Move flags and utils to internal packages

internal/flags/public_key_algorithm.go

@@ -0,0 +1,53 @@
+/*
+Copyright 2020 The Flux CD contributors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package flags
+
+import (
+	"fmt"
+	"strings"
+)
+
+var supportedPublicKeyAlgorithms = []string{"rsa", "ecdsa", "ed25519"}
+
+type PublicKeyAlgorithm string
+
+func (a *PublicKeyAlgorithm) String() string {
+	return string(*a)
+}
+
+func (a *PublicKeyAlgorithm) Set(str string) error {
+	if strings.TrimSpace(str) == "" {
+		return fmt.Errorf("no public key algorithm given, must be one of: %s",
+			strings.Join(supportedPublicKeyAlgorithms, ", "))
+	}
+	for _, v := range supportedPublicKeyAlgorithms {
+		if str == v {
+			*a = PublicKeyAlgorithm(str)
+			return nil
+		}
+	}
+	return fmt.Errorf("unsupported public key algorithm '%s', must be one of: %s",
+		str, strings.Join(supportedPublicKeyAlgorithms, ", "))
+}
+
+func (a *PublicKeyAlgorithm) Type() string {
+	return "publicKeyAlgorithm"
+}
+
+func (a *PublicKeyAlgorithm) Description() string {
+	return fmt.Sprintf("SSH public key algorithm (%s)", strings.Join(supportedPublicKeyAlgorithms, ", "))
+}