From 89622d16ad7b22579184e93a13631cd7b550ed95 Mon Sep 17 00:00:00 2001
From: ti-srebot <66930949+ti-srebot@users.noreply.github.com>
Date: Tue, 26 Jan 2021 16:52:39 +0800
Subject: [PATCH] expression: change the round rule for approximate value to
 `round to nearest even`  (#21324) (#21628)

Signed-off-by: ti-srebot <ti-srebot@pingcap.com>
---
 expression/builtin_cast_test.go |  4 ++--
 expression/builtin_math_test.go |  2 ++
 expression/builtin_time_test.go |  2 +-
 expression/integration_test.go  |  2 +-
 types/convert_test.go           |  2 +-
 types/etc_test.go               |  8 ++++----
 types/helper.go                 | 10 +++-------
 7 files changed, 14 insertions(+), 16 deletions(-)

diff --git a/expression/builtin_cast_test.go b/expression/builtin_cast_test.go
index 2969b0ab848a9..fb5cc1e8ce079 100644
--- a/expression/builtin_cast_test.go
+++ b/expression/builtin_cast_test.go
@@ -488,8 +488,8 @@ func (s *testEvaluatorSuite) TestCastFuncSig(c *C) {
 		// cast real as int.
 		{
 			&Column{RetType: types.NewFieldType(mysql.TypeDouble), Index: 0},
-			1,
-			chunk.MutRowFromDatums([]types.Datum{types.NewFloat64Datum(1)}),
+			2,
+			chunk.MutRowFromDatums([]types.Datum{types.NewFloat64Datum(2.5)}),
 		},
 		// cast Time as int.
 		{
diff --git a/expression/builtin_math_test.go b/expression/builtin_math_test.go
index 81caabfb30791..e07c7a19f5b70 100644
--- a/expression/builtin_math_test.go
+++ b/expression/builtin_math_test.go
@@ -431,6 +431,8 @@ func (s *testEvaluatorSuite) TestRound(c *C) {
 		{[]interface{}{1.298, 1}, 1.3},
 		{[]interface{}{1.298}, 1},
 		{[]interface{}{1.298, 0}, 1},
+		{[]interface{}{-1.5, 0}, -2},
+		{[]interface{}{1.5, 0}, 2},
 		{[]interface{}{23.298, -1}, 20},
 		{[]interface{}{newDec("-1.23")}, newDec("-1")},
 		{[]interface{}{newDec("-1.23"), 1}, newDec("-1.2")},
diff --git a/expression/builtin_time_test.go b/expression/builtin_time_test.go
index 9cc0cef5260c1..3ee725e78d578 100644
--- a/expression/builtin_time_test.go
+++ b/expression/builtin_time_test.go
@@ -2166,7 +2166,7 @@ func (s *testEvaluatorSuite) TestMakeTime(c *C) {
 
 		{[]interface{}{0, 58.4, 0}, "00:58:00"},
 		{[]interface{}{0, "58.4", 0}, "00:58:00"},
-		{[]interface{}{0, 58.5, 1}, "00:59:01"},
+		{[]interface{}{0, 58.5, 1}, "00:58:01"},
 		{[]interface{}{0, "58.5", 1}, "00:58:01"},
 		{[]interface{}{0, 59.5, 1}, nil},
 		{[]interface{}{0, "59.5", 1}, "00:59:01"},
diff --git a/expression/integration_test.go b/expression/integration_test.go
index da50573ddb0b2..85602d0caf8cc 100755
--- a/expression/integration_test.go
+++ b/expression/integration_test.go
@@ -579,7 +579,7 @@ func (s *testIntegrationSuite2) TestMathBuiltin(c *C) {
 
 	// for round
 	result = tk.MustQuery("SELECT ROUND(2.5), ROUND(-2.5), ROUND(25E-1);")
-	result.Check(testkit.Rows("3 -3 3")) // TODO: Should be 3 -3 2
+	result.Check(testkit.Rows("3 -3 2"))
 	result = tk.MustQuery("SELECT ROUND(2.5, NULL), ROUND(NULL, 4), ROUND(NULL, NULL), ROUND(NULL);")
 	result.Check(testkit.Rows("<nil> <nil> <nil> <nil>"))
 	result = tk.MustQuery("SELECT ROUND('123.4'), ROUND('123e-2');")
diff --git a/types/convert_test.go b/types/convert_test.go
index a2c1978dc4ffd..1a2954d9c93e8 100644
--- a/types/convert_test.go
+++ b/types/convert_test.go
@@ -970,7 +970,7 @@ func (s *testTypeConvertSuite) TestConvertJSONToInt(c *C) {
 		{`[]`, 0},
 		{`3`, 3},
 		{`-3`, -3},
-		{`4.5`, 5},
+		{`4.5`, 4},
 		{`true`, 1},
 		{`false`, 0},
 		{`null`, 0},
diff --git a/types/etc_test.go b/types/etc_test.go
index 6f061018fb390..1a5778f16b8e8 100644
--- a/types/etc_test.go
+++ b/types/etc_test.go
@@ -137,14 +137,14 @@ func (s *testTypeEtcSuite) TestRoundFloat(c *C) {
 		Input  float64
 		Expect float64
 	}{
-		{2.5, 3},
+		{2.5, 2},
 		{1.5, 2},
-		{0.5, 1},
+		{0.5, 0},
 		{0.49999999999999997, 0},
 		{0, 0},
 		{-0.49999999999999997, 0},
-		{-0.5, -1},
-		{-2.5, -3},
+		{-0.5, 0},
+		{-2.5, -2},
 		{-1.5, -2},
 	}
 
diff --git a/types/helper.go b/types/helper.go
index 2c8d4b3c8f9f8..e0ea721c53a96 100644
--- a/types/helper.go
+++ b/types/helper.go
@@ -21,16 +21,12 @@ import (
 	"github.com/pingcap/errors"
 )
 
-// RoundFloat rounds float val to the nearest integer value with float64 format, like MySQL Round function.
+// RoundFloat rounds float val to the nearest even integer value with float64 format, like MySQL Round function.
 // RoundFloat uses default rounding mode, see https://dev.mysql.com/doc/refman/5.7/en/precision-math-rounding.html
-// so rounding use "round half away from zero".
+// so rounding use "round to nearest even".
 // e.g, 1.5 -> 2, -1.5 -> -2.
 func RoundFloat(f float64) float64 {
-	if math.Abs(f) < 0.5 {
-		return 0
-	}
-
-	return math.Trunc(f + math.Copysign(0.5, f))
+	return math.RoundToEven(f)
 }
 
 // Round rounds the argument f to dec decimal places.