HEADS UP - HAMMER work

[Oliver Fromme]

Matthew Dillon wrote:
 >    64-bit directory hash encoding (for smaller filenames out of bounds
 >    indices just store a 0).
 > 
 >    aaaaa       name[0] & 0x1F
 >    bbbbb       name[1] & 0x1F
 >    ccccc       name[2] & 0x1F
 >    mmmmmm      crc32(name + 3, len - 5) and some xor magic -> 6 bits
 >    yyyyy       name[len-2] & 0x1F
 >    zzzzz       name[len-1] & 0x1F
 >    h[31]       crc32(name, len) (entire filename)
 > 
 >    0aaaaabbbbbccccc mmmmmmyyyyyzzzzz hhhhhhhhhhhhhhhh hhhhhhhhhhhhhhh0
 > [...]
 >    There are several issues but most of the trade-offs seem to be ok.  The
 >    one issue I am looking at closely is perhaps to try to do a better job
 >    sorting files that have both a prefix and a postfix.  In the above scheme
 >    any large prefix or postfix covers the a[5], b[5], c[5], y[5], and z[5]
 >    bits and devolves us back to an effectively random hash code.

You already mentioned it.  That's exactly the problem
that I'm seeing ...  I'm not sure whether a[], b[], c[],
y[] and z[] buy you anything in practice.

If a single directory contains a huge number of files,
it is likely they are all of the same type, e.g. it could
be a collection of images or whatever.  That means they
all have the same extension (e.g. .jpg), so y[] and z[]
are useless.

Furthermore, it isn't completely unlikely that they even
begin with the same prefix.  For example, all of my
digital camera pics are named "img%05d.jpg".  Admittedly
those aren't millions (but more than 10k anyway), and
I'm not stupid enough to collect them in a single
directory.  ;-)

Another example:  The cache directory of my Opera browser.
It contains several thousands of files all beginning
with "opr*".

It might be a good idea to make a small survey, i.e. find
people who actually _do_ have directories with a huge
number of files in them (and I mean more than just a few
thousands), and ask them what the filenames typically look
like.

An obvious improvement would be to store name[d-2] and
name[d-1] in y[] and z[], respectively, where d is the
location of the last dot in the filename, if any, or the
location of the terminating zero if there is no dot.
In other words:  Ignore the extension when identifying
y[] and z[].  Finding the last dot shouldn't be more
computationally expensive than strlen(name), so this
shouldn't be a problem.

Best regards
   Oliver

-- 
Oliver Fromme, secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing b. M.
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