Adding adapter pattern

Author: MrTimeout

design_patterns/structural/adapter/README.md

@@ -1 +1,6 @@
-# Adapter pattern
+# Adapter pattern
+
+This pattern is used when we have an old implementation and we need to add new functionality to it.
+By the Open/Closed principle, which means Open for extension, Closed for modification, we cannot just modify
+the "legacy" interface/struct. We should create a new interface/struct adapter so we still support "legacy" functionality
+whilst adding new to it.

design_patterns/structural/adapter/adapter.go

@@ -0,0 +1,27 @@
+package adapter
+
+type Printer interface {
+	Print(string) string
+}
+
+type MyPrinter struct{}
+
+func (m MyPrinter) Print(input string) string {
+	return "legacy printing: " + input
+}
+
+type ModernPrinter interface {
+	PrintStored() string
+}
+
+type PrinterAdapter struct {
+	Printer
+	Msg string
+}
+
+func (p PrinterAdapter) PrintStored() string {
+	if p.Printer != nil {
+		return p.Print(p.Msg)
+	}
+	return "modern printing: " + p.Msg
+}

design_patterns/structural/adapter/adapter_test.go

@@ -0,0 +1,26 @@
+package adapter
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestAdapter(t *testing.T) {
+	var input = "hello"
+
+	t.Run("adapter with old implementation", func(t *testing.T) {
+		printer := PrinterAdapter{Printer: MyPrinter{}, Msg: input}
+		want := "legacy printing: " + input
+
+		assert.Equal(t, want, printer.Print(input))
+		assert.Equal(t, want, printer.PrintStored())
+	})
+
+	t.Run("adapter with new implementation", func(t *testing.T) {
+		printer := PrinterAdapter{Msg: input}
+		want := "modern printing: " + input
+
+		assert.Equal(t, want, printer.PrintStored())
+	})
+}