add NewMongoTimestamp() and MongoTimestamp.Time(),Counter()

code is inspired by go-mgo#202

bson/bson.go

@@ -42,6 +42,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"math"
 	"os"
 	"reflect"
 	"runtime"
@@ -426,6 +427,36 @@ func Now() time.Time {
 // strange reason has its own datatype defined in BSON.
 type MongoTimestamp int64
 
+// Time returns the time part of ts which is stored with second precision.
+func (ts MongoTimestamp) Time() time.Time {
+	return time.Unix(int64(uint64(ts)>>32), 0)
+}
+
+// Counter returns the counter part of ts.
+func (ts MongoTimestamp) Counter() uint32 {
+	return uint32(ts)
+}
+
+// NewMongoTimestamp creates a timestamp using the given
+// date `t` (with second precision) and counter `c` (unique for `t`).
+//
+// Returns an error if time `t` is not between 1970-01-01T00:00:00Z
+// and 2106-02-07T06:28:15Z (inclusive).
+//
+// Note that two MongoTimestamps should never have the same (time, counter) combination:
+// the caller must ensure the counter `c` is increased if creating multiple MongoTimestamp
+// values for the same time `t` (ignoring fractions of seconds).
+func NewMongoTimestamp(t time.Time, c uint32) (MongoTimestamp, error) {
+	u := t.Unix()
+	if u < 0 || u > math.MaxUint32 {
+		return -1, errors.New("invalid value for time")
+	}
+
+	i := int64(u<<32 | int64(c))
+
+	return MongoTimestamp(i), nil
+}
+
 type orderKey int64
 
 // MaxKey is a special value that compares higher than all other possible BSON

bson/bson_test.go

@@ -32,6 +32,8 @@ import (
 	"encoding/json"
 	"encoding/xml"
 	"errors"
+	"fmt"
+	"math/rand"
 	"net/url"
 	"reflect"
 	"strings"
@@ -286,12 +288,12 @@ func (s *S) TestPtrInline(c *C) {
 		},
 		{
 			// nil embed struct
-			In: &inlinePtrStruct{A: 3},
+			In:  &inlinePtrStruct{A: 3},
 			Out: bson.M{"a": 3},
 		},
 		{
 			// nil embed struct
-			In: &inlinePtrPtrStruct{B: 5},
+			In:  &inlinePtrPtrStruct{B: 5},
 			Out: bson.M{"b": 5},
 		},
 	}
@@ -1205,18 +1207,18 @@ type inlineBadKeyMap struct {
 	M map[int]int `bson:",inline"`
 }
 type inlineUnexported struct {
-	M map[string]interface{} `bson:",inline"`
-	unexported               `bson:",inline"`
+	M          map[string]interface{} `bson:",inline"`
+	unexported `bson:",inline"`
 }
 type MStruct struct {
 	M int `bson:"m,omitempty"`
 }
 type inlinePtrStruct struct {
-	A int
+	A        int
 	*MStruct `bson:",inline"`
 }
 type inlinePtrPtrStruct struct {
-	B int
+	B                int
 	*inlinePtrStruct `bson:",inline"`
 }
 type unexported struct {
@@ -1274,11 +1276,11 @@ func (s ifaceSlice) GetBSON() (interface{}, error) {
 
 type (
 	MyString string
-	MyBytes []byte
-	MyBool bool
-	MyD []bson.DocElem
-	MyRawD []bson.RawDocElem
-	MyM map[string]interface{}
+	MyBytes  []byte
+	MyBool   bool
+	MyD      []bson.DocElem
+	MyRawD   []bson.RawDocElem
+	MyM      map[string]interface{}
 )
 
 var (
@@ -1989,3 +1991,49 @@ func (s *S) TestMarshalRespectNil(c *C) {
 	c.Assert(testStruct2.Map, NotNil)
 	c.Assert(testStruct2.MapPtr, NotNil)
 }
+
+func (s *S) TestMongoTimestampTime(c *C) {
+	t := time.Now()
+	ts, err := bson.NewMongoTimestamp(t, 123)
+	c.Assert(err, IsNil)
+	c.Assert(ts.Time().Unix(), Equals, t.Unix())
+}
+
+func (s *S) TestMongoTimestampCounter(c *C) {
+	rnd := rand.Uint32()
+	ts, err := bson.NewMongoTimestamp(time.Now(), rnd)
+	c.Assert(err, IsNil)
+	c.Assert(ts.Counter(), Equals, rnd)
+}
+
+func (s *S) TestMongoTimestampError(c *C) {
+	t := time.Date(1969, time.December, 31, 23, 59, 59, 999, time.UTC)
+	ts, err := bson.NewMongoTimestamp(t, 321)
+	c.Assert(int64(ts), Equals, int64(-1))
+	c.Assert(err, ErrorMatches, "invalid value for time")
+}
+
+func ExampleNewMongoTimestamp() {
+
+	var counter uint32 = 1
+	var t time.Time
+
+	for i := 1; i <= 3; i++ {
+
+		if c := time.Now(); t.Unix() == c.Unix() {
+			counter++
+		} else {
+			t = c
+			counter = 1
+		}
+
+		ts, err := bson.NewMongoTimestamp(t, counter)
+		if err != nil {
+			fmt.Printf("NewMongoTimestamp error: %v", err)
+		} else {
+			fmt.Printf("NewMongoTimestamp encoded timestamp: %d\n", ts)
+		}
+
+		time.Sleep(500 * time.Millisecond)
+	}
+}