concurrency/generator: implement generator pattern

concurrency/generator.md

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+# Generator Pattern
+
+[Generator](https://en.wikipedia.org/wiki/Generator_(computer_programming)) is a special routine that can be used to control the iteration behavior of a loop.
+
+# Implementation and Example
+With Go language, we can implement generator in two ways: channel and closure. Fibonacci number generation example can be found in [generators.go](generators.go).

concurrency/generators.go

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+package main
+
+import (
+	"fmt"
+)
+
+//FibonacciClosure implements fibonacci number generation using closure
+func FibonacciClosure() func() int {
+	a, b := 0, 1
+	return func() int {
+		a, b = b, a+b
+		return a
+	}
+}
+
+//FibonacciChan implements fibonacci number generation using channel
+func FibonacciChan(n int) chan int {
+	c := make(chan int)
+
+	go func() {
+		a, b := 0, 1
+		for i := 0; i < n; i++ {
+			c <- b
+			a, b = b, a+b
+		}
+		close(c)
+	}()
+
+	return c
+}
+
+func main() {
+	//closure
+	nextFib := FibonacciClosure()
+	for i := 0; i < 20; i++ {
+		fmt.Println(nextFib())
+	}
+
+	//channel
+	for i := range FibonacciChan(20) {
+		fmt.Println(i)
+	}
+}