Issue #1996: float.as_integer_ratio() should return fraction in lowest terms.
This commit is contained in:
Raymond Hettinger
parent
a51f61b555
commit
04c96d52a4
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@ -689,6 +689,14 @@ class BuiltinTest(unittest.TestCase):
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self.assertRaises(TypeError, float, Foo4(42))
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def test_floatasratio(self):
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for f, ratio in [
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(0.875, (7, 8)),
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(-0.875, (-7, 8)),
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(0.0, (0, 1)),
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(11.5, (23, 2)),
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]:
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self.assertEqual(f.as_integer_ratio(), ratio)
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R = rational.Rational
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self.assertEqual(R(0, 1),
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R(*float(0.0).as_integer_ratio()))
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@ -1154,20 +1154,18 @@ float_float(PyObject *v)
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}
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static PyObject *
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float_as_integer_ratio(PyObject *v)
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float_as_integer_ratio(PyObject *v, PyObject *unused)
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{
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double self;
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double float_part;
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int exponent;
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int is_negative;
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const int chunk_size = 28;
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PyObject *prev;
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PyObject *py_chunk = NULL;
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PyObject *py_exponent = NULL;
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PyObject *numerator = NULL;
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PyObject *denominator = NULL;
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PyObject *result_pair = NULL;
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PyNumberMethods *long_methods;
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PyNumberMethods *long_methods = PyLong_Type.tp_as_number;
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#define INPLACE_UPDATE(obj, call) \
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prev = obj; \
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@ -1189,85 +1187,22 @@ float_as_integer_ratio(PyObject *v)
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}
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#endif
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if (self == 0) {
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numerator = PyInt_FromLong(0);
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if (numerator == NULL) goto error;
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denominator = PyInt_FromLong(1);
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if (denominator == NULL) goto error;
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result_pair = PyTuple_Pack(2, numerator, denominator);
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/* Hand ownership over to the tuple. If the tuple
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wasn't created successfully, we want to delete the
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ints anyway. */
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Py_DECREF(numerator);
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Py_DECREF(denominator);
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return result_pair;
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}
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/* XXX: Could perhaps handle FLT_RADIX!=2 by using ilogb and
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scalbn, but those may not be in C89. */
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PyFPE_START_PROTECT("as_integer_ratio", goto error);
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float_part = frexp(self, &exponent);
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is_negative = 0;
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if (float_part < 0) {
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float_part = -float_part;
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is_negative = 1;
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/* 0.5 <= float_part < 1.0 */
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}
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float_part = frexp(self, &exponent); /* self == float_part * 2**exponent exactly */
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PyFPE_END_PROTECT(float_part);
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/* abs(self) == float_part * 2**exponent exactly */
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while (float_part != floor(float_part)) {
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float_part *= 2.0;
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exponent--;
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}
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/* Now, self == float_part * 2**exponent exactly and float_part is integral */
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/* Suck up chunk_size bits at a time; 28 is enough so that we
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suck up all bits in 2 iterations for all known binary
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double-precision formats, and small enough to fit in a
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long. */
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numerator = PyLong_FromLong(0);
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numerator = PyLong_FromDouble(float_part);
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if (numerator == NULL) goto error;
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long_methods = PyLong_Type.tp_as_number;
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py_chunk = PyLong_FromLong(chunk_size);
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if (py_chunk == NULL) goto error;
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while (float_part != 0) {
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/* invariant: abs(self) ==
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(numerator + float_part) * 2**exponent exactly */
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long digit;
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PyObject *py_digit;
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PyFPE_START_PROTECT("as_integer_ratio", goto error);
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/* Pull chunk_size bits out of float_part, into digits. */
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float_part = ldexp(float_part, chunk_size);
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digit = (long)float_part;
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float_part -= digit;
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/* 0 <= float_part < 1 */
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exponent -= chunk_size;
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PyFPE_END_PROTECT(float_part);
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/* Shift digits into numerator. */
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// numerator <<= chunk_size
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INPLACE_UPDATE(numerator,
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long_methods->nb_lshift(numerator, py_chunk));
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if (numerator == NULL) goto error;
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// numerator |= digit
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py_digit = PyLong_FromLong(digit);
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if (py_digit == NULL) goto error;
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INPLACE_UPDATE(numerator,
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long_methods->nb_or(numerator, py_digit));
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Py_DECREF(py_digit);
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if (numerator == NULL) goto error;
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}
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/* Add in the sign bit. */
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if (is_negative) {
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INPLACE_UPDATE(numerator,
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long_methods->nb_negative(numerator));
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if (numerator == NULL) goto error;
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}
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/* now self = numerator * 2**exponent exactly; fold in 2**exponent */
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denominator = PyLong_FromLong(1);
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py_exponent = PyLong_FromLong(labs(exponent));
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denominator = PyInt_FromLong(1);
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py_exponent = PyInt_FromLong(labs(exponent));
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if (py_exponent == NULL) goto error;
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INPLACE_UPDATE(py_exponent,
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long_methods->nb_lshift(denominator, py_exponent));
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@ -1289,7 +1224,6 @@ float_as_integer_ratio(PyObject *v)
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#undef INPLACE_UPDATE
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error:
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Py_XDECREF(py_exponent);
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Py_XDECREF(py_chunk);
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Py_XDECREF(denominator);
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Py_XDECREF(numerator);
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return result_pair;
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@ -1298,17 +1232,16 @@ error:
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PyDoc_STRVAR(float_as_integer_ratio_doc,
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"float.as_integer_ratio() -> (int, int)\n"
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"\n"
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"Returns a pair of integers, not necessarily in lowest terms, whose\n"
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"ratio is exactly equal to the original float. This method raises an\n"
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"OverflowError on infinities and a ValueError on nans. The resulting\n"
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"denominator will be positive.\n"
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"Returns a pair of integers, whose ratio is exactly equal to the original\n"
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"float and with a positive denominator.\n"
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"Raises OverflowError on infinities and a ValueError on nans.\n"
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"\n"
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">>> (10.0).as_integer_ratio()\n"
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"(167772160L, 16777216L)\n"
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"(10, 1)\n"
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">>> (0.0).as_integer_ratio()\n"
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"(0, 1)\n"
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">>> (-.25).as_integer_ratio()\n"
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"(-134217728L, 536870912L)");
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"(-1, 4)");
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static PyObject *
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