Refactor strategy pattern

behavioral/strategy.md

@@ -0,0 +1,55 @@
+#Strategy Pattern
+Strategy behavioral design pattern enables an algorithm's behavior to be selected at runtime.
+
+It defines algorithms, encapsulates them, and uses them interchangeably.
+
+## Implementation
+Implementation of an interchangeable operator object that operates on integers.
+
+```go
+type Operator interface {
+	Apply(int, int) int
+}
+
+type Operation struct {
+	Operator Operator
+}
+
+func (o *Operation) Operate(leftValue, rightValue int) int {
+	return o.Operator.Apply(leftValue, rightValue)
+}
+```
+
+## Usage
+### Addition Operator
+```go
+type Addition struct{}
+
+func (Addition) Apply(lval, rval int) int {
+	return lval + rval
+}
+```
+
+```go
+add := Operation{Addition{}}
+add.Operate(3, 5) // 8
+```
+
+### Multiplication Operator
+```go
+type Multiplication struct{}
+
+func (Multiplication) Apply(lval, rval int) int {
+	return lval * rval
+}
+```
+
+```go
+mult := Operation{Multiplication{}}
+
+mult.Operate(3, 5) // 15
+```
+
+## Rules of Thumb
+- Strategy pattern is similar to Template pattern except in its granularity.
+- Strategy pattern lets you change the guts of an object. Decorator pattern lets you change the skin.