Lib/whrandom.py

   1 """Wichman-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
  29 Multi-threading note: the random number generator used here is not
  30 thread-safe; it is possible that nearly simultaneous calls in
  31 different theads return the same random value.  To avoid this, you
  32 have to use a lock around all calls.  (I didn't want to slow this
  33 down in the serial case by using a lock here.)
  34 """
  35
  36 # Translated by Guido van Rossum from C source provided by
  37 # Adrian Baddeley.
  38
  39
  40 class whrandom:
  41     def __init__(self, x = 0, y = 0, z = 0):
  42         """Initialize an instance.
  43         Without arguments, initialize from current time.
  44         With arguments (x, y, z), initialize from them."""
  45         self.seed(x, y, z)
  46
  47     def seed(self, x = 0, y = 0, z = 0):
  48         """Set the seed from (x, y, z).
  49         These must be integers in the range [0, 256)."""
  50         if not type(x) == type(y) == type(z) == type(0):
  51             raise TypeError, 'seeds must be integers'
  52         if not (0 <= x < 256 and 0 <= y < 256 and 0 <= z < 256):
  53             raise ValueError, 'seeds must be in range(0, 256)'
  54         if 0 == x == y == z:
  55             # Initialize from current time
  56             import time
  57             t = long(time.time() * 256)
  58             t = int((t&0xffffff) ^ (t>>24))
  59             t, x = divmod(t, 256)
  60             t, y = divmod(t, 256)
  61             t, z = divmod(t, 256)
  62         # Zero is a poor seed, so substitute 1
  63         self._seed = (x or 1, y or 1, z or 1)
  64
  65     def random(self):
  66         """Get the next random number in the range [0.0, 1.0)."""
  67         # This part is thread-unsafe:
  68         # BEGIN CRITICAL SECTION
  69         x, y, z = self._seed
  70         #
  71         x = (171 * x) % 30269
  72         y = (172 * y) % 30307
  73         z = (170 * z) % 30323
  74         #
  75         self._seed = x, y, z
  76         # END CRITICAL SECTION
  77         #
  78         return (x/30269.0 + y/30307.0 + z/30323.0) % 1.0
  79
  80     def uniform(self, a, b):
  81         """Get a random number in the range [a, b)."""
  82         return a + (b-a) * self.random()
  83
  84     def randint(self, a, b):
  85         """Get a random integer in the range [a, b] including
  86         both end points.
  87
  88         (Deprecated; use randrange below.)"""
  89         return self.randrange(a, b+1)
  90
  91     def choice(self, seq):
  92         """Choose a random element from a non-empty sequence."""
  93         return seq[int(self.random() * len(seq))]
  94
  95     def randrange(self, start, stop=None, step=1, int=int, default=None):
  96         """Choose a random item from range(start, stop[, step]).
  97
  98         This fixes the problem with randint() which includes the
  99         endpoint; in Python this is usually not what you want.
 100         Do not supply the 'int' and 'default' arguments."""
 101         # This code is a bit messy to make it fast for the
 102         # common case while still doing adequate error checking
 103         istart = int(start)
 104         if istart != start:
 105             raise ValueError, "non-integer arg 1 for randrange()"
 106         if stop is default:
 107             if istart > 0:
 108                 return int(self.random() * istart)
 109             raise ValueError, "empty range for randrange()"
 110         istop = int(stop)
 111         if istop != stop:
 112             raise ValueError, "non-integer stop for randrange()"
 113         if step == 1:
 114             if istart < istop:
 115                 return istart + int(self.random() *
 116                                    (istop - istart))
 117             raise ValueError, "empty range for randrange()"
 118         istep = int(step)
 119         if istep != step:
 120             raise ValueError, "non-integer step for randrange()"
 121         if istep > 0:
 122             n = (istop - istart + istep - 1) / istep
 123         elif istep < 0:
 124             n = (istop - istart + istep + 1) / istep
 125         else:
 126             raise ValueError, "zero step for randrange()"
 127
 128         if n <= 0:
 129             raise ValueError, "empty range for randrange()"
 130         return istart + istep*int(self.random() * n)
 131
 132
 133 # Initialize from the current time
 134 _inst = whrandom()
 135 seed = _inst.seed
 136 random = _inst.random
 137 uniform = _inst.uniform
 138 randint = _inst.randint
 139 choice = _inst.choice
 140 randrange = _inst.randrange