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