Add slot option for proposer duties.

util/slot.go

@@ -0,0 +1,49 @@
+// Copyright © 2023 Weald Technology Trading
+// 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 util
+
+import (
+	"context"
+	"strconv"
+
+	"github.com/attestantio/go-eth2-client/spec/phase0"
+	"github.com/pkg/errors"
+	"github.com/wealdtech/ethdo/services/chaintime"
+)
+
+// ParseSlot parses input to calculate the desired slot.
+func ParseSlot(_ context.Context, chainTime chaintime.Service, slotStr string) (phase0.Slot, error) {
+	currentSlot := chainTime.CurrentSlot()
+	switch slotStr {
+	case "", "current", "head", "-0":
+		return currentSlot, nil
+	case "last":
+		if currentSlot > 0 {
+			currentSlot--
+		}
+		return currentSlot, nil
+	default:
+		val, err := strconv.ParseInt(slotStr, 10, 64)
+		if err != nil {
+			return 0, errors.Wrap(err, "failed to parse slot")
+		}
+		if val >= 0 {
+			return phase0.Slot(val), nil
+		}
+		if phase0.Slot(-val) > currentSlot {
+			return 0, nil
+		}
+		return currentSlot + phase0.Slot(val), nil
+	}
+}