util: Add a generic map key and value swapping function

Fun little utility function which is useful.

util/generics.go

@@ -0,0 +1,47 @@
+// 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 <https://www.gnu.org/licenses/>.
+//
+// Additional permission under GNU GPL version 3 section 7
+//
+// If you modify this program, or any covered work, by linking or combining it
+// with embedded mcl code and modules (and that the embedded mcl code and
+// modules which link with this program, contain a copy of their source code in
+// the authoritative form) containing parts covered by the terms of any other
+// license, the licensors of this program grant you additional permission to
+// convey the resulting work. Furthermore, the licensors of this program grant
+// the original author, James Shubin, additional permission to update this
+// additional permission if he deems it necessary to achieve the goals of this
+// additional permission.
+
+package util
+
+import (
+	"fmt"
+)
+
+// MapSwap returns a new map with the keys and values interchanged. It errors if
+// doing so would cause a loss of a value. This is a generic version.
+func MapSwap[M ~map[K]V, K, V comparable](m M) (map[V]K, error) {
+	ret := make(map[V]K)
+	for k, v := range m {
+		if _, exists := ret[v]; exists {
+			return nil, fmt.Errorf("duplicate value: %v", v)
+		}
+		ret[v] = k
+	}
+	return ret, nil
+}

util/generics_test.go

@@ -0,0 +1,93 @@
+// 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 <https://www.gnu.org/licenses/>.
+//
+// Additional permission under GNU GPL version 3 section 7
+//
+// If you modify this program, or any covered work, by linking or combining it
+// with embedded mcl code and modules (and that the embedded mcl code and
+// modules which link with this program, contain a copy of their source code in
+// the authoritative form) containing parts covered by the terms of any other
+// license, the licensors of this program grant you additional permission to
+// convey the resulting work. Furthermore, the licensors of this program grant
+// the original author, James Shubin, additional permission to update this
+// additional permission if he deems it necessary to achieve the goals of this
+// additional permission.
+
+//go:build !root
+
+package util
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestMapSwap1(t *testing.T) {
+	m := map[string]int{
+		"zero": 0,
+		"one":  1,
+		"two":  2,
+	}
+	ret, err := MapSwap(m)
+	if err != nil {
+		t.Errorf("test failed with: %v", err)
+		return
+	}
+
+	exp := map[int]string{
+		0: "zero",
+		1: "one",
+		2: "two",
+	}
+	if !reflect.DeepEqual(exp, ret) {
+		t.Errorf("expected: %v, actual: %v", exp, ret)
+	}
+}
+
+func TestMapSwap2(t *testing.T) {
+	m := map[int]string{
+		0: "zero",
+		1: "one",
+		2: "two",
+	}
+	ret, err := MapSwap(m)
+	if err != nil {
+		t.Errorf("test failed with: %v", err)
+		return
+	}
+
+	exp := map[string]int{
+		"zero": 0,
+		"one":  1,
+		"two":  2,
+	}
+	if !reflect.DeepEqual(exp, ret) {
+		t.Errorf("expected: %v, actual: %v", exp, ret)
+	}
+}
+
+func TestMapSwap3(t *testing.T) {
+	m := map[int]string{
+		0: "zero",
+		1: "one",
+		2: "zero", // dupe!
+	}
+	ret, err := MapSwap(m)
+	if err == nil {
+		t.Errorf("test expected failure but passed with: %v", ret)
+	}
+}