--- /dev/null
+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Generate primes up to 100 using channels, checking the results.
+// This sieve is Eratosthenesque and only considers odd candidates.
+// See discussion at <http://blog.onideas.ws/eratosthenes.go>.
+
+package main
+
+import (
+ "container/heap"
+ "container/ring"
+ "container/vector"
+)
+
+// Return a chan of odd numbers, starting from 5.
+func odds() chan int {
+ out := make(chan int, 50)
+ go func() {
+ n := 5
+ for {
+ out <- n
+ n += 2
+ }
+ }()
+ return out
+}
+
+// Return a chan of odd multiples of the prime number p, starting from p*p.
+func multiples(p int) chan int {
+ out := make(chan int, 10)
+ go func() {
+ n := p * p
+ for {
+ out <- n
+ n += 2 * p
+ }
+ }()
+ return out
+}
+
+type PeekCh struct {
+ head int
+ ch chan int
+}
+
+// Heap of PeekCh, sorting by head values.
+type PeekChHeap struct {
+ *vector.Vector
+}
+
+func (h *PeekChHeap) Less(i, j int) bool {
+ return h.At(i).(*PeekCh).head < h.At(j).(*PeekCh).head
+}
+
+// Return a channel to serve as a sending proxy to 'out'.
+// Use a goroutine to receive values from 'out' and store them
+// in an expanding buffer, so that sending to 'out' never blocks.
+func sendproxy(out chan<- int) chan<- int {
+ proxy := make(chan int, 10)
+ go func() {
+ n := 16 // the allocated size of the circular queue
+ first := ring.New(n)
+ last := first
+ var c chan<- int
+ var e int
+ for {
+ c = out
+ if first == last {
+ // buffer empty: disable output
+ c = nil
+ } else {
+ e = first.Value.(int)
+ }
+ select {
+ case e = <-proxy:
+ last.Value = e
+ if last.Next() == first {
+ // buffer full: expand it
+ last.Link(ring.New(n))
+ n *= 2
+ }
+ last = last.Next()
+ case c <- e:
+ first = first.Next()
+ }
+ }
+ }()
+ return proxy
+}
+
+// Return a chan int of primes.
+func Sieve() chan int {
+ // The output values.
+ out := make(chan int, 10)
+ out <- 2
+ out <- 3
+
+ // The channel of all composites to be eliminated in increasing order.
+ composites := make(chan int, 50)
+
+ // The feedback loop.
+ primes := make(chan int, 10)
+ primes <- 3
+
+ // Merge channels of multiples of 'primes' into 'composites'.
+ go func() {
+ h := &PeekChHeap{new(vector.Vector)}
+ min := 15
+ for {
+ m := multiples(<-primes)
+ head := <-m
+ for min < head {
+ composites <- min
+ minchan := heap.Pop(h).(*PeekCh)
+ min = minchan.head
+ minchan.head = <-minchan.ch
+ heap.Push(h, minchan)
+ }
+ for min == head {
+ minchan := heap.Pop(h).(*PeekCh)
+ min = minchan.head
+ minchan.head = <-minchan.ch
+ heap.Push(h, minchan)
+ }
+ composites <- head
+ heap.Push(h, &PeekCh{<-m, m})
+ }
+ }()
+
+ // Sieve out 'composites' from 'candidates'.
+ go func() {
+ // In order to generate the nth prime we only need multiples of
+ // primes ≤ sqrt(nth prime). Thus, the merging goroutine will
+ // receive from 'primes' much slower than this goroutine
+ // will send to it, making the buffer accumulate and block this
+ // goroutine from sending, causing a deadlock. The solution is to
+ // use a proxy goroutine to do automatic buffering.
+ primes := sendproxy(primes)
+
+ candidates := odds()
+ p := <-candidates
+
+ for {
+ c := <-composites
+ for p < c {
+ primes <- p
+ out <- p
+ p = <-candidates
+ }
+ if p == c {
+ p = <-candidates
+ }
+ }
+ }()
+
+ return out
+}
+
+func main() {
+ primes := Sieve()
+ a := []int{2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97}
+ for i := 0; i < len(a); i++ {
+ if x := <-primes; x != a[i] {
+ panic(x, " != ", a[i])
+ }
+ }
+}