rings: introduce RingID and a Ring placeholder

Signed-off-by: Alejandro Mery <amery@jpi.io>

pkg/rings/rings.go

@@ -11,6 +11,8 @@ import (
 )
 
 const (
+	// RingMax indicates the highest [Ring] identifier
+	RingMax = 4
 	// RegionMax indicates the highest number that can be used for a [RegionID].
 	RegionMax = (1 << 4) - 1
 	// ZoneMax indicates the highest number that can be used for a [ZoneID].
@@ -33,6 +35,21 @@ const (
 	RingThreeBits = 12
 )
 
+// RingID identifies a Ring
+type RingID int
+
+// Valid tells a [RingID] is within the valid range.
+func (n RingID) Valid() bool { return n > 0 && n <= RingMax }
+
+func (n RingID) String() string {
+	return idString(n)
+}
+
+// A Ring identifies what ring an address belongs to
+type Ring interface {
+	ID() RingID
+}
+
 // RegionID is the identifier of a region, valid between 1 and [RegionMax].
 type RegionID int