Source Code for Module qubx.ulp_diff

 1  import struct 
 2  from functools import partial 
 3   
 4  # (c) 2010 Eric L. Frederich 
 5  # 
 6  # Python implementation of algorithms detailed here... 
 7  # from http://www.cygnus-software.com/papers/comparingfloats/comparingfloats.htm 
 8   
 9 -def c_mem_cast(x, f=None, t=None): 
10      ''' 
11      do a c-style memory cast 
12   
13      In Python... 
14   
15      x = 12.34 
16      y = c_mem_cast(x, 'd', 'l') 
17   
18      ... should be equivilent to the following in c... 
19   
20      double x = 12.34; 
21      long   y = *(long*)&x; 
22      ''' 
23      return struct.unpack(t, struct.pack(f, x))[0] 
24   
25  dbl_to_lng = partial(c_mem_cast, f='d', t='l') 
26  lng_to_dbl = partial(c_mem_cast, f='l', t='d') 
27  flt_to_int = partial(c_mem_cast, f='f', t='i') 
28  int_to_flt = partial(c_mem_cast, f='i', t='f') 
29   
30 -def ulp_diff_maker(converter, negative_zero): 
31      ''' 
32      Getting the ulp difference of floats and doubles is similar. 
33      Only difference if the offset and converter. 
34      ''' 
35      def the_diff(a, b): 
36   
37          # Make a integer lexicographically ordered as a twos-complement int 
38          ai = converter(a) 
39          if ai < 0: 
40              ai = negative_zero - ai 
41   
42          # Make b integer lexicographically ordered as a twos-complement int 
43          bi = converter(b) 
44          if bi < 0: 
45              bi = negative_zero - bi 
46   
47          return abs(ai - bi) 
48   
49      return the_diff 
50   
51  # double ULP difference 
52  dulpdiff = ulp_diff_maker(dbl_to_lng, 0x8000000000000000) 
53  # float  ULP difference 
54  fulpdiff = ulp_diff_maker(flt_to_int, 0x80000000        ) 
55   
56  # default to double ULP difference 
57  ulpdiff = dulpdiff 
58  ulpdiff.__doc__ = ''' 
59  Get the number of doubles between two doubles. 
60  ''' 
61