Hi All,
I have been trying to write this email for a day or two but the
content keeps shifting as my understanding of the problem develops.
I have encountered a bit of code that I am struggling to understand
completely. The code is for dealing with database fields that are
bigint(20) type.
| allowcountry1 | bigint(20) | NO | | -1
| |
| allowcountry2 | bigint(20) | NO | | -1
| |
| allowcountry3 | bigint(20) | NO | | -1
| |
| allowcountry5 | bigint(20) | NO | | -1
| |
| allowcountry4 | bigint(20) | NO | | -1
| |
| allowcountry6 | bigint(20) | NO | | -1
| |
| allowcountry7 | bigint(20) | NO | | -1
| |
| allowcountry0 | bigint(20) | NO | | -1
| |
The DB table is for a product. The product can only be sold in
particularly countries and I think these fields/columns use a bit
pattern to represent that.
I have tracked down a piece of code that I believe insert the data
into each column. I have taken as small a snippet as possible and
tried to test it. It produces this:
$VAR1 = {
'allowcountry6' => '0xffffffffffffffff',
'allowcountry1' => '0xffffffffffffffff',
'allowcountry5' => '0xffffffffffffffff',
'allowcountry2' => '0xffffffffffffffff',
'allowcountry3' => '0xffffffffffffffff',
'allowcountry4' => '0xffffffffffffffff',
'allowcountry0' => '0xffffffffefffffff',
'allowcountry7' => '0xffffffffffffffff'
};
The code that produced this is below and it should run as stood alone.
I can understand parts of it but the overall logic is...well, I think
beyond me.
The routine expect a countryid (156) in this case and a string within
a set of tags. The string can be negated so
RS.CY1> ANTARCTICA</ means available in all countries EXCEPT ANTARCTICA
RS.CY1>* ANTARCTICA</ mean only IN ANTARCTICA.
I have put some questions inline and I would really appreciate an help
understanding the logic behind this. There are problems elsewhere in
the program and I can't really tackling them until I understand how
this bit pattern works. Also if anyone has an pointers to reference
material that might help with this sort of programming, I'd appreciate
it. Mastrering Perl does touch on it but not to this extent.
Thanx in advance.
Dp.
#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;
my %hr;
my $countryid = 156;
my $val = '<RS.CY1> ANTARCTICA</RS.CY1>';
create_bit_mask($val, $countryid, \%hr);
print Dumper(\%hr);
sub create_bit_mask {
my $DEBUG = 1;
my $val = shift;
my $countryid=shift;
my $hr = shift;
my $isinclude = 0;
# Figure out whether this is an 'allow' or 'deny' entry
if( $val =~ /\*/ ){
$val =~ s/\*//;
$val =~ s/^\s+(\S.*)/$1/;
$isinclude = 1;
print STDERR "ProcessCountry ISINC $isinclude\n" if ($DEBUG == 1);
}
# Have we already got bitmaps? (Authors' comment)
if( not exists $hr->{'allowcountry0'} )
{
## Is
this how the pattern is negated?
my $bmval = $isinclude? ## So it's hex not base 2?
'0x0000000000000000'
:'0xFFFFFFFFFFFFFFFF';
for(my $i=0;$i<8;$i++) ## Set all the field to there
default; either all 0's or F's
{
$hr->{'allowcountry'.$i} = $bmval;
}
}
# The highest
countryid is 243. How can fieldno ever be greater than 3?
my $fieldno=int($countryid/64); # 156/64 2.43, fieldno=2
my $str="";
my $currentno=$fieldno*64; # 2 * 64
# Amend the bitmaps. This is necessarily a bit ugly. (Authors comment).
for(my $i=0;$i<8;$i++)
{
my $digits=$hr->{"allowcountry$i"}; ## Changing the bit pattern !!!
for(my $j=0;$j<16;$j++)
{
my $bits=hex( substr($digits,17-$j,1));
if(($countryid - $currentno) < 4 && ($countryid >= $currentno))
{
if($isinclude)
{
$bits |= 1<< ($countryid - $currentno); ###
What
is this left shift doing? Is that a or complement?
}
else
{
$bits &= 15- (1<< ($countryid - $currentno)); ### ditto
}
}
$currentno+=4;
substr($digits,17-$j,1,sprintf("%1x",$bits));
}
$hr->{"allowcountry$i"}=$digits;
print STDERR "allowed ", $digits, "\n";
}
}
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