<SOLVED!! (not the index lookup but the actual problem from my original example). Reversal is possible with almost no time difference.> I also was looking for a method to do this, preferably the fastest possible. With a relatively small array (256), the use List::Util qw(first); method was actually significantly slower than simply iterating through the array and breaking(last) out once the match was found. In my test example on a 2.5MB (2560000bytes) file doing a sort of Sbox type lookup, the iterating took about 3m45s on my old Atom netbook where as the use List::Util qw(first); method required over 5m. Does anyone know the fastest way to perform these index lookups? A few of the examples shown appear to be basically identical. Hashing an array(did I say that right?) is a concept I am not yet familiar with.

I just tried the index hashing method described in the original post I found info on this as well and on the same file went at a grueling pace...Around 15mins. Is there really no other way to perform this operation faster than simply iterating through the array?

Looks like I solved the timing issue in my posted code by re-ordering the array by it's own values. Fixed code shown below. 

#!/usr/bin/env perl
use strict; #use warnings;

my $key="a5e0d57354e5fb898a44a979582536141ca3819d1c457db21e3cdf64bdc90
+01d1903ea7d9814649b2e6edee9674d57a5e47a3b0fcb5dbc8de3181c617c88650a88
+f9ff4a9852538087dabf35a596d657c86715d89701edefee55453302488ec3aae6a53
+20319a13ffbd11eb31a7e5c921421a4728b86523889fa20a44f19c4d4abf302490e39
+b5fa50619a18acf9785b6f1bd55158e1405776fb0ee2f3fa5f56aa828b8c22bd6f4e6
+70c23edefb08add86cc5d3415aaa64a4fcf677d2198f1801d800c6762a84e7b0e1053
+b95c38c94fdd637757c2f0ef5d0c628ddf1e83e4f178a00cefd41f58c3b4c2fa71c5e
+d56bfa6aa321120978fbe6f8dfa7fc2b5f109";
$key=pack('H*',$key);

my @sbase;
for my $i (0..255){$sbase[$i]=chr($i)};

my @sbox=sbox_gen(@sbase);

if ( $ARGV[2] eq "fwd" ) {
    swap(@sbox);
} elsif ( $ARGV[2] eq "rev" ) {
    @sbox=sbox_reindex(@sbox);
    swap(@sbox);
}

sub swap {
my ($n,$infile,$buf,$data,$outfile,$tmp); my @sbox=@_;
open($infile,"<",$ARGV[0]) or die $!; binmode($infile);
open($outfile,">",$ARGV[1]) or die $!; binmode($outfile);
while (($n=sysread $infile,$buf,256)!=0){
    for my $i (0..255) {
        $tmp=ord(substr($buf,$i,1));
        $data.=$sbox[$tmp];
    }
    print $outfile $data;$data="";
}
close($outfile);
close($infile);
}

sub sbox_gen {
my @sbox; my @sary=@_;
for my $i (0..255) {
    my $tmp=ord(substr($key,$i,1))**3;
    $tmp=$tmp%scalar(@sary);
    push @sbox,$sary[$tmp];
    splice(@sary,$tmp,1);
}
return @sbox;
}

sub sbox_reindex {
my @sbox=@_; my @out;
for my $i (0..255) {
    my $a=ord($sbox[$i]);
    $out[$a]=chr($i);
}
return @out;
}

[download]

Timings now:

$ time ./sbox.pl testfile outfile fwd real 0m3.424s user 0m3.330s sys 0m0.073s

$ time ./sbox.pl outfile revfile rev real 0m3.529s user 0m3.413s sys 0m0.083s

$ diff testfile revfile

$

In reply to How do I find the index of a specific array value? by SM177Y

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