Issue #1772851. Alters long.__hash__ from being *almost* completely

predictable to being completely predictable.  The value of hash(n)
is unchanged for any n that's small enough to be representable as an
int, and also unchanged for the vast majority of long integers n of
reasonable size.

Lib/test/test_hash.py

@@ -18,10 +18,21 @@ class HashEqualityTestCase(unittest.TestCase):
 
     def test_numeric_literals(self):
         self.same_hash(1, 1L, 1.0, 1.0+0.0j)
+        self.same_hash(0, 0L, 0.0, 0.0+0.0j)
+        self.same_hash(-1, -1L, -1.0, -1.0+0.0j)
+        self.same_hash(-2, -2L, -2.0, -2.0+0.0j)
 
     def test_coerced_integers(self):
         self.same_hash(int(1), long(1), float(1), complex(1),
                        int('1'), float('1.0'))
+        self.same_hash(int(-2**31), long(-2**31), float(-2**31))
+        self.same_hash(int(1-2**31), long(1-2**31), float(1-2**31))
+        self.same_hash(int(2**31-1), long(2**31-1), float(2**31-1))
+        # for 64-bit platforms
+        self.same_hash(int(2**31), long(2**31), float(2**31))
+        self.same_hash(int(-2**63), long(-2**63), float(-2**63))
+        self.same_hash(int(1-2**63), long(1-2**63))
+        self.same_hash(int(2**63-1), long(2**63-1))
 
     def test_coerced_floats(self):
         self.same_hash(long(1.23e300), float(1.23e300))

Objects/longobject.c

@@ -1937,10 +1937,18 @@ long_hash(PyLongObject *v)
 		i = -(i);
 	}
 #define LONG_BIT_SHIFT	(8*sizeof(long) - SHIFT)
+	/* The following loop produces a C long x such that (unsigned long)x
+	   is congruent to the absolute value of v modulo ULONG_MAX.  The
+	   resulting x is nonzero if and only if v is. */
 	while (--i >= 0) {
 		/* Force a native long #-bits (32 or 64) circular shift */
 		x = ((x << SHIFT) & ~MASK) | ((x >> LONG_BIT_SHIFT) & MASK);
 		x += v->ob_digit[i];
+		/* If the addition above overflowed (thinking of x as
+		   unsigned), we compensate by incrementing.  This preserves
+		   the value modulo ULONG_MAX. */
+		if ((unsigned long)x < v->ob_digit[i])
+			x++;
 	}
 #undef LONG_BIT_SHIFT
 	x = x * sign;