[docs] The computation of pi in Decimal fixed point and floating point arithmetic

[bernard greening]

This is not a bug, its a question.

on the web page

http://docs.python.org/library/decimal.html


  9.4 Decimal fixed point and floating point arithmetic


in the


    9.4.7. Recipes¶ <http://docs.python.org/library/decimal.html#recipes>

section is a function to compute the value of pi

def  pi():
     """Compute Pi to the current precision.

     >>>  print pi()
     3.141592653589793238462643383

     """
     getcontext().prec  +=  2   # extra digits for intermediate steps
     three  =  Decimal(3)       # substitute "three=3.0" for regular floats
     lasts,  t,  s,  n,  na,  d,  da  =  0,  three,  3,  1,  0,  0,  24
     while  s  !=  lasts:
         lasts  =  s
         n,  na  =  n+na,  na+8
         d,  da  =  d+da,  da+32
         t  =  (t  *  n)  /  d
         s  +=  t
     getcontext().prec  -=  2
     return  +s                # unary plus applies the new precision


I would like to ask the author what algorithm this is based upon and any references as to how it works.



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