What is the best way to hash the number of unique colors in a 219MB, uint48, raw RGB file?

DON'T!

What is the single most significant number characterizing the quality of a camera RAW image?
The number (or percentage) of unique colors contained in the image has to be THEE NUMBER!

At each step in the processing, quickly checking unique color count can tell you if
whatever you just did damaged your image. This number is indispensable for comparing
different color spaces or different raw processors or before and after optimizations.

Hashing in a domain with 281,474,976,710,656 possible combinations is Quixotic and
catastrophically inefficient. About 24 seconds was the best time with hashes.

Sometimes, there is no better solution than brute force.
Here is the Monster Truck Counting Solution:

 
&sysread_uint48($RGB, \@rgbz);  # Store RGB in uint64, RGBZ array
&RADIX_SORT(\@rgbz);  # Sort by digits!
$std=$rgbz[0];  # Make first uint64 the STanDard 
$unique=1;      # First one is Always Unique!
for($ii=1; $ii < $num_pix; $ii++)  {
    next  if($rgbz[$ii] == $std);  # Same Old value as last time!
    $unique++;                    # New value, Increment Unique
    $std=$rgbz[$ii];              # Make new THIS the STD
}
printf("%d distinct values\n", $unique);
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 I:\exp\raw>cdrc pf-2015.0531-249465.pprgb.7360x4912.raw
CDRC: 27645898 distinct RGBs = 76.471% in 0.946 sec

I:\exp\raw>cdrc pf-2015.0531-249465.pprgb.7360x4912.raw -d
CDRC: Read 216.913920MB, 36.2MP in 73 msec->2964 MB/s, CPU@3.421GHz
CDRC: 36.152320M MemCpy in 67 msec
RS: R_Sort-> 775 ms, C_Unique-> 32 ms, total-> 808 ms
CDRC: 27645898 distinct RGBs = 76.471% in 0.950 sec
[download]

Let the Radix Sort do all of the heavy lifting.

Here is the hash way implemented by PerlMagick:

Running C:\bin\bb2.pl   Sun Dec 13 16:30:19 2015
Unique colors=27.645898M =  76.471%  <<< EXACT!

SE: Sum of Event Times = 6.789 min, now-init = 6.78937 min
ETime=239.024 ms =  0.059%, 1 Hits, Event 'PerlMagic Read Raw File'
ETime=  6.785 min = 99.941%, 1 Hits, Event 'Quantize'
99.94% -> Quantize
 0.06% -> PerlMagic Read Raw File
Elapsed C:\bin\bb2.pl time = 6.789 min


# Here is the hash way implemented by ImageMagick Bone-Nary[*]:
Running C:\bin\bb.pl   Sun Dec 13 15:19:29 2015;  
Conv convert.exe -size 7360x4912 -depth 16 \
RGB:i:/exp/raw/pf-2015.0531-249465.pprgb.7360x4912.raw \
-unique-colors \
i:/exp/raw/pf-2015.0531-249465.pprgb.7360x4912.raw.tif ->
        size=165875652
Elapsed C:\bin\bb.pl time = 6.936 min

I:\exp\raw>mult 165875652 /27645898
6.0000    <<  Very close agreement on Unique Count

I:\exp\raw>mult 6.936 60 /0.946
439.915433
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946 MilliSeconds makes this analysis affordable.

# =============================================================
sub unique_colors_perl()  {
use Image::Magick;  # 
my($debug, $raw, $xres, $yres, $xysize, $unique, $xysize, %e2at);
    $debug=0;
    %e2at = ();
    $raw='i:/exp/raw/pf-2015.0531-249465.pprgb.7360x4912.raw';
    &time_event('init', \%e2at, $debug*0);  # Initialize timing 
    &time_event('PerlMagic Read Raw File', \%e2at, $debug*0);
    ($xres, $yres)=&fname_to_xyres($raw, $debug);
    $xysize=$xres . "x$yres";  # 7360x4912
    # Create a new rgb48 with just the right size
    $im=Image::Magick->new(size=>"$xysize", type=>'TrueColor',\
depth=>16);  
    $err=$im->Read(filename=>"RGB:$raw");  warn $err  if $err;
    &time_event('Quantize', \%e2at, $debug*0);
    ($unique) = $im->Get('colors');
    printf("Unique colors=%.6fM = %6.3f%%\n",
        $unique/1E6, $unique/(7360*4912));
    &time_event('term', \%e2at, $debug*0);  # Finalize timing 
    &show_event(\%e2at, $debug*0);  # Event timing
}  # End Unique_Colors_Perl().

NOTE: Bone-Nary: Special build for BoneHeads who can't \
 make their own! :)
[download]

B