#!/usr/local/bin/perl -w

##  set up some globals

$BASE  = '/base/of-the/image/path';     ##  base image directory
$LINK  = "$BASE/desktop.jpg";           ##  the target image

$GO_DOWN    = 0.70;                 ##  dig deeper 70% of the time
$TRY_LIMIT  = 1000;                 ##  stop after 1000 tries, max

$current = "$BASE";

##  set up a path heirarchy
@P = ($current);

##  set up a loop counter
$N = 0;

while (($N++ < $TRY_LIMIT) && (-d $current)) {

##  read the directory
    opendir (DIR, $current) or die qq(can't read "$current": $!);
    @D = grep { 
        (!/^\./)    &&      ##  exclude hidden files
        (!/^#/)             ##  exclude user-selected files
    } readdir DIR;
    closedir DIR;

##  get a list of subirs
    $d = [];  @$d = grep { -d "$current/$_" } @D;
    
##  get a list of '*.jpg' files
    $j = [];  @$j = grep { /.jpg$/i } @D;

##  assemble a list of weights..
#
#   the basic idea is to give some items a higher chance
#   of being chosen than others.   if a filename begins 
#   with a number N, the program's decision table will 
#   contain N copies of that filename.
#
#   we start by building a table with N-1 entries for 
#   each item that does start with a number, then finish
#   off by adding the whole file list to the table.
#
    @w = ();
    for $f (@$d) {
        if ($f =~ /^(\d+)/) {
            push @w, ($f) x ($1 - 1);
        }
    }
    push @$d, @w;


##  decide whether to choose a file from this directory,
##  or to pick a random directory and drill down again.
#
#   note that we may end up choosing an empty list.. 
#   therefore we check the list we choose to make sure 
#   it does contain something, and if it doesn't, we 
#   deal with it.
#
#   the first stage of dealing with it is to fall back
#   to the other option.. if we chose to drill deeper and 
#   in a directory with no subdirs, we fall back to 
#   selecting a random image.   if we chose to select an
#   image in a directory that only contains subdirs, we
#   fall back to drilling deeper.
#
#   the second stage of dealing with it.. which occurs 
#   if we hit an empty directory.. is to back out one
#   level and try again.
#
#   in the pathological case, where you run this loop
#   on the base of a file tree that contains no .jpg
#   files at all, the loop counter will force termination
#   after $TRY_LIMIT tries.
#
    ($l, $r) = (rand() < $GO_DOWN) ? ($d, $j) : ($j, $d);
    $list = (@$l) ? $l : $r;
    
    if (@$list) {
        push @P, $list->[ rand (@$list) ];
    } else {
        pop @P;
    }
    
    $current = join ('/', @P);
}

unlink $LINK;
link $current, $LINK if (-f $current);