#!/usr/bin/perl -w

require 5.006; # i'm using "open my $fh ..." construct

use strict;

# =================================================
# constants

# put magic constants at top, so people can easily change them

my $CHARGE_CODES_FILE = "mnta2/bmp/table/smsc/chg_indicator.hdb";

my $INPUT_TEMPLATE = "A12 A4 A20 A20
                      A2  A2 A2  A2
                      A12 A4 A24 A24";

my $SOURCE_IX = 2;
my $DEST_IX   = 3;

# =================================================

# first, load up the charge database.
my %charge_codes;
{
    open my $fh, $CHARGE_CODE_FILE
      or die "$0: opening '$CHARGE_CODE_FILE': $!";
    while ( <$fh> )
    {
        next if /^#/;      # skip comments
        s/\s+$/;           # remove end of line chars

        # get fields from line, clean them up.
        my ( $src, $dest, $code ) = split /,/;
        $src  =~ s/^0+//;
        $dest =~ s/^0+//;

        $charge_codes{$src}{$dest} = $code;
    }
}

# -------------------------------------------------
# process standard input, accumulate charges.

my %charges_against;

while ( my $input = <> )
{
    # extract source and destination values
    my @fields = unpack $INPUT_TEMPLATE, $input;
    my $src  = $fields[ $SOURCE_IX ];
    my $dest = $fields[ $DEST_IX ];

    # remove leading zeros
    $src  =~ s/^0+//;
    $dest =~ s/^0+//;

    # see if there are any charge codes associated with this pair.
    my $code = $charge_codes{$src}{$dest};
    if ( defined $code )
    {
        ++$charges_against{$code};
    }
}

# -------------------------------------------------
# output the results

foreach my $code ( sort keys %charges_against )
{
    my $count = $charges_against{$code};
    print "Total calls for Billing Code [$code] : $count \n";
}

# note that skip_zero is no longer necessary.

##</code><code>##

        # the original code indicates there can never
        # be more than one charge code associated with
        # any given $src and $dest pair; if there are,
        # we would have to use an array ref here and
        # a foreach loop in the main processing.

        # only one charge code:
        $charge_codes{$src}{$dest} = $code;

        # # for multiple charge codes:
        # push @{ $charge_codes{$src}{$dest} }, $code;

##</code><code>##

    # # if you know that there is no charge code 0 or
    # # empty string, you could compress that like so:
    # if ( my $code = $charge_codes{$src}{$dest} )
    # {
    #     ++$charges_against{$code};
    # }

    # # if you need multiple codes per src/dest pair:
    # my $charge_code_aref = $charge_codes{$src}{$dest}
    #   or next;
    # # increment the number of charges against each code
    # foreach my $code ( @$charge_code_aref )
    # {
    #     ++$charges_against{$code};
    # }

##</code><code>##

# construct the template in a more self-documenting way.
# this is a list of name => width values.  you can add
# more info here if you like; just adjust the loops
# that pull information out of it.  having all this
# information in one place means that you only have to
# change one place...

my @INPUT_COLUMN_INFO =
  ( unknown1 => { width => 12 },
    unknown2 => { width =>  4 },
    source   => { width => 20 },
    dest     => { width => 20 },
    ... );

# you can also use this structure to do clever things
# such as building a hash out of each input line to
# extract values by name instead of by numeric index.

# =================================================
# derived constants

my @INPUT_NAMES;    # names in original order
my $INPUT_TEMPLATE; # build up an 'unpack' template
my %INPUT_POS;      # map from name to index

for ( my $i = 0; $i < @INPUT_COLUMN_INFO; $i += 2 )
{
    my ( $name, $info ) = @INPUT_COLUMN_INFO[$i, $i+1];
    push @INPUT_NAMES, $name;
    $INPUT_TEMPLATE .= "A" . $info->{width};
    $INPUT_POS{$name} = @INPUT_NAMES-1;
}

# =================================================
# use during processing

while (<>)
{
    # construct hash using slice
    my %input;
    @input{@INPUT_NAMES} = unpack $INPUT_TEMPLATE, $_;

    # now access info from the hash
    my $src  = $input{source};
    my $dest = $input{dest};

    # ...
}