add sequence bounds (#21)

sequence.go

@@ -35,6 +35,21 @@ func (seq *Sequence) Next() (KSUID, error) {
 		return Nil, errors.New("too many IDs were generated")
 	}
 	seq.count++
-	binary.BigEndian.PutUint16(id[len(id)-2:], uint16(count))
-	return id, nil
+	return withSequenceNumber(id, uint16(count)), nil
+}
+
+// Bounds returns the inclusive min and max bounds of the KSUIDs that may be
+// generated by the sequence. If all ids have been generated already then the
+// returned min value is equal to the max.
+func (seq *Sequence) Bounds() (min KSUID, max KSUID) {
+	count := seq.count
+	if count > math.MaxUint16 {
+		count = math.MaxUint16
+	}
+	return withSequenceNumber(seq.Seed, uint16(count)), withSequenceNumber(seq.Seed, math.MaxUint16)
+}
+
+func withSequenceNumber(id KSUID, n uint16) KSUID {
+	binary.BigEndian.PutUint16(id[len(id)-2:], n)
+	return id
 }

sequence_test.go

@@ -9,6 +9,10 @@ import (
 func TestSequence(t *testing.T) {
 	seq := Sequence{Seed: New()}
 
+	if min, max := seq.Bounds(); min == max {
+		t.Error("min and max of KSUID range must differ when no ids have been generated")
+	}
+
 	for i := 0; i <= math.MaxUint16; i++ {
 		id, err := seq.Next()
 		if err != nil {
@@ -22,4 +26,8 @@ func TestSequence(t *testing.T) {
 	if _, err := seq.Next(); err == nil {
 		t.Fatal("no error returned after exhausting the id generator")
 	}
+
+	if min, max := seq.Bounds(); min != max {
+		t.Error("after all KSUIDs were generated the min and max must be equal")
+	}
 }