done that was fun

main.go

@@ -0,0 +1,100 @@
+package main
+
+import (
+	"hash/fnv"
+	"log"
+	"os"
+	"strconv"
+	"sync"
+	"sync/atomic"
+	"time"
+)
+
+func main() {
+	log.Println(host_10b)
+	log.Println(epoch)
+	log.Println(epoch.UnixNano() / int64(time.Millisecond) / 10)
+	log.Println(ts_41b())
+	for i := 0; i < 7; i++ {
+		time.Sleep(time.Duration(i) * time.Millisecond)
+		log.Printf("%v = ts_41b/%v/seq_13b/%v/host_10b/%v", id(), ts_41b(), seq_13b(), host_10b)
+	}
+}
+
+func id() uint64 {
+	ts := ts_41b() << (64 - 41)
+	seq := seq_13b() << (64 - 41 - 13)
+	host := host_10b
+	return ts | seq | host
+}
+
+func ts_41b() uint64 {
+	durationSinceEpoch := time.Now().Sub(epoch)
+	return uint64(durationSinceEpoch/time.Millisecond) / 10
+}
+
+var seqLock = &sync.RWMutex{}
+var seqV = &atomic.Uint64{}
+var seqTS = &atomic.Int64{}
+
+func seq_13b() uint64 {
+	_seq_13b_rollover()
+	got := func() uint64 {
+		seqLock.RLock()
+		defer seqLock.RUnlock()
+		return seqV.Add(1)
+	}()
+	if got > (1 << 13) {
+		time.Sleep(time.Millisecond * 10)
+		return seq_13b()
+	}
+	return got
+}
+
+func _seq_13b_now() int64 {
+	return time.Now().UnixNano() / int64(time.Millisecond) / 10
+}
+
+func _seq_13b_rollover() {
+	now := _seq_13b_now()
+	if seqTS.Load() == now {
+		return
+	}
+	func() {
+		seqLock.Lock()
+		defer seqLock.Unlock()
+		if seqTS.Load() == now {
+			return
+		}
+		seqTS.Store(now)
+		seqV.Store(0)
+	}()
+}
+
+var epoch = func() time.Time {
+	t := time.Unix(0, 0)
+	if v := os.Getenv("SNOWFLAKE_UID_SERVICE_EPOCH"); v != "" {
+		// 	RFC3339     = "2006-01-02T15:04:05Z07:00"
+		t2, err := time.ParseInLocation(time.RFC3339, v, time.UTC)
+		if err != nil {
+			panic(err)
+		}
+		t = t2
+	}
+	return t // uint64(t.UnixNano() / int64(time.Millisecond) / 10)
+}()
+
+var host_10b = func() uint64 {
+	if v := os.Getenv("SNOWFLAKE_UID_SERVICE_HOST"); v != "" {
+		v, err := strconv.ParseUint(v, 10, 10)
+		if err != nil {
+			panic(err)
+		}
+		ui64 := uint64(v)
+		return uint64(ui64 << 54 >> 54)
+	}
+	hash := fnv.New32()
+	hash.Write([]byte(os.Getenv("HOSTNAME")))
+	ui32 := hash.Sum32()
+	return uint64((ui32 << 22) >> 22)
+}()

main_test.go

@@ -0,0 +1,45 @@
+package main
+
+import (
+	"testing"
+	"time"
+)
+
+func TestID(t *testing.T) {
+	last := id()
+	for i := 0; i < 1000; i++ {
+		next := id()
+		if next <= last {
+			t.Error(i, last, next)
+		}
+		last = next
+	}
+}
+
+func TestTS(t *testing.T) {
+	got := int64(ts_41b())
+	if got != ((got << (64 - 41)) >> (64 - 41)) {
+		t.Error("got > 41b")
+	}
+	gotTime := time.Unix(got/100, 10*(got%100))
+	if time.Since(gotTime) > time.Second {
+		t.Error(gotTime)
+	}
+}
+
+func TestSeq(t *testing.T) {
+	last := seq_13b()
+	for i := 0; i < 1000; i++ {
+		next := seq_13b()
+		if next <= last {
+			t.Error(i, last, next)
+		}
+		last = next
+	}
+
+	seqV.Store(1 << 14)
+	got := seq_13b()
+	if got > 2 {
+		t.Error(got)
+	}
+}