feat: add command domain invariants

internal/domain/revision.go

@@ -0,0 +1,63 @@
+package domain
+
+import (
+	"errors"
+	"math"
+)
+
+var (
+	ErrInvalidRevision  = errors.New("command revision must be nonzero")
+	ErrRevisionOverflow = errors.New("command revision overflow")
+	ErrStaleRevision    = errors.New("stale command revision")
+	ErrFutureRevision   = errors.New("unexpected future command revision")
+)
+
+type CommandRevision uint64
+
+const InitialCommandRevision CommandRevision = 1
+
+type RevisionRelation uint8
+
+const (
+	RevisionInvalid RevisionRelation = iota
+	RevisionStale
+	RevisionCurrent
+	RevisionFuture
+)
+
+func CompareRevision(current, incoming CommandRevision) RevisionRelation {
+	if current == 0 || incoming == 0 {
+		return RevisionInvalid
+	}
+	if incoming < current {
+		return RevisionStale
+	}
+	if incoming > current {
+		return RevisionFuture
+	}
+	return RevisionCurrent
+}
+
+func ValidateCurrentRevision(current, incoming CommandRevision) error {
+	switch CompareRevision(current, incoming) {
+	case RevisionCurrent:
+		return nil
+	case RevisionStale:
+		return ErrStaleRevision
+	case RevisionFuture:
+		return ErrFutureRevision
+	default:
+		return ErrInvalidRevision
+	}
+}
+
+// NextRevision durably orders a new cancel or signal intent.
+func NextRevision(current CommandRevision) (CommandRevision, error) {
+	if current == 0 {
+		return 0, ErrInvalidRevision
+	}
+	if current == math.MaxUint64 {
+		return 0, ErrRevisionOverflow
+	}
+	return current + 1, nil
+}