_PyLong_NumBits(): The definition of this was too specific to the quirky

tim-one · tim-one · commit 5b8132ffa342 · 2003-01-31T15:52:05.000Z
needs of pickling longs.  Backed off to a definition that's much easier
to understand.  The pickler will have to work a little harder, but other
uses are more likely to be correct &lt;0.5 wink&gt;.

_PyLong_Sign():  New teensy function to characterize a long, as to &lt;0, ==0,
or &gt;0.
diff --git a/Include/longobject.h b/Include/longobject.h
@@ -44,11 +44,17 @@ PyAPI_FUNC(PyObject *) PyLong_FromString(char *, char **, int);
 PyAPI_FUNC(PyObject *) PyLong_FromUnicode(Py_UNICODE*, int, int);
 #endif
 
-/* _PyLong_NumBits.  Return the number of bits needed to represent a long
-   in contiguous 2's-complement form, including 1 for the sign bit.  For
-   example, this returns 1 for 0, and 2 for 1 and -1.  Note that the
-   ceiling of this divided by 8 is the number of bytes needed by
-   _PyLong_AsByteArray to store the long in 256's-complement form.
+/* _PyLong_Sign.  Return 0 if v is 0, -1 if v < 0, +1 if v > 0.
+   v must not be NULL, and must be a normalized long.
+   There are no error cases.
+*/
+PyAPI_FUNC(int) _PyLong_Sign(PyObject *v);
+
+
+PyAPI_FUNC(size_t) _PyLong_NumBits(PyObject *v);
+/* _PyLong_NumBits.  Return the number of bits needed to represent the
+   absolute value of a long.  For example, this returns 1 for 1 and -1, 2
+   for 2 and -2, and 2 for 3 and -3.  It returns 0 for 0.
    v must not be NULL, and must be a normalized long.
    (size_t)-1 is returned and OverflowError set if the true result doesn't
    fit in a size_t.
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
@@ -334,37 +334,43 @@ test_u_code(PyObject *self)
 
 #endif
 
-/* Simple test of _PyLong_NumBits. */
+/* Simple test of _PyLong_NumBits and _PyLong_Sign. */
 static PyObject *
 test_long_numbits(PyObject *self)
 {
-	struct pair {
+	struct triple {
 		long input;
-		size_t output;
-	} testcases[] = {{0, 1},
-			 {1L, 2},
-			 {-1L, 2},
-			 {2L, 3},
-			 {-2L, 3},
-			 {3L, 3},
-			 {-3L, 3},
-			 {4L, 4},
-			 {-4L, 4},
-			 {0x7fffL, 16},		/* one Python long digit */
-			 {-0x7fffL, 16},
-			 {0xfffffffL, 29},
-			 {-0xfffffffL, 29}};
+		size_t nbits;
+		int sign;
+	} testcases[] = {{0, 0, 0},
+			 {1L, 1, 1},
+			 {-1L, 1, -1},
+			 {2L, 2, 1},
+			 {-2L, 2, -1},
+			 {3L, 2, 1},
+			 {-3L, 2, -1},
+			 {4L, 3, 1},
+			 {-4L, 3, -1},
+			 {0x7fffL, 15, 1},	/* one Python long digit */
+			 {-0x7fffL, 15, -1},
+			 {0xffffL, 16, 1},
+			 {-0xffffL, 16, -1},
+			 {0xfffffffL, 28, 1},
+			 {-0xfffffffL, 28, -1}};
 	int i;
 
-	for (i = 0; i < sizeof(testcases) / sizeof(struct pair); ++i) {
-		long input = testcases[i].input;
-		PyObject *plong = PyLong_FromLong(input);
+	for (i = 0; i < sizeof(testcases) / sizeof(struct triple); ++i) {
+		PyObject *plong = PyLong_FromLong(testcases[i].input);
 		size_t nbits = _PyLong_NumBits(plong);
+		int sign = _PyLong_Sign(plong);
 
 		Py_DECREF(plong);
-		if (nbits != testcases[i].output)
+		if (nbits != testcases[i].nbits)
 			return raiseTestError("test_long_numbits",
-					      "wrong result");
+					"wrong result for _PyLong_NumBits");
+		if (sign != testcases[i].sign)
+			return raiseTestError("test_long_numbits",
+					"wrong result for _PyLong_Sign");
 	}
 	Py_INCREF(Py_None);
 	return Py_None;
diff --git a/Objects/longobject.c b/Objects/longobject.c
@@ -260,25 +260,34 @@ PyLong_AsUnsignedLong(PyObject *vv)
 	return x;
 }
 
+int
+_PyLong_Sign(PyObject *vv)
+{
+	PyLongObject *v = (PyLongObject *)vv;
+	const int ndigits = v->ob_size;
+
+	assert(v != NULL);
+	assert(PyLong_Check(v));
+	assert(ndigits == 0 || v->ob_digit[ndigits - 1] != 0);
+
+	return ndigits == 0 ? 0 : (ndigits < 0 ? -1 : 1);
+}
+
 size_t
 _PyLong_NumBits(PyObject *vv)
 {
 	PyLongObject *v = (PyLongObject *)vv;
-	size_t result = 1;	/* for the sign bit */
-	size_t ndigits = ABS(v->ob_size);
+	size_t result = 0;
+	int ndigits = ABS(v->ob_size);
 
 	assert(v != NULL);
 	assert(PyLong_Check(v));
 	assert(ndigits == 0 || v->ob_digit[ndigits - 1] != 0);
 	if (ndigits > 0) {
-		size_t product;
 		digit msd = v->ob_digit[ndigits - 1];
 
-		product = (ndigits - 1) * SHIFT;
-		if (product / SHIFT != ndigits - 1)
-			goto Overflow;
-		result += product;
-		if (result < product)
+		result = (ndigits - 1) * SHIFT;
+		if (result / SHIFT != ndigits - 1)
 			goto Overflow;
 		do {
 			++result;
