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# WICHMANN-HILL RANDOM NUMBER GENERATOR
#
# Wichmann, B. A. & Hill, I. D. (1982)
# Algorithm AS 183:
# An efficient and portable pseudo-random number generator
# Applied Statistics 31 (1982) 188-190
#
# see also:
# Correction to Algorithm AS 183
# Applied Statistics 33 (1984) 123
#
# McLeod, A. I. (1985)
# A remark on Algorithm AS 183
# Applied Statistics 34 (1985),198-200
#
#
# USE:
# whrandom.random() yields double precision random numbers
# uniformly distributed between 0 and 1.
#
# whrandom.seed(x, y, z) must be called before whrandom.random()
# to seed the generator
#
# There is also an interface to create multiple independent
# random generators, and to choose from other ranges.
# Translated by Guido van Rossum from C source provided by
# Adrian Baddeley.
# Multi-threading note: the random number generator used here is not
# thread-safe; it is possible that nearly simultaneous calls in
# different theads return the same random value. To avoid this, you
# have to use a lock around all calls. (I didn't want to slow this
# down in the serial case by using a lock here.)
class whrandom:
#
# Initialize an instance.
# Without arguments, initialize from current time.
# With arguments (x, y, z), initialize from them.
#
def __init__(self, x = 0, y = 0, z = 0):
self.seed(x, y, z)
#
# Set the seed from (x, y, z).
# These must be integers in the range [0, 256).
#
def seed(self, x = 0, y = 0, z = 0):
if not type(x) == type(y) == type(z) == type(0):
raise TypeError, 'seeds must be integers'
if not (0 <= x < 256 and 0 <= y < 256 and 0 <= z < 256):
raise ValueError, 'seeds must be in range(0, 256)'
if 0 == x == y == z:
# Initialize from current time
import time
t = long(time.time() * 256)
t = int((t&0xffffff) ^ (t>>24))
t, x = divmod(t, 256)
t, y = divmod(t, 256)
t, z = divmod(t, 256)
# Zero is a poor seed, so substitute 1
self._seed = (x or 1, y or 1, z or 1)
#
# Get the next random number in the range [0.0, 1.0).
#
def random(self):
# This part is thread-unsafe:
# BEGIN CRITICAL SECTION
x, y, z = self._seed
#
x = (171 * x) % 30269
y = (172 * y) % 30307
z = (170 * z) % 30323
#
self._seed = x, y, z
# END CRITICAL SECTION
#
return (x/30269.0 + y/30307.0 + z/30323.0) % 1.0
#
# Get a random number in the range [a, b).
#
def uniform(self, a, b):
return a + (b-a) * self.random()
#
# Get a random integer in the range [a, b] including both end points.
# (Deprecated; use randrange below.)
#
def randint(self, a, b):
return self.randrange(a, b+1)
#
# Choose a random element from a non-empty sequence.
#
def choice(self, seq):
return seq[int(self.random() * len(seq))]
#
# Choose a random item from range([start,] step[, stop]).
# This fixes the problem with randint() which includes the
# endpoint; in Python this is usually not what you want.
#
def randrange(self, start, stop=None, step=1,
# Do not supply the following arguments
int=int, default=None):
# This code is a bit messy to make it fast for the
# common case while still doing adequate error checking
istart = int(start)
if istart != start:
raise ValueError, "non-integer arg 1 for randrange()"
if stop is default:
if istart > 0:
return int(self.random() * istart)
raise ValueError, "empty range for randrange()"
istop = int(stop)
if istop != stop:
raise ValueError, "non-integer stop for randrange()"
if step == 1:
if istart < istop:
return istart + int(self.random() *
(istop - istart))
raise ValueError, "empty range for randrange()"
istep = int(step)
if istep != step:
raise ValueError, "non-integer step for randrange()"
if istep > 0:
n = (istop - istart + istep - 1) / istep
elif istep < 0:
n = (istop - istart + istep + 1) / istep
else:
raise ValueError, "zero step for randrange()"
if n <= 0:
raise ValueError, "empty range for randrange()"
return istart + istep*int(self.random() * n)
# Initialize from the current time
#
_inst = whrandom()
seed = _inst.seed
random = _inst.random
uniform = _inst.uniform
randint = _inst.randint
choice = _inst.choice
randrange = _inst.randrange
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