evaluator,parser: support ASCII(). (pingcap#309)

* evaluator: add a builtin function wanted by pingcap#310.

* parser: update parser for ASCII().

evaluator/builtin.go

@@ -72,6 +72,7 @@ var Funcs = map[string]Func{
 	"date_arith":        {builtinDateArith, 3, 3},
 
 	// string functions
+	"ascii":           {builtinASCII, 1, 1},
 	"concat":          {builtinConcat, 1, -1},
 	"concat_ws":       {builtinConcatWS, 2, -1},
 	"left":            {builtinLeft, 2, 2},

evaluator/builtin_string.go

@@ -46,6 +46,25 @@ func builtinLength(args []types.Datum, _ context.Context) (d types.Datum, err er
 	}
 }
 
+// See: https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_ascii
+func builtinASCII(args []types.Datum, _ context.Context) (d types.Datum, err error) {
+	switch args[0].Kind() {
+	case types.KindNull:
+		return d, nil
+	default:
+		s, err := args[0].ToString()
+		if err != nil {
+			return d, errors.Trace(err)
+		}
+		if len(s) == 0 {
+			d.SetInt64(0)
+			return d, nil
+		}
+		d.SetInt64(int64(s[0]))
+		return d, nil
+	}
+}
+
 // See: https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_concat
 func builtinConcat(args []types.Datum, _ context.Context) (d types.Datum, err error) {
 	var s []byte

evaluator/builtin_string_test.go

@@ -55,6 +55,33 @@ func (s *testEvaluatorSuite) TestLength(c *C) {
 	}
 }
 
+func (s *testEvaluatorSuite) TestASCII(c *C) {
+	defer testleak.AfterTest(c)()
+	v, err := builtinASCII(types.MakeDatums([]interface{}{nil}...), nil)
+	c.Assert(err, IsNil)
+	c.Assert(v.Kind(), Equals, types.KindNull)
+
+	for _, t := range []struct {
+		Input    interface{}
+		Expected int64
+	}{
+		{"", 0},
+		{"A", 65},
+		{"你好", 228},
+		{1, 49},
+		{1.2, 49},
+		{true, 49},
+		{false, 48},
+	} {
+		v, err = builtinASCII(types.MakeDatums(t.Input), nil)
+		c.Assert(err, IsNil)
+		c.Assert(v.GetInt64(), Equals, t.Expected)
+	}
+
+	v, err = builtinASCII(types.MakeDatums([]interface{}{errors.New("must error")}...), nil)
+	c.Assert(err, NotNil)
+}
+
 func (s *testEvaluatorSuite) TestConcat(c *C) {
 	defer testleak.AfterTest(c)()
 	args := []interface{}{nil}