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Bash - array loop performance

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Top Forums Shell Programming and Scripting Bash - array loop performance
# 15  
Old 10-02-2014
Quote:
Originally Posted by Chubler_XL
Bash isn't very efficient looking up large arrays I found using bash 4 assoicative arrays gave me a significant boost in speed if you have bash 4 consider using them:

Code:
declare -A sn
n=80000
k4=2
k3=6
for((i=0;i<n;i++))
do  sn[i]=$((RANDOM%256))
done

for(( i=0; i<n-2; i++ ))
do sn[i]=$(( (sn[i] ^ sn[i+1] ^ sn[i+2] * k4) % 256 ))
done

for(( i=(n-1); i>1; i--))
do sn[i]=$(( (sn[i] ^ sn[i-2] ^ sn[i-1] * k3) % 256))
done

Interesting.... I did some further testing with bash 3, bash 4 and ksh93 (for the associative array I used the synonym typeset -A instead of declare -A which works both in bash4 and ksh93:


Code:
indexed arrays                  bash 3          bash 4          ksh93
                        real    2m4.457s        1m6.420s        0m1.855s
                        user    1m53.857s       0m57.251s       0m1.764s
                        sys     0m0.501s        0m0.399s        0m0.007s

Associative arrays              bash 3          bash 4          ksh93   
                        real    -               0m4.553s        0m1.871s
                        user    -               0m3.867s        0m1.567s
                        sys     -               0m0.171s        0m0.003s

There seems to be an issue with regular arrays in bash that is absent in ksh93 which processes both types of arrays at the same speed...


---
Testing on OSX 10.9.5, bash 3.2.51(1), bash 4.2.0(1), ksh 93u 2011-02-08
Last edited by Scrutinizer; 10-02-2014 at 03:58 AM..
# 16  
Old 10-02-2014
I have rewrite the function in perl: wow, very fast!
I like perl seem easy to understand.
If someone is interested...

Code:
sub decrypt {
    my ($f, $key) = @_;
    my $s;
    {
      local *FH;
      open FH, "$f" or die $!;
      -f FH and sysread FH, $s, -s FH;
    }
    my $n=length($s);
    my $k1=11+($key % 233); my $k2= 7+($key % 239);
    my $k3= 5+($key % 241); my $k4= 3+($key % 251);
    my @sn = unpack("C*", $s);   
  # debug: print "@sn\n";
    my $i=0;
    for ($i=0; $i<($n-2); $i++) { $sn[$i]=(($sn[$i] ^ $sn[$i+2] ^ ($k4 * $sn[$i+1])) % 256); }
    for ($i=($n-1); $i>1; $i--) { $sn[$i]=(($sn[$i] ^ $sn[$i-2] ^ ($k3 * $sn[$i-1])) % 256); }
    for ($i=0; $i<($n-1); $i++) { $sn[$i]=(($sn[$i] ^ $sn[$i+1] ^ ($k2 * $sn[$i+1])) % 256); }
    for ($i=($n-1); $i>0; $i--) { $sn[$i]=(($sn[$i] ^ $sn[$i-1] ^ ($k1 * $sn[$i-1])) % 256); }
    $s=""; for ($i=0; $i<$n; $i++) { $s.=chr( $sn[$i] ); }  
}
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