diff --git a/README.md b/README.md
index d5ec407..b66d05c 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,3 @@
 # algorithm
-搜集的大量算法库
+
+A large collection of algorithms.
diff --git a/chinaid/README.md b/chinaid/README.md
new file mode 100644
index 0000000..bec7e6d
--- /dev/null
+++ b/chinaid/README.md
@@ -0,0 +1,3 @@
+# chinaid
+
+Generate and verify the Chinese identity card number.
\ No newline at end of file
diff --git a/chinaid/china_id.go b/chinaid/china_id.go
new file mode 100644
index 0000000..bd72c40
--- /dev/null
+++ b/chinaid/china_id.go
@@ -0,0 +1,259 @@
+package chinaid
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"strconv"
+	"unicode"
+	"unsafe"
+)
+
+// ErrInvalidChinaID error
+var ErrInvalidChinaID = errors.New("invalid Chinese identity card number")
+
+// ChinaID Chinese identity card number
+type ChinaID struct {
+	whole []byte
+	body  []byte
+	addr  []byte
+	birth []byte
+	seq   []byte
+	sex   int // 1 male / 0 female
+	check []byte
+}
+
+// Make18 generates an 18-character length number
+func Make18(addr [6]byte, birth [8]byte, seq [3]byte) (*ChinaID, bool) {
+	body := string(addr[:]) + string(birth[:]) + string(seq[:])
+	if !isDigit(body, -1) {
+		return nil, false
+	}
+	c := &ChinaID{
+		body:  []byte(body),
+		addr:  addr[:],
+		birth: birth[:],
+		seq:   seq[:],
+	}
+	c.check = []byte{c.createCheckCode()}
+	c.whole = append(c.body, c.check...)
+	c.setSex()
+	return c, true
+}
+
+// Make15 generates an 15-character length number
+func Make15(addr [6]byte, birth [6]byte, seq [3]byte) (*ChinaID, bool) {
+	body := string(addr[:]) + string(birth[:]) + string(seq[:])
+	if !isDigit(body, -1) {
+		return nil, false
+	}
+	c := &ChinaID{
+		body:  []byte(body),
+		addr:  addr[:],
+		birth: birth[:],
+		seq:   seq[:],
+	}
+	c.whole = c.body
+	c.setSex()
+	return c, true
+}
+
+// Validate judge whether chinese identity card number is valid.
+func Validate(id string) bool {
+	l := len(id)
+	switch l {
+	case 15:
+		return true
+	case 18:
+		var sum int
+		for k, v := range id[:l-1] {
+			d, _ := strconv.Atoi(string(v))
+			sum += weight[k] * d
+		}
+		return validate[sum%11] == id[l-1]
+	default:
+		return false
+	}
+}
+
+// CheckCode generates check code of chinese identity card number.
+func CheckCode(idBody string) (string, bool) {
+	if len(idBody) == 17 {
+		var sum int
+		for k, v := range idBody {
+			d, _ := strconv.Atoi(string(v))
+			sum += weight[k] * d
+		}
+		return string(validate[sum%11]), true
+	}
+	return "", false
+}
+
+// Parse parse chinese identity card number to *ChinaID
+func Parse(id string) (*ChinaID, error) {
+	if !isDigit(id, 17) {
+		return nil, ErrInvalidChinaID
+	}
+	whole := []byte(id)
+	switch len(whole) {
+	case 18:
+		c := &ChinaID{
+			whole: whole,
+			body:  whole[:17],
+			addr:  whole[:6],
+			birth: whole[6:14],
+			seq:   whole[14:17],
+			check: whole[17:],
+		}
+		if !c.validate() {
+			return nil, ErrInvalidChinaID
+		}
+		c.setSex()
+		return c, nil
+
+	case 15:
+		c := &ChinaID{
+			whole: whole,
+			body:  whole,
+			addr:  whole[:6],
+			birth: whole[6:12],
+			seq:   whole[12:],
+		}
+		c.setSex()
+		return c, nil
+	default:
+		return nil, ErrInvalidChinaID
+	}
+}
+
+// String returns chinese identity card number
+func (c *ChinaID) String() string {
+	return bytes2String(c.whole)
+}
+
+// MarshalJSON implements json.Marshaler
+func (c *ChinaID) MarshalJSON() ([]byte, error) {
+	var m = map[string]interface{}{
+		"whole": c.String(),
+		"body":  c.Body(),
+		"addr":  c.Addr(),
+		"birth": c.Birth(),
+		"seq":   c.Seq(),
+		"sex":   c.sex,
+	}
+	if len(c.check) > 0 {
+		m["check"] = c.Check()
+	}
+	return json.Marshal(m)
+}
+
+// func (c *ChinaID) GoString() string {
+// 	b, _ := c.MarshalJSON()
+// 	return bytes2String(b)
+// }
+
+// Format implements fmt.Formatter
+func (c *ChinaID) Format(f fmt.State, r rune) {
+	if r == 'v' && (f.Flag('+') || f.Flag('#')) {
+		b, _ := c.MarshalJSON()
+		f.Write(b)
+		return
+	}
+	f.Write(c.whole)
+}
+
+// Body returns main part of chinese identity card number
+func (c *ChinaID) Body() string {
+	return bytes2String(c.body)
+}
+
+// Addr returns address code
+func (c *ChinaID) Addr() string {
+	return bytes2String(c.addr)
+}
+
+// Birth returns date of birth
+func (c *ChinaID) Birth() string {
+	return bytes2String(c.birth)
+}
+
+// Seq returns sequence number in the persons whose date of birth are same
+func (c *ChinaID) Seq() string {
+	return bytes2String(c.seq)
+}
+
+// Check returns check code
+func (c *ChinaID) Check() string {
+	return bytes2String(c.check)
+}
+
+// Sex returns sex
+// 1 male / 0 female
+func (c *ChinaID) Sex() int {
+	return c.sex
+}
+
+// SexText returns sex text, male or female
+func (c *ChinaID) SexText() string {
+	if c.IsMale() {
+		return "male"
+	}
+	return "female"
+}
+
+// IsMale judge whether is male or not
+func (c *ChinaID) IsMale() bool {
+	return c.sex == 1
+}
+
+// IsMale judge whether is female or not
+func (c *ChinaID) IsFemale() bool {
+	return c.sex == 0
+}
+
+func (c *ChinaID) setSex() {
+	i, _ := strconv.Atoi(bytes2String(c.seq[2:]))
+	c.sex = i % 2
+}
+
+var (
+	weight   = [17]int{7, 9, 10, 5, 8, 4, 2, 1, 6, 3, 7, 9, 10, 5, 8, 4, 2}
+	validate = [17]byte{'1', '0', 'X', '9', '8', '7', '6', '5', '4', '3', '2'}
+)
+
+func (c *ChinaID) createCheckCode() byte {
+	var sum int
+	for k, v := range c.body {
+		d, _ := strconv.Atoi(string(v))
+		sum += weight[k] * d
+	}
+	return validate[sum%11]
+}
+
+func (c *ChinaID) validate() bool {
+	return c.check[0] == c.createCheckCode()
+}
+
+func isDigit(s string, maxLen int) bool {
+	if maxLen < 0 {
+		for _, r := range s {
+			if !unicode.IsDigit(r) {
+				return false
+			}
+		}
+	} else {
+		for i, r := range s {
+			if i >= maxLen {
+				break
+			}
+			if !unicode.IsDigit(r) {
+				return false
+			}
+		}
+	}
+	return true
+}
+
+func bytes2String(b []byte) string {
+	return *(*string)(unsafe.Pointer(&b))
+}
diff --git a/chinaid/chinaid_test.go b/chinaid/chinaid_test.go
new file mode 100644
index 0000000..0025f46
--- /dev/null
+++ b/chinaid/chinaid_test.go
@@ -0,0 +1,78 @@
+package chinaid
+
+import (
+	"testing"
+)
+
+func Test18(t *testing.T) {
+	// make ChinaID18
+
+	addr := [6]byte{}
+	for k, v := range "326221" {
+		addr[k] = byte(v)
+	}
+	birth := [8]byte{}
+	for k, v := range "19880102" {
+		birth[k] = byte(v)
+	}
+	seq := [3]byte{}
+	for k, v := range "124" {
+		seq[k] = byte(v)
+	}
+	id18, ok := Make18(addr, birth, seq)
+	t.Logf("Make18(%v): %s", ok, id18)
+
+	// check ChinaID18
+	chinaId, err := Parse(id18.String())
+	if err != nil {
+		t.Fatalf("%v", err)
+	}
+	t.Logf("id18 format(%%v): %v, sex: %s\n\n", chinaId, chinaId.SexText())
+	t.Logf("id18 format(%%+v): %#v\n\n", chinaId)
+}
+
+func Test15(t *testing.T) {
+	// make ChinaID15
+
+	addr := [6]byte{}
+	for k, v := range "326221" {
+		addr[k] = byte(v)
+	}
+	birth := [6]byte{}
+	for k, v := range "880102" {
+		birth[k] = byte(v)
+	}
+	seq := [3]byte{}
+	for k, v := range "124" {
+		seq[k] = byte(v)
+	}
+	id15, ok := Make15(addr, birth, seq)
+	t.Logf("Make15(%v): %s", ok, id15)
+
+	// check ChinaID15
+	chinaId, err := Parse(id15.String())
+	if err != nil {
+		t.Fatalf("%v", err)
+	}
+	t.Logf("id15 format(%%v): %v, sex: %s\n\n", chinaId, chinaId.SexText())
+	t.Logf("id15 format(%%+v): %#v\n\n", chinaId)
+}
+
+func TestValidate(t *testing.T) {
+	ok := Validate("32622119880102124X")
+	if !ok {
+		t.Fail()
+	}
+}
+
+func TestCheckCode(t *testing.T) {
+	checkCode, ok := CheckCode("32622119880102124")
+	if !ok || checkCode != "X" {
+		t.Fatalf("get '%s', expect 'X'", checkCode)
+	}
+
+	checkCode, ok = CheckCode("326221880102124")
+	if ok {
+		t.Fatalf("get '%s', expect ''", checkCode)
+	}
+}