BUG: Fix rollover handling in json encoding

doc/source/whatsnew/v0.20.0.txt

@@ -988,6 +988,7 @@ I/O
 - Bug in ``pd.read_hdf()`` passing a ``Timestamp`` to the ``where`` parameter with a non date column (:issue:`15492`)
 - Bug in ``DataFrame.to_stata()`` and ``StataWriter`` which produces incorrectly formatted files to be produced for some locales (:issue:`13856`)
 - Bug in ``StataReader`` and ``StataWriter`` which allows invalid encodings (:issue:`15723`)
+- Bug in ``pd.to_json()`` for the C engine where rollover was not correctly handled for case where frac is odd and diff is exactly 0.5 (:issue:`15716`, :issue:`15864`)
 
 Plotting
 ^^^^^^^^

pandas/_libs/src/ujson/lib/ultrajsonenc.c

@@ -823,17 +823,19 @@ int Buffer_AppendDoubleUnchecked(JSOBJ obj, JSONObjectEncoder *enc,
 
     if (diff > 0.5) {
         ++frac;
-        /* handle rollover, e.g.  case 0.99 with prec 1 is 1.0  */
-        if (frac >= pow10) {
-            frac = 0;
-            ++whole;
-        }
     } else if (diff == 0.5 && ((frac == 0) || (frac & 1))) {
         /* if halfway, round up if odd, OR
         if last digit is 0.  That last part is strange */
         ++frac;
     }
 
+    // handle rollover, e.g.
+    // case 0.99 with prec 1 is 1.0 and case 0.95 with prec is 1.0 as well
+    if (frac >= pow10) {
+        frac = 0;
+        ++whole;
+    }
+
     if (enc->doublePrecision == 0) {
         diff = value - whole;
 

pandas/tests/io/json/test_pandas.py

@@ -380,6 +380,31 @@ def test_frame_from_json_nones(self):
         unser = read_json(df.to_json(), dtype=False)
         self.assertTrue(np.isnan(unser[2][0]))
 
+    def test_frame_to_json_float_precision(self):
+        df = pd.DataFrame([dict(a_float=0.95)])
+        encoded = df.to_json(double_precision=1)
+        self.assertEqual(encoded, '{"a_float":{"0":1.0}}')
+
+        df = pd.DataFrame([dict(a_float=1.95)])
+        encoded = df.to_json(double_precision=1)
+        self.assertEqual(encoded, '{"a_float":{"0":2.0}}')
+
+        df = pd.DataFrame([dict(a_float=-1.95)])
+        encoded = df.to_json(double_precision=1)
+        self.assertEqual(encoded, '{"a_float":{"0":-2.0}}')
+
+        df = pd.DataFrame([dict(a_float=0.995)])
+        encoded = df.to_json(double_precision=2)
+        self.assertEqual(encoded, '{"a_float":{"0":1.0}}')
+
+        df = pd.DataFrame([dict(a_float=0.9995)])
+        encoded = df.to_json(double_precision=3)
+        self.assertEqual(encoded, '{"a_float":{"0":1.0}}')
+
+        df = pd.DataFrame([dict(a_float=0.99999999999999944)])
+        encoded = df.to_json(double_precision=15)
+        self.assertEqual(encoded, '{"a_float":{"0":1.0}}')
+
     def test_frame_to_json_except(self):
         df = DataFrame([1, 2, 3])
         self.assertRaises(ValueError, df.to_json, orient="garbage")

pandas/tests/io/json/test_ujson.py

@@ -43,6 +43,48 @@ def test_encodeDecimal(self):
         decoded = ujson.decode(encoded)
         self.assertEqual(decoded, 1337.1337)
 
+        sut = decimal.Decimal("0.95")
+        encoded = ujson.encode(sut, double_precision=1)
+        self.assertEqual(encoded, "1.0")
+        decoded = ujson.decode(encoded)
+        self.assertEqual(decoded, 1.0)
+
+        sut = decimal.Decimal("0.94")
+        encoded = ujson.encode(sut, double_precision=1)
+        self.assertEqual(encoded, "0.9")
+        decoded = ujson.decode(encoded)
+        self.assertEqual(decoded, 0.9)
+
+        sut = decimal.Decimal("1.95")
+        encoded = ujson.encode(sut, double_precision=1)
+        self.assertEqual(encoded, "2.0")
+        decoded = ujson.decode(encoded)
+        self.assertEqual(decoded, 2.0)
+
+        sut = decimal.Decimal("-1.95")
+        encoded = ujson.encode(sut, double_precision=1)
+        self.assertEqual(encoded, "-2.0")
+        decoded = ujson.decode(encoded)
+        self.assertEqual(decoded, -2.0)
+
+        sut = decimal.Decimal("0.995")
+        encoded = ujson.encode(sut, double_precision=2)
+        self.assertEqual(encoded, "1.0")
+        decoded = ujson.decode(encoded)
+        self.assertEqual(decoded, 1.0)
+
+        sut = decimal.Decimal("0.9995")
+        encoded = ujson.encode(sut, double_precision=3)
+        self.assertEqual(encoded, "1.0")
+        decoded = ujson.decode(encoded)
+        self.assertEqual(decoded, 1.0)
+
+        sut = decimal.Decimal("0.99999999999999944")
+        encoded = ujson.encode(sut, double_precision=15)
+        self.assertEqual(encoded, "1.0")
+        decoded = ujson.decode(encoded)
+        self.assertEqual(decoded, 1.0)
+
     def test_encodeStringConversion(self):
         input = "A string \\ / \b \f \n \r \t </script> &"
         not_html_encoded = ('"A string \\\\ \\/ \\b \\f \\n '