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 class whrandom:
  33         #
  34         # Initialize an instance.
  35         # Without arguments, initialize from current time.
  36         # With arguments (x, y, z), initialize from them.
  37         #
  38         def __init__(self, x = 0, y = 0, z = 0):
  39                 self.seed(x, y, z)
  40         #
  41         # Set the seed from (x, y, z).
  42         # These must be integers in the range [0, 256).
  43         #
  44         def seed(self, x = 0, y = 0, z = 0):
  45                 if not type(x) == type(y) == type(z) == type(0):
  46                         raise TypeError, 'seeds must be integers'
  47                 if not 0 <= x < 256 and 0 <= y < 256 and 0 <= z < 256:
  48                         raise ValueError, 'seeds must be in range(0, 256)'
  49                 if 0 == x == y == z:
  50                         # Initialize from current time
  51                         import time
  52                         t = int(time.time() % 0x80000000)
  53                         t, x = divmod(t, 256)
  54                         t, y = divmod(t, 256)
  55                         t, z = divmod(t, 256)
  56                 self._seed = (x, y, z)
  57         #
  58         # Get the next random number in the range [0.0, 1.0).
  59         #
  60         def random(self):
  61                 x, y, z = self._seed
  62                 #
  63                 x1, x2 = divmod(x, 177)
  64                 y1, y2 = divmod(y, 176)
  65                 z1, z2 = divmod(z, 178)
  66                 #
  67                 x = (171 * x2 -  2 * x1) % 30269
  68                 y = (172 * y2 - 35 * y1) % 30307
  69                 z = (170 * z2 - 63 * z1) % 30323
  70                 #
  71                 self._seed = x, y, z
  72                 #
  73                 return (x/30269.0 + y/30307.0 + z/30323.0) % 1.0
  74         #
  75         # Get a random number in the range [a, b).
  76         #
  77         def uniform(self, a, b):
  78                 return a + (b-a) * self.random()
  79         #
  80         # Get a random integer in the range [a, b] including both end points.
  81         #
  82         def randint(self, a, b):
  83                 return a + int(self.random() * (b+1-a))
  84         #
  85         # Choose a random element from a non-empty sequence.
  86         #
  87         def choice(self, seq):
  88                 return seq[int(self.random() * len(seq))]
  89
  90
  91 # Initialize from the current time
  92 #
  93 _inst = whrandom()
  94 seed = _inst.seed
  95 random = _inst.random
  96 uniform = _inst.uniform
  97 randint = _inst.randint
  98 choice = _inst.choice