module HL03TUOLP where

evens = [x | x <- [0 ..], even x]

prime :: Integer -> Bool
prime n
  | n < 1 = error "not a positive integer"
  | n == 1 = False
  | otherwise = ldp n == n
  where
    ldp = ldpf primes
    ldpf (p : ps) m
      | rem m p == 0 = p
      | p ^ 2 > m = m
      | otherwise = ldpf ps m
    primes = 2 : filter prime [3 ..]

sieve :: [Integer] -> [Integer]
sieve (0 : xs) = sieve xs
sieve (n : xs) = n : sieve (mark xs 1 n)
  where
    mark :: [Integer] -> Integer -> Integer -> [Integer]
    mark (y : ys) k m
      | k == m = 0 : mark ys 1 m
      | otherwise = y : mark ys (k + 1) m

primes :: [Integer]
primes = sieve [2 ..]

oddsFrom3 :: [Integer]
oddsFrom3 = 3 : map (+ 2) oddsFrom3

mersenne = [(p, 2 ^ p - 1) | p <- primes, prime (2 ^ p - 1)]

notmersenne = [(p, 2 ^ p - 1) | p <- primes, not (prime (2 ^ p - 1))]

pdivisors :: Integer -> [Integer]
pdivisors n = [d | d <- [1 .. (n - 1)], rem n d == 0]

primePairs :: [(Integer, Integer)]
primePairs = pairs primes
  where
    pairs (x : y : xys)
      | x + 2 == y = (x, y) : pairs (y : xys)
      | otherwise = pairs (y : xys)

primeTriples :: [(Integer, Integer, Integer)]
primeTriples = triples primes
  where
    triples (x : y : z : xyzs)
      | x + 2 == y && y + 2 == z = (x, y, z) : triples (y : z : xyzs)
      | otherwise = triples (y : z : xyzs)