primes.go

package main

import "fmt"


// Sends the sequence n, n+1, n+2, ... to the `out` channel
func from(n int, out chan<- int) {
        for i := n; ; i++ {
                out <- i
        }
}

// Reads from the `in` channel and sends only those numbers
// that satisfy the predicate `pred` to the `out` channel
func filter(in <-chan int, out chan<- int, pred func (int) bool) {
        for n := range in {
                if pred(n) {
                        out <- n
                }
        }
}

// Implements Sieve of Eratosthenes
// by creating a chain of sieves, one for each found prime
func sieve(numbers <-chan int, primes chan<- int) {

        // Get first number; that's a prime
        p := <-numbers
        primes <- p

        // Send sifted numbers to next sieve
        // through channel `filtered`
        filtered := make(chan int)
        go sieve(filtered, primes)

        filter(numbers, filtered, func (n int) bool {
                return n % p != 0
        })
}

// Reads from the `primes` channel and prints to console
func output(primes chan int) {
        for p := range primes {
                fmt.Printf("%d\n", p)
        }
}

// The main function
func main() {
        // Create channels;
        // `numbers` for unsifted numbers
        // `primes` for primes
        numbers := make(chan int)
        primes  := make(chan int)

        // Start first sieve
        go sieve(numbers, primes)

        // Start reading and printing primes
        go output(primes)

        // Feed unsifted numbers 2, 3, 4, 5, ... into `numbers`
        from(2, numbers)
}