Issue #3166: Make long -> float (and int -> float) conversions

correctly rounded, using round-half-to-even.  This ensures that the
value of float(n) doesn't depend on whether we're using 15-bit digits
or 30-bit digits for Python longs.

1 files changed, 34 insertions, 0 deletions

diff --git a/Lib/test/test_int.py b/Lib/test/test_int.py
index ce18ad2bde..514a98e74c 100644
--- a/Lib/test/test_int.py
+++ b/Lib/test/test_int.py
@@ -275,6 +275,40 @@ class IntTestCases(unittest.TestCase):
             self.assertEqual((a+1).bit_length(), i+1)
             self.assertEqual((-a-1).bit_length(), i+1)
 
+    @unittest.skipUnless(float.__getformat__("double").startswith("IEEE"),
+                         "test requires IEEE 754 doubles")
+    def test_float_conversion(self):
+        # values exactly representable as floats
+        exact_values = [-2, -1, 0, 1, 2, 2**52, 2**53-1, 2**53, 2**53+2,
+                         2**53+4, 2**54-4, 2**54-2, 2**63, -2**63, 2**64,
+                         -2**64, 10**20, 10**21, 10**22]
+        for value in exact_values:
+            self.assertEqual(int(float(int(value))), value)
+
+        # test round-half-to-even
+        self.assertEqual(int(float(2**53+1)), 2**53)
+        self.assertEqual(int(float(2**53+2)), 2**53+2)
+        self.assertEqual(int(float(2**53+3)), 2**53+4)
+        self.assertEqual(int(float(2**53+5)), 2**53+4)
+        self.assertEqual(int(float(2**53+6)), 2**53+6)
+        self.assertEqual(int(float(2**53+7)), 2**53+8)
+
+        self.assertEqual(int(float(-2**53-1)), -2**53)
+        self.assertEqual(int(float(-2**53-2)), -2**53-2)
+        self.assertEqual(int(float(-2**53-3)), -2**53-4)
+        self.assertEqual(int(float(-2**53-5)), -2**53-4)
+        self.assertEqual(int(float(-2**53-6)), -2**53-6)
+        self.assertEqual(int(float(-2**53-7)), -2**53-8)
+
+        self.assertEqual(int(float(2**54-2)), 2**54-2)
+        self.assertEqual(int(float(2**54-1)), 2**54)
+        self.assertEqual(int(float(2**54+2)), 2**54)
+        self.assertEqual(int(float(2**54+3)), 2**54+4)
+        self.assertEqual(int(float(2**54+5)), 2**54+4)
+        self.assertEqual(int(float(2**54+6)), 2**54+8)
+        self.assertEqual(int(float(2**54+10)), 2**54+8)
+        self.assertEqual(int(float(2**54+11)), 2**54+12)
+
     def test_intconversion(self):
         # Test __int__()
         class ClassicMissingMethods: