lang: Unnested the core package from the functions dir

The core package could contain non-functions, so we might as well move
it upwards.

lang/core/datetime/weekday_func.go

@@ -0,0 +1,49 @@
+// 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 coredatetime
+
+import (
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/purpleidea/mgmt/lang/funcs/simple"
+	"github.com/purpleidea/mgmt/lang/types"
+)
+
+func init() {
+	simple.ModuleRegister(ModuleName, "weekday", &types.FuncValue{
+		T: types.NewType("func(a int) str"),
+		V: Weekday,
+	})
+}
+
+// Weekday returns the lowercased day of the week corresponding to the input
+// time. The time is the number of seconds since the epoch, and matches what
+// comes from our Now function.
+func Weekday(input []types.Value) (types.Value, error) {
+	epochDelta := input[0].Int()
+	if epochDelta < 0 {
+		return nil, fmt.Errorf("epoch delta must be positive")
+	}
+
+	weekday := time.Unix(epochDelta, 0).Weekday()
+	return &types.StrValue{
+		V: strings.ToLower(weekday.String()),
+	}, nil
+}