util/riffle.py

   1
   2 def riffle(lol):
   3     """
   4     given list of lists return flat list of elements, riffled:
   5     - order of individual lists is preserved
   6     - elements of the same list are far apart
   7
   8     (this really should be called "false riffle" because
   9     it doesn't shuffle, or "perfect riffle" for the same
  10     reason. consult youtube for the card shuffle technique
  11     that inspired the name)
  12     """
  13     def merge(a, b):
  14         "does not modify either, returns merged list"
  15         if len(a) == 0:
  16             return b
  17         elif len(b) == 0:
  18             return a
  19         elif len(a) > len(b):
  20             x = a
  21             a = b
  22             b = x
  23         c = []
  24         r = float(len(a)) / len(b)
  25         i_a = 0
  26         for i_b in xrange(len(b)):
  27             if r*(i_b) > i_a:
  28                 c.append(a[i_a])
  29                 i_a += 1
  30             c.append(b[i_b])
  31         return c
  32
  33     res = lol[0]
  34     for i in lol[1:]:
  35         res = merge(res,i)
  36     return res
  37
  38 def test_riffle():
  39     def gen_taglist(prefix, amount):
  40         return [prefix+str(i) for i in xrange(amount)]
  41
  42     def print_lst(lst):
  43         for x in lst:
  44             print x, " ",
  45         print
  46
  47     for i in xrange(10):
  48         print_lst( riffle( [gen_taglist(chr(a+ord('a')),
  49                             random.randint(2,13))
  50                             for a in range( random.randint(1,10))] ))
  51         print
  52