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A Directory traversal is a very complex tree traversal!

In a normal tree structure you have three basic ways of traversal...

     Depth First            Top-Down                Breath
          7                    1                       1
        /   \                /   \                   /   \
       3     6              2     5                 2     3
      / \   / \            / \   / \               / \   / \
     1   2 4   5          3   4 6   7             4   5 6   7


In a directory traversal, each node consists of a very long list of filenames
of both directory and non-directory filenames, in that node (directory).
This vastly complicates matters and the definition of exactly what order
a directory tree is traversed.

However you also have the 'order' of the sub-directories to consider,
as many UNIX filesystems just returns the filenames in a specific
directory in unsorted psuedo-random (stored list) order.

As such within just one directory (node of the tree) the names could be
processed in...

  unsorted "as found" order
  alphetically sorted (case sensitive or insensitive?)
  a user specified order.
    EG: certian files first (readme?), numerically, then anything else?

Then you have the question on if you want to seperate sub-directory components
from the list before or during processing (forming two seperate traversals of
each directory node).  So with the filename list do you...

  do each entry in the order given.
  do all file entries  THEN do all directory entries
  do all directory entries  THEN  do all file entries

None of this actually involves the traversal of the sub-directory only
the handling of a single directory of sub-directory and filename entries.

So when do you actually traverse the sub-directories?

  before/after processing ALL the filenames in the parent directory?
  only before/after looking at non-directory filenames?
  traverse a sub-directory immediately before/after that directory?

As you can see directory traversal has a lot of posibilities.

This is not just for file directories, but also other sorts of
directories, such as LDAP directory tree of entries, or some other complex
data structure (that may even contain loops).

In summery...

Any tree recursion has
  sort order of the directory entries
  process files and directory entries seperatally? and in what order
  when to traveral sub-directories (depth, top-down, breath)
    which may even be a different sort order!

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Specific directory traversal programs...

UNIX "find" command...
  Traverse in unsorted directory listing order (files/directory names mixed).
  By default traversal each sub-directory immediatally after processing
  the filename of the current directory.

  What this means is that when a directory is being processes only some of the
  non-directory files in the parent directory may have been processed, and
  what has or has not been processed depends on the randomised file creation
  order.

  The "-depth" changes sub-directory traversal from immediately before
  to immediately after the contents of that directory


Perl File::Find
  Perl version 5.7
      Optional sorting of the retrieved directory listing,
      Process all file filenames in a directory first
      Then process each sub-direcory (in reverse order)
        processing a dir filename immediately before traversing it.

  Version 5.8 File::Find is exactly as 5.7 but a patch I provided makes the
  directory handling in the correct order.   The file - directory seperation
  is still present (no option), and in most cases desirable.

  The "bydepth" makes the directory filename processing AFTER that
  directory is processed.  That is all it does, filenames are still
  processed in the same order as before,  non-directory files first, then
  directory files.

Perl File::Find::Parallel
  From cpan.. parallel directory searches

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Anthony Thyssen  7 November 2003
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