lang: funcs: core: math: Add minus1 function

This is a useful template hack until we get something better.

lang/funcs/core/math/minus1_func.go

@@ -0,0 +1,38 @@
+// Mgmt
+// Copyright (C) 2013-2023+ 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 coremath
+
+import (
+	"github.com/purpleidea/mgmt/lang/funcs/simple"
+	"github.com/purpleidea/mgmt/lang/types"
+)
+
+func init() {
+	simple.ModuleRegister(ModuleName, "minus1", &types.FuncValue{
+		T: types.NewType("func(x int) int"),
+		V: Minus1,
+	})
+}
+
+// Minus1 takes an int and subtracts one from it.
+func Minus1(input []types.Value) (types.Value, error) {
+	// TODO: check for overflow
+	return &types.IntValue{
+		V: input[0].Int() - 1,
+	}, nil
+}