util: fix codec for negative zero (#57343)

close #41878

Author: wshwsh12

pkg/util/codec/codec_test.go

@@ -1333,3 +1333,22 @@ func TestDatumHashEquals(t *testing.T) {
 	require.NotEqual(t, hasher1.Sum64(), hasher2.Sum64())
 	require.False(t, tests[len(tests)-1].d1.Equals(tests[len(tests)-1].d2))
 }
+
+func TestEncodeFloatForNegativeZero(t *testing.T) {
+	floatNum := 0.0
+	floatNum = -floatNum
+
+	b := EncodeFloat(nil, floatNum)
+	_, v, err := DecodeFloat(b)
+	require.NoError(t, err)
+	require.Equal(t, floatNum, v)
+	require.Equal(t, math.Signbit(floatNum), math.Signbit(v))
+	require.Equal(t, math.Atan2(0, v), math.Pi)
+
+	b = EncodeFloatDesc(nil, floatNum)
+	_, v, err = DecodeFloatDesc(b)
+	require.NoError(t, err)
+	require.Equal(t, floatNum, v)
+	require.Equal(t, math.Signbit(floatNum), math.Signbit(v))
+	require.Equal(t, math.Atan2(0, v), math.Pi)
+}

pkg/util/codec/float.go

@@ -22,9 +22,17 @@ import (
 
 func encodeFloatToCmpUint64(f float64) uint64 {
 	u := math.Float64bits(f)
-	if f >= 0 {
+	// Check the sign bit (the highest bit in the IEEE 754 representation).
+	// If the sign bit is 0, the number is non-negative (+0 is considered non-negative).
+	if u&signMask == 0 {
+		// For non-negative numbers (+0 included), set the sign bit to 1.
+		// This ensures non-negative numbers are ordered after negative numbers
+		// when compared as unsigned integers.
 		u |= signMask
 	} else {
+		// For negative numbers (-0 included), invert all bits.
+		// This reorders negative numbers so that the smallest (closest to 0)
+		// has the smallest encoded value, and the most negative has the largest.
 		u = ^u
 	}
 	return u

tests/integrationtest/r/expression/issues.result

@@ -3230,3 +3230,9 @@ INSERT INTO test.t VALUES (1234567890123456);
 SELECT IFNULL(id, 'abcdef') FROM test.t;
 IFNULL(id, 'abcdef')
 1234567890123456
+DROP TABLE IF EXISTS test.t;
+CREATE TABLE test.t (c0 decimal(10,0));
+INSERT INTO test.t VALUES (0);
+SELECT c0 FROM test.t WHERE CAST(ATAN2(((t.c0) IS NULL), (- (''))) AS TIME);
+c0
+0

tests/integrationtest/t/expression/issues.test

@@ -2182,3 +2182,9 @@ DROP TABLE IF EXISTS test.t;
 CREATE TABLE test.t (id bigint(11) UNSIGNED PRIMARY KEY);
 INSERT INTO test.t VALUES (1234567890123456);
 SELECT IFNULL(id, 'abcdef') FROM test.t;
+
+# TestIssue41878
+DROP TABLE IF EXISTS test.t;
+CREATE TABLE test.t (c0 decimal(10,0));
+INSERT INTO test.t VALUES (0);
+SELECT c0 FROM test.t WHERE CAST(ATAN2(((t.c0) IS NULL), (- (''))) AS TIME);