Re: pgbench - add pseudo-random permutation function

Hello Hironobu-san,

> However, the implementation of the scatter operation in this patch overflows 
> in many cases if the variable:size is 38 bit integer or greater. Because the 
> variable:size and the item of the array:primes[] which stores 27-29 bit 
> integers are multiplicated. If overflow occurs, the scatter operation does 
> not satisfy bijective.

Indeed. Again, thanks for the debug! As you contributed some code, I added 
you as a co-author in the CF entry.

Attached a v3, based on your fix, plus some additional changes:
  - explicitly declare unsigned variables where appropriate, to avoid casts
  - use smaller 24 bits primes instead of 27-29 bits
  - add a shortcut for multiplier below 24 bits and y value below 40 bits,
    which should avoid the manually implemented multiplication in most
    practical cases (tables with over 2^40 rows are pretty rare...).
  - change the existing shortcut to look a the number of bits instead of
    using 32 limits.
  - add a test for minimal code coverage with over 40 bits sizes
  - attempt to improve the documentation
  - some comments were updates, hopefully for the better

The main idea behind the smaller primes is to avoid the expensive modmul 
implementation on most realistic cases.

-- 
Fabien.