Add Compose method and corresponding tests (#79)

* Implemented the Compose function to create a Sonyflake ID from its components, including validation for start time, sequence, and machine ID.
* Added a new test, TestCompose, to verify the functionality of the Compose method, ensuring correct decomposition of generated IDs.
* Introduced a new error for invalid sequence numbers to enhance error handling in the Sonyflake package.

sonyflake.go

@@ -57,6 +57,7 @@ var (
 	ErrNoPrivateAddress = errors.New("no private ip address")
 	ErrOverTimeLimit    = errors.New("over the time limit")
 	ErrInvalidMachineID = errors.New("invalid machine id")
+	ErrInvalidSequence  = errors.New("invalid sequence number")
 )
 
 var defaultInterfaceAddrs = net.InterfaceAddrs
@@ -213,6 +214,25 @@ func MachineID(id uint64) uint64 {
 	return id & maskMachineID
 }
 
+// Compose creates a Sonyflake ID from its parts.
+func Compose(sf *Sonyflake, t time.Time, sequence uint16, machineID uint16) (uint64, error) {
+	elapsedTime := toSonyflakeTime(t.UTC()) - sf.startTime
+	if elapsedTime < 0 {
+		return 0, ErrStartTimeAhead
+	}
+	if elapsedTime >= 1<<BitLenTime {
+		return 0, ErrOverTimeLimit
+	}
+
+	if sequence >= 1<<BitLenSequence {
+		return 0, ErrInvalidSequence
+	}
+
+	return uint64(elapsedTime)<<(BitLenSequence+BitLenMachineID) |
+		uint64(sequence)<<BitLenMachineID |
+		uint64(machineID), nil
+}
+
 // Decompose returns a set of Sonyflake ID parts.
 func Decompose(id uint64) map[string]uint64 {
 	msb := id >> 63

sonyflake_test.go

@@ -312,3 +312,36 @@ func TestSonyflakeTimeUnit(t *testing.T) {
 		t.Errorf("unexpected time unit")
 	}
 }
+
+func TestCompose(t *testing.T) {
+	var st Settings
+	st.StartTime = time.Date(2023, 1, 1, 0, 0, 0, 0, time.UTC)
+	sf, err := New(st)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	now := time.Now()
+	sequence := uint16(123)
+	machineID := uint16(456)
+
+	id, err := Compose(sf, now, sequence, machineID)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	parts := Decompose(id)
+
+	actualTime := toSonyflakeTime(now) - toSonyflakeTime(st.StartTime)
+	if parts["time"] != uint64(actualTime) {
+		t.Errorf("unexpected time: %d", parts["time"])
+	}
+
+	if parts["sequence"] != uint64(sequence) {
+		t.Errorf("unexpected sequence: %d", parts["sequence"])
+	}
+
+	if parts["machine-id"] != uint64(machineID) {
+		t.Errorf("unexpected machine id: %d", parts["machine-id"])
+	}
+}