// Streams
//
// Chapter 12, Scala Programming by Example
//


def sumPrimes(start: Int, end: Int): Int = { 
  var i = start 
  var acc = 0 
  while (i < end) { 
    if (isPrime(i)) acc += i 
    i += 1 
  } 
  acc 
}

// or 

def sumPrimes(start: Int, end: Int) = 
  sum(range(start, end) filter isPrime) 

// more concise, but also less efficient ...

// generate lists "on demand" -> streams

// has first, tail, isEmpty, range methods

def print[A](xs: Stream[A]) { 
  if (!xs.isEmpty) { Console.println(xs.head); print(xs.tail) } 
} 


(Stream.range(1000, 10000) filter isPrime).tail.head

// Streams do not support  ::  or ::: ,   use Stream.cons(x,xs)   and    xs append ys  instead

// see also chapter 13:  Iterators ...


// Stream-based Sieve of Eratosthenes
// from http://en.literateprograms.org/Sieve_of_Eratosthenes_(Scala)

def ints(n: Int): Stream[Int] = Stream.cons(n, ints(n+1))

def primes(nums: Stream[Int]): Stream[Int] =
    Stream.cons(nums.head,
                primes ((nums tail) filter (x => x % nums.head != 0)) )