Rewrite fan-in/out messaging patterns

Author: tmrts

README.md

@@ -77,8 +77,8 @@ __Messaging Patterns__:
 
 | Pattern | Description |
 |:-------:| ----------- |
-| [Fan-In](fan/fan_in.go) | Funnels tasks to a work sink (e.g. server) |
-| [Fan-Out](fan/fan_out.go) | Distributes tasks amongs workers |
+| [Fan-In](messaging/fan_in.md) | Funnels tasks to a work sink (e.g. server) |
+| [Fan-Out](messaging/fan_out.md) | Distributes tasks amongs workers (e.g. producer) |
 | [Futures & Promises](futures_promises.go) | Acts as a place-holder of a result that is initally unknown for synchronization purposes |
 | [Publish/Subscribe](messaging/publish_subscribe.md) | Passes information to a collection of recipients who subscribed to a topic |
 | [Push & Pull](push_pull.go) | Distributes messages to multiple workers, arranged in a pipeline |

fan/fan_in.go

@@ -0,0 +1,40 @@
+Fan-In Messaging Patterns
+===================================
+Fan-In is a messaging pattern used to create a funnel for work amongst workers (clients: source, server: destination).
+
+We can model fan-in using the Go channels.
+
+```go
+// Merge different channels in one channel
+func Merge(cs ...<-chan int) <-chan int {
+	var wg sync.WaitGroup
+
+	out := make(chan int)
+
+	// Start an send goroutine for each input channel in cs. send
+	// copies values from c to out until c is closed, then calls wg.Done.
+	send := func(c <-chan int) {
+		for n := range c {
+			out <- n
+		}
+		wg.Done()
+	}
+
+	wg.Add(len(cs))
+	for _, c := range cs {
+		go send(c)
+	}
+
+	// Start a goroutine to close out once all the send goroutines are
+	// done.  This must start after the wg.Add call.
+	go func() {
+		wg.Wait()
+		close(out)
+	}()
+	return out
+}
+```
+
+The `Merge` function converts a list of channels to a single channel by starting a goroutine for each inbound channel that copies the values to the sole outbound channel.
+
+Once all the output goroutines have been started, `Merge` a goroutine is started to close the main channel.

messaging/fan_in.md

@@ -1,9 +1,12 @@
-package fan
+Fan-Out Messaging Pattern
+=========================
+Fan-Out is a messaging pattern used for distributing work amongst workers (producer: source, consumers: destination).
 
-// Out implements fan.Out messaging pattern
-// Split a channel into n channels that receive messages
-// in a round-robin fashion.
-func Out(ch <-chan int, n int) []<-chan int {
+We can model fan-out using the Go channels.
+
+```go
+// Split a channel into n channels that receive messages in a round-robin fashion.
+func Split(ch <-chan int, n int) []<-chan int {
 	cs := make([]chan int)
 	for i := 0; i < n; i++ {
 		cs = append(cs, make(chan int))
@@ -38,3 +41,7 @@ func Out(ch <-chan int, n int) []<-chan int {
 
 	return cs
 }
+```
+
+The `Split` function converts a single channel into a list of channels by using 
+a goroutine to copy received values to channels in the list in a round-robin fashion.