OpenEuphoria

Re: Derangements

• Posted by Patrick Barnes <mrtrick at gmail.com> Jun 30, 2004
• 546 views

> Something like this?
> 
> sequence perm_set
> perm_set = "ABCDEFGHIJKLMNOPQR"
> 
> function perm(atom n)
>   sequence result
>   atom pstep
>   integer level, i2
>   sequence remains
> 
>   result = perm_set
>   remains = perm_set
>   pstep = factorial(length(perm_set))
>   if (n >= pstep) then
>     return -1
>   elsif (n = floor(n)) then
>     return -1
>   elsif (0 > n) then
>     return -1
>   else
>     i2 = length(perm_set)
>     for i = 1 to length(perm_set)-1 do
>       pstep /= i2
>       i2 -= 1
>       level = floor(n/pstep) + 1
>       n = remainder(n, pstep)
>       result[i] = remains[level]
>       remains = remains[1..level-1] & remains[level+1..length(remains)]
>     end for
>     result[length(perm_set)] = remains[1]
>     return result
>   end if
> end function

So, what does it do??
or how does it work??

The application I want to use this for involves positions in a large
2D array, and a reproducible way to step through those positions. 100
percent coverage is not required, but no position should be visited
twice, as that would overwrite existing data.

Speed of access is not that important, as most of the time it will be
accessed sequentially.


-- 
MrTrick