codewars/go/kyu5/primes_in_numbers/sieve1.go

package kata

// Based on the Go classical concurrent prime sieve
// See https://golang.org/test/chan/sieve1.go
func Primes(limit int) []int {
	// Generate numbers from 2 to limit, included.
	gen := func(ch chan<- int, limit int) {
		for i := 2; i <= limit; i++ {
			ch <- i
		}
		close(ch)
	}
	// Remove divisible numbers
	//  - in: numbers source
	//  - out: numbers sink
	//  - prime: prime by which to attempt modulo
	filter := func(in <-chan int, out chan<- int, prime int) {
		for {
			candidate, ok := <-in
			// Source was closed.
			if !ok {
				break
			}
			if candidate%prime != 0 {
				out <- candidate
			}
		}
		close(out)
	}

	genChan := make(chan int)
	go gen(genChan, limit)
	var primes []int
	for i := 0; ; i++ {
		prime, ok := <-genChan
		if !ok {
			break
		}
		primes = append(primes, prime)
		filterChan := make(chan int)
		go filter(genChan, filterChan, prime)
		genChan = filterChan
	}
	return primes
}