SF bug #812202: randint is always even
* Added C coded getrandbits(k) method that runs in linear time.
* Call the new method from randrange() for ranges >= 2**53.
* Adds a warning for generators not defining getrandbits() whenever they
have a call to randrange() with too large of a population.
diff --git a/Doc/lib/librandom.tex b/Doc/lib/librandom.tex
index 5483ff2..2b686ec 100644
--- a/Doc/lib/librandom.tex
+++ b/Doc/lib/librandom.tex
@@ -41,6 +41,10 @@
different basic generator of your own devising: in that case, override
the \method{random()}, \method{seed()}, \method{getstate()},
\method{setstate()} and \method{jumpahead()} methods.
+Optionally, a new generator can supply a \method{getrandombits()}
+method --- this allows \method{randrange()} to produce selections
+over an arbitrarily large range.
+\versionadded[the \method{getrandombits()} method]{2.4}
As an example of subclassing, the \module{random} module provides
the \class{WichmannHill} class which implements an alternative generator
@@ -92,6 +96,14 @@
separated by many steps.]{2.3}
\end{funcdesc}
+\begin{funcdesc}{getrandbits}{k}
+ Returns a python \class{long} int with \var{k} random bits.
+ This method is supplied with the MersenneTwister generator and some
+ other generators may also provide it as an optional part of the API.
+ When available, \method{getrandbits()} enables \method{randrange()}
+ to handle arbitrarily large ranges.
+ \versionadded{2.4}
+\end{funcdesc}
Functions for integers:
diff --git a/Lib/random.py b/Lib/random.py
index 2530c39..5916864 100644
--- a/Lib/random.py
+++ b/Lib/random.py
@@ -39,6 +39,8 @@
"""
+from warnings import warn as _warn
+from types import MethodType as _MethodType, BuiltinMethodType as _BuiltinMethodType
from math import log as _log, exp as _exp, pi as _pi, e as _e
from math import sqrt as _sqrt, acos as _acos, cos as _cos, sin as _sin
from math import floor as _floor
@@ -47,12 +49,14 @@
"randrange","shuffle","normalvariate","lognormvariate",
"expovariate","vonmisesvariate","gammavariate",
"gauss","betavariate","paretovariate","weibullvariate",
- "getstate","setstate","jumpahead"]
+ "getstate","setstate","jumpahead", "WichmannHill", "getrandbits",
+ "Random"]
NV_MAGICCONST = 4 * _exp(-0.5)/_sqrt(2.0)
TWOPI = 2.0*_pi
LOG4 = _log(4.0)
SG_MAGICCONST = 1.0 + _log(4.5)
+BPF = 53 # Number of bits in a float
# Translated by Guido van Rossum from C source provided by
# Adrian Baddeley. Adapted by Raymond Hettinger for use with
@@ -72,6 +76,8 @@
Class Random can also be subclassed if you want to use a different basic
generator of your own devising: in that case, override the following
methods: random(), seed(), getstate(), setstate() and jumpahead().
+ Optionally, implement a getrandombits() method so that randrange()
+ can cover arbitrarily large ranges.
"""
@@ -131,12 +137,13 @@
## -------------------- integer methods -------------------
- def randrange(self, start, stop=None, step=1, int=int, default=None):
+ def randrange(self, start, stop=None, step=1, int=int, default=None,
+ maxwidth=1L<<BPF):
"""Choose a random item from range(start, stop[, step]).
This fixes the problem with randint() which includes the
endpoint; in Python this is usually not what you want.
- Do not supply the 'int' and 'default' arguments.
+ Do not supply the 'int', 'default', and 'maxwidth' arguments.
"""
# This code is a bit messy to make it fast for the
@@ -146,6 +153,8 @@
raise ValueError, "non-integer arg 1 for randrange()"
if stop is default:
if istart > 0:
+ if istart >= maxwidth:
+ return self._randbelow(istart)
return int(self.random() * istart)
raise ValueError, "empty range for randrange()"
@@ -153,36 +162,43 @@
istop = int(stop)
if istop != stop:
raise ValueError, "non-integer stop for randrange()"
- if step == 1 and istart < istop:
+ width = istop - istart
+ if step == 1 and width > 0:
# Note that
- # int(istart + self.random()*(istop - istart))
+ # int(istart + self.random()*width)
# instead would be incorrect. For example, consider istart
# = -2 and istop = 0. Then the guts would be in
# -2.0 to 0.0 exclusive on both ends (ignoring that random()
# might return 0.0), and because int() truncates toward 0, the
# final result would be -1 or 0 (instead of -2 or -1).
- # istart + int(self.random()*(istop - istart))
+ # istart + int(self.random()*width)
# would also be incorrect, for a subtler reason: the RHS
# can return a long, and then randrange() would also return
# a long, but we're supposed to return an int (for backward
# compatibility).
- return int(istart + int(self.random()*(istop - istart)))
+
+ if width >= maxwidth:
+ return int(istart + self._randbelow(width))
+ return int(istart + int(self.random()*width))
if step == 1:
- raise ValueError, "empty range for randrange()"
+ raise ValueError, "empty range for randrange() (%d,%d, %d)" % (istart, istop, width)
# Non-unit step argument supplied.
istep = int(step)
if istep != step:
raise ValueError, "non-integer step for randrange()"
if istep > 0:
- n = (istop - istart + istep - 1) / istep
+ n = (width + istep - 1) / istep
elif istep < 0:
- n = (istop - istart + istep + 1) / istep
+ n = (width + istep + 1) / istep
else:
raise ValueError, "zero step for randrange()"
if n <= 0:
raise ValueError, "empty range for randrange()"
+
+ if n >= maxwidth:
+ return istart + self._randbelow(n)
return istart + istep*int(self.random() * n)
def randint(self, a, b):
@@ -191,6 +207,33 @@
return self.randrange(a, b+1)
+ def _randbelow(self, n, _log=_log, int=int, _maxwidth=1L<<BPF,
+ _Method=_MethodType, _BuiltinMethod=_BuiltinMethodType):
+ """Return a random int in the range [0,n)
+
+ Handles the case where n has more bits than returned
+ by a single call to the underlying generator.
+ """
+
+ try:
+ getrandbits = self.getrandbits
+ except AttributeError:
+ pass
+ else:
+ # Only call self.getrandbits if the original random() builtin method
+ # has not been overridden or if a new getrandbits() was supplied.
+ # This assures that the two methods correspond.
+ if type(self.random) is _BuiltinMethod or type(getrandbits) is _Method:
+ k = int(1.00001 + _log(n-1, 2.0)) # 2**k > n-1 > 2**(k-2)
+ r = getrandbits(k)
+ while r >= n:
+ r = getrandbits(k)
+ return r
+ if n >= _maxwidth:
+ _warn("Underlying random() generator does not supply \n"
+ "enough bits to choose from a population range this large")
+ return int(self.random() * n)
+
## -------------------- sequence methods -------------------
def choice(self, seq):
@@ -757,6 +800,7 @@
getstate = _inst.getstate
setstate = _inst.setstate
jumpahead = _inst.jumpahead
+getrandbits = _inst.getrandbits
if __name__ == '__main__':
_test()
diff --git a/Lib/test/test_random.py b/Lib/test/test_random.py
index fbd4184..3796c3b 100644
--- a/Lib/test/test_random.py
+++ b/Lib/test/test_random.py
@@ -4,6 +4,7 @@
import random
import time
import pickle
+import warnings
from math import log, exp, sqrt, pi
from sets import Set
from test import test_support
@@ -153,6 +154,13 @@
self.assertEqual(x1, x2)
self.assertEqual(y1, y2)
+ def test_bigrand(self):
+ # Verify warnings are raised when randrange is too large for random()
+ oldfilters = warnings.filters[:]
+ warnings.filterwarnings("error", "Underlying random")
+ self.assertRaises(UserWarning, self.gen.randrange, 2**60)
+ warnings.filters[:] = oldfilters
+
class MersenneTwister_TestBasicOps(TestBasicOps):
gen = random.Random()
@@ -219,6 +227,76 @@
seed = (1L << (10000 * 8)) - 1 # about 10K bytes
self.gen.seed(seed)
+ def test_53_bits_per_float(self):
+ # This should pass whenever a C double has 53 bit precision.
+ span = 2 ** 53
+ cum = 0
+ for i in xrange(100):
+ cum |= int(self.gen.random() * span)
+ self.assertEqual(cum, span-1)
+
+ def test_bigrand(self):
+ # The randrange routine should build-up the required number of bits
+ # in stages so that all bit positions are active.
+ span = 2 ** 500
+ cum = 0
+ for i in xrange(100):
+ r = self.gen.randrange(span)
+ self.assert_(0 <= r < span)
+ cum |= r
+ self.assertEqual(cum, span-1)
+
+ def test_bigrand_ranges(self):
+ for i in [40,80, 160, 200, 211, 250, 375, 512, 550]:
+ start = self.gen.randrange(2 ** i)
+ stop = self.gen.randrange(2 ** (i-2))
+ if stop <= start:
+ return
+ self.assert_(start <= self.gen.randrange(start, stop) < stop)
+
+ def test_rangelimits(self):
+ for start, stop in [(-2,0), (-(2**60)-2,-(2**60)), (2**60,2**60+2)]:
+ self.assertEqual(Set(range(start,stop)),
+ Set([self.gen.randrange(start,stop) for i in xrange(100)]))
+
+ def test_genrandbits(self):
+ # Verify cross-platform repeatability
+ self.gen.seed(1234567)
+ self.assertEqual(self.gen.getrandbits(100),
+ 97904845777343510404718956115L)
+ # Verify ranges
+ for k in xrange(1, 1000):
+ self.assert_(0 <= self.gen.getrandbits(k) < 2**k)
+
+ # Verify all bits active
+ getbits = self.gen.getrandbits
+ for span in [1, 2, 3, 4, 31, 32, 32, 52, 53, 54, 119, 127, 128, 129]:
+ cum = 0
+ for i in xrange(100):
+ cum |= getbits(span)
+ self.assertEqual(cum, 2**span-1)
+
+ def test_randbelow_logic(self, _log=log, int=int):
+ # check bitcount transition points: 2**i and 2**(i+1)-1
+ # show that: k = int(1.001 + _log(n, 2))
+ # is equal to or one greater than the number of bits in n
+ for i in xrange(1, 1000):
+ n = 1L << i # check an exact power of two
+ numbits = i+1
+ k = int(1.00001 + _log(n, 2))
+ self.assertEqual(k, numbits)
+ self.assert_(n == 2**(k-1))
+
+ n += n - 1 # check 1 below the next power of two
+ k = int(1.00001 + _log(n, 2))
+ self.assert_(k in [numbits, numbits+1])
+ self.assert_(2**k > n > 2**(k-2))
+
+ n -= n >> 15 # check a little farther below the next power of two
+ k = int(1.00001 + _log(n, 2))
+ self.assertEqual(k, numbits) # note the stronger assertion
+ self.assert_(2**k > n > 2**(k-1)) # note the stronger assertion
+
_gammacoeff = (0.9999999999995183, 676.5203681218835, -1259.139216722289,
771.3234287757674, -176.6150291498386, 12.50734324009056,
-0.1385710331296526, 0.9934937113930748e-05, 0.1659470187408462e-06)
diff --git a/Misc/NEWS b/Misc/NEWS
index f4ca158..5a8379e 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -84,6 +84,14 @@
seed. Modified to match Py2.2 behavior and use fractional seconds so
that successive runs are more likely to produce different sequences.
+- random.Random has a new method, getrandbits(k), which returns an int
+ with k random bits. This method is now an optional part of the API
+ for user defined generators. Any generator that defines genrandbits()
+ can now use randrange() for ranges with a length >= 2**53. Formerly,
+ randrange would return only even numbers for ranges that large (see
+ SF bug #812202). Generators that do not define genrandbits() now
+ issue a warning when randrange() is called with a range that large.
+
- itertools.izip() with no arguments now returns an empty iterator instead
of raising a TypeError exception.
diff --git a/Modules/_randommodule.c b/Modules/_randommodule.c
index 4a87a8e..1189036 100644
--- a/Modules/_randommodule.c
+++ b/Modules/_randommodule.c
@@ -435,6 +435,47 @@
}
static PyObject *
+random_getrandbits(RandomObject *self, PyObject *args)
+{
+ int k, i, bytes;
+ unsigned long r;
+ unsigned char *bytearray;
+ PyObject *result;
+
+ if (!PyArg_ParseTuple(args, "i:getrandbits", &k))
+ return NULL;
+
+ if (k <= 0) {
+ PyErr_SetString(PyExc_ValueError,
+ "number of bits must be greater than zero");
+ return NULL;
+ }
+
+ bytes = ((k - 1) / 32 + 1) * 4;
+ bytearray = (unsigned char *)PyMem_Malloc(bytes);
+ if (bytearray == NULL) {
+ PyErr_NoMemory();
+ return NULL;
+ }
+
+ /* Fill-out whole words, byte-by-byte to avoid endianness issues */
+ for (i=0 ; i<bytes ; i+=4, k-=32) {
+ r = genrand_int32(self);
+ if (k < 32)
+ r >>= (32 - k);
+ bytearray[i+0] = (unsigned char)r;
+ bytearray[i+1] = (unsigned char)(r >> 8);
+ bytearray[i+2] = (unsigned char)(r >> 16);
+ bytearray[i+3] = (unsigned char)(r >> 24);
+ }
+
+ /* little endian order to match bytearray assignment order */
+ result = _PyLong_FromByteArray(bytearray, bytes, 1, 0);
+ PyMem_Free(bytearray);
+ return result;
+}
+
+static PyObject *
random_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
RandomObject *self;
@@ -464,6 +505,9 @@
{"jumpahead", (PyCFunction)random_jumpahead, METH_O,
PyDoc_STR("jumpahead(int) -> None. Create new state from "
"existing state and integer.")},
+ {"getrandbits", (PyCFunction)random_getrandbits, METH_VARARGS,
+ PyDoc_STR("getrandbits(k) -> x. Generates a long int with "
+ "k random bits.")},
{NULL, NULL} /* sentinel */
};