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

my   $EmployeeDataFilename = 'emp.dat';
my   %EmployeeInfo = ();

# -----[ Initialize ]-----------------------------------------------------------
# We cold have also demonstrated how to code up the population of a hash from
# its declarative statement above, but I felt this would make more sense to
# someone new to Perl:
# ------------------------------------------------------------------------------
{
     $EmployeeInfo{'A001'}{'FIRSTNAME'} = 'George';
     $EmployeeInfo{'A001'}{'LASTNAME'} = 'Burns';
     $EmployeeInfo{'A001'}{'AGE'} = 99;

     $EmployeeInfo{'A002'}{'FIRSTNAME'} = 'Luke';
     $EmployeeInfo{'A002'}{'LASTNAME'} = 'Skywalker';
     $EmployeeInfo{'A002'}{'AGE'} = 21;
}

# -----[ Process ]--------------------------------------------------------------
{
     &readEmployeeFile();

     # Modify one of the elements -- pretend it's George's birthday.
     $EmployeeInfo{'A001'}{'AGE'}++;

     &writeEmployeeFile();
}

exit;

sub readEmployeeFile
{
     # --------------------------------------------------------------------------
     # For the simple version, we will simply load the entire file into a hash
     # each time, then write the whole thing back out each time.
     #
     # This is NOT efficient, will NOT scale well, and is generally a poor design
     # for a production product.
     #
     # It does, however, have the advantage of demonstrating a number of
     # necessary techniques for writing a better system without cluttering up the
     # process with record management logic.
     # --------------------------------------------------------------------------
     if (!open EMPFIL, '<', $EmployeeDataFilename)
     {
          print "Cannot open input file \"$EmployeeDataFilename\"\n";
     }
     else
     {
          # File sucessfully opened.  Read all employee data and store in the hash
          while (my $empbuf = <EMPFIL>)
          {
               chomp $empbuf;

               my ($employeeID, $lastName, $firstName, $age, $garbage) = split /\t/, $empbuf, 4;
               my $empkey = uc $employeeID;
               $EmployeeInfo{$empkey}{'LASTNAME'} = $lastName;
               $EmployeeInfo{$empkey}{'FIRSTNAME'}     = $firstName;
               $EmployeeInfo{$empkey}{'AGE'}      = $age;
          }
          close EMPFIL;
     }
}

sub writeEmployeeFile
{
     # --------------------------------------------------------------------------
     # For the simple version, we will simply rewrite the entire file each time
     # Doing otherwise requires either use of deprecated functionality, modules,
     #     or more complex coding techniques which exceed the initial scope of
     #     this example.
     # --------------------------------------------------------------------------
     if (!open EMPFIL, '>', $EmployeeDataFilename)
     {
          print "Cannot open output file \"$EmployeeDataFilename\"\n";
     }
     else
     {
          # File sucessfully opened.  Write each employee data to the file
          foreach my $empkey (sort keys %EmployeeInfo)
          {
               print "Storing employee ID $empkey\n";
               # ------------------------------------------------------------------
               # First convert the hash data to a tab-separated values format
               # NOTE:  This requires that the data contains no tabs or that a way
               #        to encapsulate tabs is taken into consideration.  To keep
               #        this example simple, we will intentionally make the poor
               #        engineering choice of presuming the data to have no tabs.
               # ------------------------------------------------------------------
               my $tsvdat = &makeTsvData(
                    $empkey,
                    $EmployeeInfo{$empkey}{'LASTNAME'},
                    $EmployeeInfo{$empkey}{'FIRSTNAME'},
                    $EmployeeInfo{$empkey}{'AGE'}
               );
               # Now converted to a single line of text, write the data to the file
               print EMPFIL "$tsvdat\n";
          }
          close EMPFIL;
     }
}

# -----[ makeTsvData() ]--------------------------------------------------------
# This is a largely-useless subroutine, but it could be moulded into something
# very useful.  Right now it presumes exactly four parameters, knows what they
# are supposed to be, and the only service it really provides is converting any
# undef values to blank or zero depending on whether it is intended as string or
# numeric.  The crux of the routine is the join statement, which could simply
# have been done in the main code, but I wanted to demonstrate the ability to
# use a subroutine to perform more complex data validation and manipulation
# without muddying up your main script.  This is kept deliberately simple as it
# is meant to be an example to inspire other uses and to demonstrate the
# technique; in the real world, I would have made this routine much more generic
# and much more robust.
# ------------------------------------------------------------------------------

sub makeTsvData
{
     my ($employeeID, $lastName, $firstName, $age) = @_;
     # Cleans the input
     if (!defined $employeeID)     { $employeeID = ''; }
     if (!defined $lastName)       { $lastName = ''; }
     if (!defined $firstName) { $firstName = ''; }
     if (!defined $age)            { $age = 0; }

     my $tsvData = join "\t", $employeeID, $lastName, $firstName, $age;

     return $tsvData;
}

__END__

##</code><code>##

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

my   $EmployeeDataFilename = 'emp.dat';
my   %EmployeeInfo = ();

{
     if (!open EMPFIL, '<', $EmployeeDataFilename)
     {
          print "Cannot open input file \"$EmployeeDataFilename\"\n";
     }
     else
     {
          # File sucessfully opened.  Read all employee data and store in the hash
          while (my $empbuf = <EMPFIL>)
          {
               chomp $empbuf;

               my ($employeeID, $lastName, $firstName, $age, $garbage) = split /\t/, $empbuf, 4;
               my $empkey = uc $employeeID;
               $EmployeeInfo{$empkey}{'LASTNAME'} = $lastName;
               $EmployeeInfo{$empkey}{'FIRSTNAME'}     = $firstName;
               $EmployeeInfo{$empkey}{'AGE'}      = $age;

               print "\n";
               print "Employee ID:  $employeeID\n";
               print "    Name:  $firstName $lastName\n";
               print "     Age:  $age\n";
          }
          close EMPFIL;
     }
}


__END__