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Issue #3439: add bit_length method to int and long.

Browse source 
Thanks Fredrik Johansson and Victor Stinner for code,
Raymond Hettinger for review.
This commit is contained in:
Mark Dickinson 2008-12-17 16:14:37 +00:00
parent d0c3515bc5
commit 1a707981c8
8 changed files with 239 additions and 1 deletions
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71
Objects/longobject.c
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@ -3451,6 +3451,75 @@ long_sizeof(PyLongObject *v)
return PyInt_FromSsize_t(res);
}
static const unsigned char BitLengthTable[32] = {
0, 1, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4,
5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5
};
static PyObject *
long_bit_length(PyLongObject *v)
{
PyLongObject *result, *x, *y;
Py_ssize_t ndigits, msd_bits = 0;
digit msd;
assert(v != NULL);
assert(PyLong_Check(v));
ndigits = ABS(Py_SIZE(v));
if (ndigits == 0)
return PyInt_FromLong(0);
msd = v->ob_digit[ndigits-1];
while (msd >= 32) {
msd_bits += 6;
msd >>= 6;
}
msd_bits += (long)(BitLengthTable[msd]);
if (ndigits <= PY_SSIZE_T_MAX/PyLong_SHIFT)
return PyInt_FromSsize_t((ndigits-1)*PyLong_SHIFT + msd_bits);
/* expression above may overflow; use Python integers instead */
result = (PyLongObject *)PyLong_FromSsize_t(ndigits - 1);
if (result == NULL)
return NULL;
x = (PyLongObject *)PyLong_FromLong(PyLong_SHIFT);
if (x == NULL)
goto error;
y = (PyLongObject *)long_mul(result, x);
Py_DECREF(x);
if (y == NULL)
goto error;
Py_DECREF(result);
result = y;
x = (PyLongObject *)PyLong_FromLong(msd_bits);
if (x == NULL)
goto error;
y = (PyLongObject *)long_add(result, x);
Py_DECREF(x);
if (y == NULL)
goto error;
Py_DECREF(result);
result = y;
return (PyObject *)result;
error:
Py_DECREF(result);
return NULL;
}
PyDoc_STRVAR(long_bit_length_doc,
"long.bit_length() -> int or long\n\
\n\
Number of bits necessary to represent self in binary.\n\
>>> bin(37L)\n\
'0b100101'\n\
>>> (37L).bit_length()\n\
6");
#if 0
static PyObject *
long_is_finite(PyObject *v)
@ -3462,6 +3531,8 @@ long_is_finite(PyObject *v)
static PyMethodDef long_methods[] = {
{"conjugate", (PyCFunction)long_long, METH_NOARGS,
"Returns self, the complex conjugate of any long."},
{"bit_length", (PyCFunction)long_bit_length, METH_NOARGS,
long_bit_length_doc},
#if 0
{"is_finite", (PyCFunction)long_is_finite, METH_NOARGS,
"Returns always True."},