fix: Monotonicity in UUIDv7 (google#150)

* Monotonicity in UUIDv7

* fix Monotonicity

* fix comment

* Monotonicity 2

* lock

* fix comment

* fix comment

uuid_test.go

@@ -825,7 +825,7 @@ func TestVersion6(t *testing.T) {
 func TestVersion7(t *testing.T) {
 	SetRand(nil)
 	m := make(map[string]bool)
-	for x := 1; x < 32; x++ {
+	for x := 1; x < 128; x++ {
 		uuid, err := NewV7()
 		if err != nil {
 			t.Fatalf("could not create UUID: %v", err)
@@ -874,20 +874,25 @@ func TestVersion7_pooled(t *testing.T) {
 func TestVersion7FromReader(t *testing.T) {
 	myString := "8059ddhdle77cb52"
 	r := bytes.NewReader([]byte(myString))
-	r2 := bytes.NewReader([]byte(myString))
-	uuid1, err := NewV7FromReader(r)
+	_, err := NewV7FromReader(r)
 	if err != nil {
 		t.Errorf("failed generating UUID from a reader")
 	}
 	_, err = NewV7FromReader(r)
 	if err == nil {
 		t.Errorf("expecting an error as reader has no more bytes. Got uuid. NewV7FromReader may not be using the provided reader")
 	}
-	uuid3, err := NewV7FromReader(r2)
-	if err != nil {
-		t.Errorf("failed generating UUID from a reader")
-	}
-	if uuid1 != uuid3 {
-		t.Errorf("expected duplicates, got %q and %q", uuid1, uuid3)
+}
+
+func TestVersion7Monotonicity(t *testing.T) {
+	length := 10000
+	u1 := Must(NewV7()).String()
+	for i := 0; i < length; i++ {
+		u2 := Must(NewV7()).String()
+		if u2 <= u1 {
+			t.Errorf("monotonicity failed at #%d: %s(next) < %s(before)", i, u2, u1)
+			break
+		}
+		u1 = u2
 	}
 }

version7.go

@@ -44,15 +44,15 @@ func NewV7FromReader(r io.Reader) (UUID, error) {
 
 // makeV7 fill 48 bits time (uuid[0] - uuid[5]), set version b0111 (uuid[6])
 // uuid[8] already has the right version number (Variant is 10)
-// see function  NewV7 and NewV7FromReader
+// see function NewV7 and NewV7FromReader
 func makeV7(uuid []byte) {
 	/*
 		 0                   1                   2                   3
 		 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 		|                           unix_ts_ms                          |
 		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-		|          unix_ts_ms           |  ver  |       rand_a          |
+		|          unix_ts_ms           |  ver  |  rand_a (12 bit seq)  |
 		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 		|var|                        rand_b                             |
 		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
@@ -61,7 +61,7 @@ func makeV7(uuid []byte) {
 	*/
 	_ = uuid[15] // bounds check
 
-	t := timeNow().UnixMilli()
+	t, s := getV7Time()
 
 	uuid[0] = byte(t >> 40)
 	uuid[1] = byte(t >> 32)
@@ -70,6 +70,35 @@ func makeV7(uuid []byte) {
 	uuid[4] = byte(t >> 8)
 	uuid[5] = byte(t)
 
-	uuid[6] = 0x70 | (uuid[6] & 0x0F)
-	// uuid[8] has already has right version
+	uuid[6] = 0x70 | (0x0F & byte(s>>8))
+	uuid[7] = byte(s)
+}
+
+// lastV7time is the last last time we returned stored as:
+//
+//	52 bits of time in milliseconds since epoch
+//	12 bits of (fractional nanoseconds) >> 8
+var lastV7time int64
+
+const nanoPerMilli = 1000000
+
+// getV7Time returns the time in milliseconds and nanoseconds / 256.
+// The returned (milli << 12 + seq) is guarenteed to be greater than
+// (milli << 12 + seq) returned by any previous call to getV7Time.
+func getV7Time() (milli, seq int64) {
+	timeMu.Lock()
+	defer timeMu.Unlock()
+
+	nano := timeNow().UnixNano()
+	milli = nano / nanoPerMilli
+	// Sequence number is between 0 and 3906 (nanoPerMilli>>8)
+	seq = (nano - milli*nanoPerMilli) >> 8
+	now := milli<<12 + seq
+	if now <= lastV7time {
+		now = lastV7time + 1
+		milli = now >> 12
+		seq = now & 0xfff
+	}
+	lastV7time = now
+	return milli, seq
 }