Lib/whrandom.py

   1 #       WICHMANN-HILL RANDOM NUMBER GENERATOR
   2 #
   3 #       Wichmann, B. A. & Hill, I. D. (1982)
   4 #       Algorithm AS 183:
   5 #       An efficient and portable pseudo-random number generator
   6 #       Applied Statistics 31 (1982) 188-190
   7 #
   8 #       see also:
   9 #               Correction to Algorithm AS 183
  10 #               Applied Statistics 33 (1984) 123
  11 #
  12 #               McLeod, A. I. (1985)
  13 #               A remark on Algorithm AS 183
  14 #               Applied Statistics 34 (1985),198-200
  15 #
  16 #
  17 #       USE:
  18 #       whrandom.random()       yields double precision random numbers
  19 #                               uniformly distributed between 0 and 1.
  20 #
  21 #       whrandom.seed(x, y, z)  must be called before whrandom.random()
  22 #                               to seed the generator
  23 #
  24 #       There is also an interface to create multiple independent
  25 #       random generators, and to choose from other ranges.
  26
  27
  28 #       Translated by Guido van Rossum from C source provided by
  29 #       Adrian Baddeley.
  30
  31
  32 # Multi-threading note: the random number generator used here is not
  33 # thread-safe; it is possible that nearly simultaneous calls in
  34 # different theads return the same random value.  To avoid this, you
  35 # have to use a lock around all calls.  (I didn't want to slow this
  36 # down in the serial case by using a lock here.)
  37
  38
  39 class whrandom:
  40         #
  41         # Initialize an instance.
  42         # Without arguments, initialize from current time.
  43         # With arguments (x, y, z), initialize from them.
  44         #
  45         def __init__(self, x = 0, y = 0, z = 0):
  46                 self.seed(x, y, z)
  47         #
  48         # Set the seed from (x, y, z).
  49         # These must be integers in the range [0, 256).
  50         #
  51         def seed(self, x = 0, y = 0, z = 0):
  52                 if not type(x) == type(y) == type(z) == type(0):
  53                         raise TypeError, 'seeds must be integers'
  54                 if not (0 <= x < 256 and 0 <= y < 256 and 0 <= z < 256):
  55                         raise ValueError, 'seeds must be in range(0, 256)'
  56                 if 0 == x == y == z:
  57                         # Initialize from current time
  58                         import time
  59                         t = long(time.time() * 256)
  60                         t = int((t&0xffffff) ^ (t>>24))
  61                         t, x = divmod(t, 256)
  62                         t, y = divmod(t, 256)
  63                         t, z = divmod(t, 256)
  64                 # Zero is a poor seed, so substitute 1
  65                 self._seed = (x or 1, y or 1, z or 1)
  66         #
  67         # Get the next random number in the range [0.0, 1.0).
  68         #
  69         def random(self):
  70                 # This part is thread-unsafe:
  71                 # BEGIN CRITICAL SECTION
  72                 x, y, z = self._seed
  73                 #
  74                 x = (171 * x) % 30269
  75                 y = (172 * y) % 30307
  76                 z = (170 * z) % 30323
  77                 #
  78                 self._seed = x, y, z
  79                 # END CRITICAL SECTION
  80                 #
  81                 return (x/30269.0 + y/30307.0 + z/30323.0) % 1.0
  82         #
  83         # Get a random number in the range [a, b).
  84         #
  85         def uniform(self, a, b):
  86                 return a + (b-a) * self.random()
  87         #
  88         # Get a random integer in the range [a, b] including both end points.
  89         # (Deprecated; use randrange below.)
  90         #
  91         def randint(self, a, b):
  92                 return self.randrange(a, b+1)
  93         #
  94         # Choose a random element from a non-empty sequence.
  95         #
  96         def choice(self, seq):
  97                 return seq[int(self.random() * len(seq))]
  98         #
  99         # Choose a random item from range([start,] step[, stop]).
 100         # This fixes the problem with randint() which includes the
 101         # endpoint; in Python this is usually not what you want.
 102         #
 103         def randrange(self, start, stop=None, step=1,
 104                       # Do not supply the following arguments
 105                       int=int, default=None):
 106                 # This code is a bit messy to make it fast for the
 107                 # common case while still doing adequate error checking
 108                 istart = int(start)
 109                 if istart != start:
 110                         raise ValueError, "non-integer arg 1 for randrange()"
 111                 if stop is default:
 112                         if istart > 0:
 113                                 return int(self.random() * istart)
 114                         raise ValueError, "empty range for randrange()"
 115                 istop = int(stop)
 116                 if istop != stop:
 117                         raise ValueError, "non-integer stop for randrange()"
 118                 if step == 1:
 119                         if istart < istop:
 120                                 return istart + int(self.random() *
 121                                                    (istop - istart))
 122                         raise ValueError, "empty range for randrange()"
 123                 istep = int(step)
 124                 if istep != step:
 125                         raise ValueError, "non-integer step for randrange()"
 126                 if istep > 0:
 127                         n = (istop - istart + istep - 1) / istep
 128                 elif istep < 0:
 129                         n = (istop - istart + istep + 1) / istep
 130                 else:
 131                         raise ValueError, "zero step for randrange()"
 132
 133                 if n <= 0:
 134                         raise ValueError, "empty range for randrange()"
 135                 return istart + istep*int(self.random() * n)
 136
 137
 138 # Initialize from the current time
 139 #
 140 _inst = whrandom()
 141 seed = _inst.seed
 142 random = _inst.random
 143 uniform = _inst.uniform
 144 randint = _inst.randint
 145 choice = _inst.choice
 146 randrange = _inst.randrange