gistfile1.hs

-- LFSR, en formato compatible con 
-- http://homepage.mac.com/afj/taplist.html

-- uso (en ghci):
-- > let t = ciclo 7 $ drop 7 $ leer $ generar "1111111" [7,6,4,2]
-- "1111111" es cadena inicial con 7 registros
-- [7, 6, 4, 2] es cuarteta en http://homepage.mac.com/afj/7bits.txt
-- 'ciclo 7' se detiene cuando se repite la secuencia de los 7 primeros símbolos
-- > t
-- "00111110110111000000101000011011
--  00100000111100010110101100001000
--  10011101111010001100110100110001
--  1100101010111010100100101111111"
-- > length t 
-- 127 (2^7 - 1)

zipWithN f = foldl1 (zipWith f)                                                 
                                                                                
generar :: String -> [Int] -> [Bool]                                            
generar inicial indices = ts where                                              
    n = length inicial                                                          
    ts = xs ++ zipWithN (/=) (g indices)                                        
    xs = map (== '1') inicial                                                   
    g = map (((flip drop) ts) . (n -))                                                                                                          
                                                                                
ciclo n (x:xs) = x : takeWhileNoCycle marca xs where                         
    marca = take n (x:xs)                                 
                                                                                
takeWhileNoCycle :: (Eq a) => [a] -> [a] -> [a]                                 
takeWhileNoCycle mark (x:xs) = if (take (length mark) (x:xs)) == mark           
    then []                                                                     
    else x : takeWhileNoCycle mark xs                                           
                                                                                
leer = map (\k -> if k then '1' else '0')