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JPEG Zig-Zag

by blackle (Beadle)
on Oct 29, 2012 at 17:07 UTC ( #1001394=obfuscated: print w/replies, xml ) Need Help??

For the last week I've been trying to figure out an algorithm for JPEG's zig-zag spacefilling curve. Specifically, I wanted an algorithm that maps all elements in an NxN matrix to a N*N array given their x and y values. I finally figured it out with the help of my analysis 101 TA. He's not a computer scientist, but he knows what pairing functions are. Anyway, after a few hours messing with perl, I made this subroutine:

sub getPos{ ($x,$y,$s)=@_;($d=$x+$y)*($d++)/2 +($d%2?$y:$x)-($d>$s)*($d-$s)**2 }

To use it, you do something like this:

for(my $x = 0; $x < $size; $x++){ for(my $y = 0; $y < $size; $y++){ print getPos($x, $y, $size)." "; } print "\n"; }

For $size = 10 the loop will yield:

0 1 5 6 14 15 27 28 44 45 2 4 7 13 16 26 29 43 46 63 3 8 12 17 25 30 42 47 62 64 9 11 18 24 31 41 48 61 65 78 10 19 23 32 40 49 60 66 77 79 20 22 33 39 50 59 67 76 80 89 21 34 38 51 58 68 75 81 88 90 35 37 52 57 69 74 82 87 91 96 36 53 56 70 73 83 86 92 95 97 54 55 71 72 84 85 93 94 98 99

The body of getPos has 65 (non-whitespace) characters, any ideas on how to get it even smaller?


UPDATE: This prints a JPEG zig-zag when $ARGV[0] is the size of the matrix:
$==pop;print map{$y=$_;map(($d=$y+$_)*$d++/2-($d>$=)*($d-$=)**2+($d&1? +$_:$y).$",@m),$/}@m=0..$=-1
97 characters! I started at 125!

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[Corion]: marioroy: Oh, that's always cool, having API-compatible modules. This makes testing and comparing things much easier
[marioroy]: IPC in MCE::Shared can handle 400k (sends) per second. That's seems a lot for being a pure-Perl module. After making the release, will come back and post a solution for a node by a fellow wanting faster logging.
[Corion]: While working on WWW::Mechanize:: Chrome, I had the suspicion that AnyEvent was doing something wrong, but I was able to swap it out for Mojolicious and the error persisted.
[Corion]: Of course, the error was in my own code ;)
[marioroy]: Corion, start and start_child in MCE::Hobo::Manager return a MCE::Hobo object, whereas P::FM returns the PID. I can have it return the PID though. I tried Hobo::Manager with several P::FM modules, just changed P::FM to MCE::Hobo::Manager and it works.
[marioroy]: I also have a Hobo driver for Forklift allowing folks to use in multiple classes, no conflicts with one another. That's not possible for P::FM.
[Discipulus]: congrats marioroy!
[marioroy]: CORE::wait works well eventhough multiple instances or classes using Hobo::Manager.
[Corion]: marioroy: I'm not sure what the normal use for the PID is in P:FM, but I guess that most programs just ignore or log it

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