Add support for b64decode

Signed-off-by: Knut Ahlers <knut@ahlers.me>

README.md

@@ -15,6 +15,12 @@
   $ echo "{{ .foo }}" | korvike -v foo=bar
   bar
   ```
+- `{{ b64decode <string> }}`  
+  Decodes the string with base64 [StdEncoding](https://golang.org/pkg/encoding/base64/#pkg-variables)
+  ```console
+  $ echo '{{ b64decode "SGVsbG8gV29ybGQ=" }}' | korvike
+  Hello World
+  ```
 - `{{ b64encode <string> }}`  
   Encodes the string with base64 [StdEncoding](https://golang.org/pkg/encoding/base64/#pkg-variables)
   ```console

functions/func_base64.go

@@ -3,6 +3,10 @@ package functions
 import "encoding/base64"
 
 func init() {
+	registerFunction("b64decode", func(name string, v ...string) (string, error) {
+		b, err := base64.StdEncoding.DecodeString(name)
+		return string(b), err
+	})
 	registerFunction("b64encode", func(name string, v ...string) string {
 		return base64.StdEncoding.EncodeToString([]byte(name))
 	})

functions/func_test.go

@@ -47,6 +47,21 @@ func Test_GetFunctionMap(t *testing.T) {
 	}
 }
 
+func Test_base64(t *testing.T) {
+	var (
+		b64   = "aGVsbG8="
+		plain = "hello"
+	)
+
+	if r := renderHelper(fmt.Sprintf(`{{b64decode "%s"}}`, b64), nil); r != plain {
+		t.Errorf("[b64decode] did not yield expected string: %q (expected %q)", r, plain)
+	}
+
+	if r := renderHelper(fmt.Sprintf(`{{b64encode "%s"}}`, plain), nil); r != b64 {
+		t.Errorf("[b64encode] did not yield expected string: %q (expected %q)", r, b64)
+	}
+}
+
 func Test_env(t *testing.T) {
 	result := randomString()
 	os.Setenv("KORVIKE_TESTING", result)