cli: Lookup subcommand names dynamically

This lets us get the chosen subcommands dynamically and removes the
eventual need for three package imports.

cli/util/args.go

@@ -0,0 +1,58 @@
+// Mgmt
+// Copyright (C) 2013-2024+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package util
+
+import (
+	"reflect"
+	"strings"
+)
+
+// LookupSubcommand returns the name of the subcommand in the obj, of a struct.
+// This is useful for determining the name of the subcommand that was activated.
+// It returns an empty string if a specific name was not found.
+func LookupSubcommand(obj interface{}, st interface{}) string {
+	val := reflect.ValueOf(obj)
+	if val.Kind() == reflect.Ptr { // max one de-referencing
+		val = val.Elem()
+	}
+
+	v := reflect.ValueOf(st) // value of the struct
+	typ := val.Type()
+	for i := 0; i < typ.NumField(); i++ {
+		f := val.Field(i) // value of the field
+		if f.Interface() != v.Interface() {
+			continue
+		}
+
+		field := typ.Field(i)
+		alias, ok := field.Tag.Lookup("arg")
+		if !ok {
+			continue
+		}
+
+		// XXX: `arg` needs a split by comma first or fancier parsing
+		prefix := "subcommand"
+		split := strings.Split(alias, ":")
+		if len(split) != 2 || split[0] != prefix {
+			continue
+		}
+
+		return split[1] // found
+	}
+	return "" // not found
+}