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Moved Rational._binary_float_to_ratio() to float.as_integer_ratio() because 

it's useful outside of rational numbers.

This is my first C code that had to do anything significant. Please be more
careful when looking over it.
This commit is contained in:
Jeffrey Yasskin 2008-01-27 23:08:46 +00:00
parent 56eadd9d0d
commit 3ea7b41b58
3 changed files with 180 additions and 56 deletions
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56
Lib/rational.py
View File

@ -25,60 +25,6 @@ def gcd(a, b):
return a
def _binary_float_to_ratio(x):
"""x -> (top, bot), a pair of ints s.t. x = top/bot.
The conversion is done exactly, without rounding.
bot > 0 guaranteed.
Some form of binary fp is assumed.
Pass NaNs or infinities at your own risk.
>>> _binary_float_to_ratio(10.0)
(10, 1)
>>> _binary_float_to_ratio(0.0)
(0, 1)
>>> _binary_float_to_ratio(-.25)
(-1, 4)
"""
# XXX Move this to floatobject.c with a name like
# float.as_integer_ratio()
if x == 0:
return 0, 1
f, e = math.frexp(x)
signbit = 1
if f < 0:
f = -f
signbit = -1
assert 0.5 <= f < 1.0
# x = signbit * f * 2**e exactly
# Suck up CHUNK bits at a time; 28 is enough so that we suck
# up all bits in 2 iterations for all known binary double-
# precision formats, and small enough to fit in an int.
CHUNK = 28
top = 0
# invariant: x = signbit * (top + f) * 2**e exactly
while f:
f = math.ldexp(f, CHUNK)
digit = trunc(f)
assert digit >> CHUNK == 0
top = (top << CHUNK) | digit
f = f - digit
assert 0.0 <= f < 1.0
e = e - CHUNK
assert top
# Add in the sign bit.
top = signbit * top
# now x = top * 2**e exactly; fold in 2**e
if e>0:
return (top * 2**e, 1)
else:
return (top, 2 ** -e)
_RATIONAL_FORMAT = re.compile(
r'^\s*(?P<sign>[-+]?)(?P<num>\d+)'
r'(?:/(?P<denom>\d+)|\.(?P<decimal>\d+))?\s*$')
@ -163,7 +109,7 @@ class Rational(RationalAbc):
(cls.__name__, f, type(f).__name__))
if math.isnan(f) or math.isinf(f):
raise TypeError("Cannot convert %r to %s." % (f, cls.__name__))
return cls(*_binary_float_to_ratio(f))
return cls(*f.as_integer_ratio())
@classmethod
def from_decimal(cls, dec):

21
Lib/test/test_builtin.py
View File

@ -5,7 +5,7 @@ from test.test_support import fcmp, have_unicode, TESTFN, unlink, \
run_unittest, run_with_locale
from operator import neg
import sys, warnings, cStringIO, random, UserDict
import sys, warnings, cStringIO, random, rational, UserDict
warnings.filterwarnings("ignore", "hex../oct.. of negative int",
FutureWarning, __name__)
warnings.filterwarnings("ignore", "integer argument expected",
@ -688,6 +688,25 @@ class BuiltinTest(unittest.TestCase):
self.assertAlmostEqual(float(Foo3(21)), 42.)
self.assertRaises(TypeError, float, Foo4(42))
def test_floatasratio(self):
R = rational.Rational
self.assertEqual(R(0, 1),
R(*float(0.0).as_integer_ratio()))
self.assertEqual(R(5, 2),
R(*float(2.5).as_integer_ratio()))
self.assertEqual(R(1, 2),
R(*float(0.5).as_integer_ratio()))
self.assertEqual(R(4728779608739021, 2251799813685248),
R(*float(2.1).as_integer_ratio()))
self.assertEqual(R(-4728779608739021, 2251799813685248),
R(*float(-2.1).as_integer_ratio()))
self.assertEqual(R(-2100, 1),
R(*float(-2100.0).as_integer_ratio()))
self.assertRaises(OverflowError, float('inf').as_integer_ratio)
self.assertRaises(OverflowError, float('-inf').as_integer_ratio)
self.assertRaises(ValueError, float('nan').as_integer_ratio)
def test_getattr(self):
import sys
self.assert_(getattr(sys, 'stdout') is sys.stdout)

159
Objects/floatobject.c
View File

@ -1161,6 +1161,163 @@ float_float(PyObject *v)
return v;
}
static PyObject *
float_as_integer_ratio(PyObject *v)
{
double self;
double float_part;
int exponent;
int is_negative;
const int chunk_size = 28;
PyObject *prev;
PyObject *py_chunk = NULL;
PyObject *py_exponent = NULL;
PyObject *numerator = NULL;
PyObject *denominator = NULL;
PyObject *result_pair = NULL;
PyNumberMethods *long_methods;
#define INPLACE_UPDATE(obj, call) \
prev = obj; \
obj = call; \
Py_DECREF(prev); \
CONVERT_TO_DOUBLE(v, self);
if (Py_IS_INFINITY(self)) {
PyErr_SetString(PyExc_OverflowError,
"Cannot pass infinity to float.as_integer_ratio.");
return NULL;
}
#ifdef Py_NAN
if (Py_IS_NAN(self)) {
PyErr_SetString(PyExc_ValueError,
"Cannot pass nan to float.as_integer_ratio.");
return NULL;
}
#endif
if (self == 0) {
numerator = PyInt_FromLong(0);
if (numerator == NULL) goto error;
denominator = PyInt_FromLong(1);
if (denominator == NULL) goto error;
result_pair = PyTuple_Pack(2, numerator, denominator);
/* Hand ownership over to the tuple. If the tuple
wasn't created successfully, we want to delete the
ints anyway. */
Py_DECREF(numerator);
Py_DECREF(denominator);
return result_pair;
}
/* XXX: Could perhaps handle FLT_RADIX!=2 by using ilogb and
scalbn, but those may not be in C89. */
PyFPE_START_PROTECT("as_integer_ratio", goto error);
float_part = frexp(self, &exponent);
is_negative = 0;
if (float_part < 0) {
float_part = -float_part;
is_negative = 1;
/* 0.5 <= float_part < 1.0 */
}
PyFPE_END_PROTECT(float_part);
/* abs(self) == float_part * 2**exponent exactly */
/* Suck up chunk_size bits at a time; 28 is enough so that we
suck up all bits in 2 iterations for all known binary
double-precision formats, and small enough to fit in a
long. */
numerator = PyLong_FromLong(0);
if (numerator == NULL) goto error;
long_methods = PyLong_Type.tp_as_number;
py_chunk = PyLong_FromLong(chunk_size);
if (py_chunk == NULL) goto error;
while (float_part != 0) {
/* invariant: abs(self) ==
(numerator + float_part) * 2**exponent exactly */
long digit;
PyObject *py_digit;
PyFPE_START_PROTECT("as_integer_ratio", goto error);
/* Pull chunk_size bits out of float_part, into digits. */
float_part = ldexp(float_part, chunk_size);
digit = (long)float_part;
float_part -= digit;
/* 0 <= float_part < 1 */
exponent -= chunk_size;
PyFPE_END_PROTECT(float_part);
/* Shift digits into numerator. */
// numerator <<= chunk_size
INPLACE_UPDATE(numerator,
long_methods->nb_lshift(numerator, py_chunk));
if (numerator == NULL) goto error;
// numerator |= digit
py_digit = PyLong_FromLong(digit);
if (py_digit == NULL) goto error;
INPLACE_UPDATE(numerator,
long_methods->nb_or(numerator, py_digit));
Py_DECREF(py_digit);
if (numerator == NULL) goto error;
}
/* Add in the sign bit. */
if (is_negative) {
INPLACE_UPDATE(numerator,
long_methods->nb_negative(numerator));
if (numerator == NULL) goto error;
}
/* now self = numerator * 2**exponent exactly; fold in 2**exponent */
denominator = PyLong_FromLong(1);
py_exponent = PyLong_FromLong(labs(exponent));
if (py_exponent == NULL) goto error;
INPLACE_UPDATE(py_exponent,
long_methods->nb_lshift(denominator, py_exponent));
if (py_exponent == NULL) goto error;
if (exponent > 0) {
INPLACE_UPDATE(numerator,
long_methods->nb_multiply(numerator,
py_exponent));
if (numerator == NULL) goto error;
}
else {
Py_DECREF(denominator);
denominator = py_exponent;
py_exponent = NULL;
}
result_pair = PyTuple_Pack(2, numerator, denominator);
#undef INPLACE_UPDATE
error:
Py_XDECREF(py_exponent);
Py_XDECREF(py_chunk);
Py_XDECREF(denominator);
Py_XDECREF(numerator);
return result_pair;
}
PyDoc_STRVAR(float_as_integer_ratio_doc,
"float.as_integer_ratio() -> (int, int)\n"
"\n"
"Returns a pair of integers, not necessarily in lowest terms, whose\n"
"ratio is exactly equal to the original float. This method raises an\n"
"OverflowError on infinities and a ValueError on nans. The resulting\n"
"denominator will be positive.\n"
"\n"
">>> (10.0).as_integer_ratio()\n"
"(167772160L, 16777216L)\n"
">>> (0.0).as_integer_ratio()\n"
"(0, 1)\n"
">>> (-.25).as_integer_ratio()\n"
"(-134217728L, 536870912L)");
static PyObject *
float_subtype_new(PyTypeObject *type, PyObject *args, PyObject *kwds);
@ -1349,6 +1506,8 @@ static PyMethodDef float_methods[] = {
"Returns self, the complex conjugate of any float."},
{"__trunc__", (PyCFunction)float_trunc, METH_NOARGS,
"Returns the Integral closest to x between 0 and x."},
{"as_integer_ratio", (PyCFunction)float_as_integer_ratio, METH_NOARGS,
float_as_integer_ratio_doc},
{"__getnewargs__", (PyCFunction)float_getnewargs, METH_NOARGS},
{"__getformat__", (PyCFunction)float_getformat,
METH_O|METH_CLASS, float_getformat_doc},