByteString -> Int
findSequenceCustom ss = count Empty (PS ss (B.length ss)) 0
    where count !_ (PS _ 0) !res = res
          count Empty (PS s n) res = count (OneIn $! B.head s) (PS (B.tail s) (n-1)) (res+1)
          count (OneIn a) (PS s n) res
            | B.head s == a = count (OneIn a) (PS (B.tail s) (n-1)) (res+1)
            | otherwise = count (TwoIn a (B.head s)) (PS (B.tail s) (n-1)) (res+1)
          count (TwoIn a b) (PS s n) res
            | B.head s == a || B.head s == b = count (TwoIn a b) (PS (B.tail s) (n-1)) (res+1)
            | otherwise = res