Main Program

Enough discussion--let's look at the program:

#!/usr/local/bin/perl5
use GD;
use Sprite;
$webmaster = "Shishir Gundavaram (shishir\@bu\.edu)";
$cal = "/usr/bin/cal";

The UNIX cal utility displays a text version of the calendar. See the draw_text_calendar subroutine to see what the output of this command looks like.

$database = "/home/shishir/calendar.db";
$delimiter = "::";

The database uses the "::" string as a delimiter and contains six fields for each calendar event: ID, Month, Day, Year, Keywords, and Description. The ID field uniquely identifies an appointment based on the time of creation. The Month (numerical), Day, and Year are self-explanatory. One thing to note here is that the Year is stored as a four-digit number (i.e., 1995, not 95).

The Keywords field is a short description of the appointment. This is what is displayed on the graphic calendar. And finally, the Description field should contain a more lengthy explanation regarding the appointment. Here is the format for a typical appointment file:

ID::Month::Day::Year::Keywords::Description 
796421318::11::02::1995::See Professor::It is important that I see the professor
806421529::11::03::1995::ABC Enterprises::Meet Drs. Bird and McHale about job!!
805762393::11::03::1995::Luncheon Meeting::Travel associates

Now to create and manipulate the data:

($current_month, $current_year) = (localtime(time))[4,5];
$current_month += 1;
$current_year += 1900;

These three statements determine the current month and year. Remember, the month number, as returned by localtime, is zero-based (0-11, instead of 1-12). And the year is returned as a two-digit number (95, instead of 1995).

$action_types = '^(add|delete|modify|search)$';
$delete_password = "CGI Super Source";

The $action_types variable consists of four options that the user can select from the Calendar Manager. The user is asked for a password when the delete option is chosen. Replace this with a password of your choice.

&check_database ();
&parse_query_and_form_data (*CALENDAR); 

The check_database subroutine checks for the existence of the calendar database. The database is created if it does not already exist. The parse_query_and_form_data subroutine is called to parse all information from the Calendar Manager, handling both POST and GET queries. As in so many other examples, an associative array proves useful, so that's what CALENDAR is.

$action = $CALENDAR{'action'};
$month = $CALENDAR{'month'};
($temp_month, $temp_year) = split ("/", $month, 2); 

The action and month fields are stored in variables. The month and year are split from the month field. As you saw near the beginning of this section, the month field has a format like 11/1995.

if ( ($temp_month =~ /^\d{1,2}$/) && ($temp_year =~ /^\d{4}$/) ) {
    if ( ($temp_month >= 1) && ($temp_month <= 12) ) {
        $current_month = $temp_month;
        $current_year = $temp_year;
    }
}

If the month and year values as specified in the query string are valid numbers, they are stored in $current_month and $current_year. Otherwise, these variables will reflect the current month and year (as defined above). One feature of this program is that it remembers the month that the user most recently clicked or entered in a search form. The month chosen by the user is stored in $current_month so that it becomes the default for future searches.

@month_names = ('January', 'February', 'March', 'April', 'May', 'June',  'July', 'August', 'September', 'October', 'November',  'December');
$weekday_names = "Sun,Mon,Tue,Wed,Thu,Fri,Sat";
$current_month_name = $month_names[$current_month - 1];
$current_month_year = join ("/", $current_month, $current_year);

The $current_month_name variable contains the full name of the specified month. $current_month_year is a string containing the month and year (e.g.,"11/1995").

This completes the initialization. Remember that the program is called afresh each time the user submits a form or clicks on a date, so it runs through this initialization again and potentially changes the current month. But now it is time to handle the action that the user passed in the query.

if ($action eq "full") {
    &display_year_calendar ();

If the user passed the full field, display_year_calendar is called to display the full year calendar.

} elsif ($action eq "view") {
    $date = $CALENDAR{'date'};
    &display_all_appointments ($date);

If the user selects to view the appointments for a certain date, the display_all_appointments routine displays all of the appointments for that date.

} elsif ($action =~ /$action_types/) {
    $type = $CALENDAR{'type'};
    if ($type eq "form") {
        $dynamic_sub = "display_${action}_form";
        &$dynamic_sub ();
    } elsif ($type eq "execute") {
        $dynamic_sub = "${action}_appointment";
        &$dynamic_sub ();
    } else {
        &return_error (500, "Calendar Manager", "An invalid query was passed!");
    }

If the action field contains one of the four actions defined near the beginning of the program, the appropriate subroutine is executed. This is an example of a dynamic subroutine call. For example, if the action is "add" and the type is "form," the $dynamic_sub variable will call the display_add_form subroutine. This is much more compact than to conditionally compare all possible values.

} else {
    &display_month_calendar ();
}
exit (0);

If no query is passed (or the query does not match the ones above), the display_month_calendar subroutine is called to output the current calendar in the appropriate format, either as a graphic imagemap or as plain text.

The Database

In the rest of this chapter I'm going to explain the various subroutines that set and retrieve data, create a display, and parse input. We'll start with some database functions. You'll also find incidental routines here, which I've written as conveniences because their functions appear so often.

The following subroutine checks to see if the calendar database exists. If not, we create one. This job is simple, since we're using a flat file with Sprite as an interface: we just open a file with the desired name and write a one-line header.

sub check_database
{
        local ($exclusive_lock, $unlock, $header);
        $exclusive_lock = 2;
        $unlock = 8;
        if (! (-e $database) ) {
                if ( open (DATABASE, ">" . $database) ) {
                        flock (DATABASE, $exclusive_lock);
                        $header = join ($delimiter, "ID", "Month", "Day",
                                        "Year", "Keywords", "Description");
                        print DATABASE $header, "\n";
                        flock (DATABASE, $unlock);
                        close (DATABASE);
                } else {
                        &return_error (500, "Calendar Manager",
                                "Cannot create new calendar database.");
                }
        }
}

If the database does not exist, a header line is output:

ID::Month::Day::Year::Keywords::Description 

The following subroutine just returns an error; it is defined for convenience and used in open_database.

sub Sprite_error
{
    &return_error (500, "Calendar Manager",
          "Sprite Database Error. Check the server log file.");
}

The open_database subroutine passes an SQL statement to the Sprite database.

sub open_database
{
    local (*INFO, $command, $rdb_query) = @_;
    local ($rdb, $status, $no_matches);

This subroutine accepts three arguments: a reference to an array, the SQL command name, and the actual query to execute. A typical call to the subroutine looks like:

    &open_database (undef, "insert", <<End_of_Insert);
insert into $database
   (ID, Day, Month, Year, Keywords, Description)
values
   ($time, $date, $current_month, $current_year, '$keywords', '$description')
End_of_Insert

The third argument looks strange because it's telling the subroutine to read the query on the following lines. In other words, the SQL query lies between the call to open_database and the text on the closing line, End_of_Insert. The effect is to insert a new appointment containing information passed by the user. Remember, we would also have to escape single and double quotes in the field values.

    $rdb = new Sprite ();
    $rdb->set_delimiter ("Read",  $delimiter);
    $rdb->set_delimiter ("Write", $delimiter);

This creates a new Sprite database object, and sets the read and write delimiters to the value stored in $delimiter (in this case, "::").

    if ($command eq "select") {
        @INFO = $rdb->sql ($rdb_query);
        $status = shift (@INFO);
        $no_matches = scalar (@INFO);
        $rdb->close ();

If the user passed a select command, the query is executed with the sql method (in object-oriented programming, "method" is a glorified term for a subroutine). We treat the select commands separately from other commands because it doesn't change the database, but just returns data. All other commands modify the database.

The INFO array contains the status of the request (success or failure) in its first element, followed by other elements containing the records that matched the specified criteria. The status and the number of matches are stored.

        if (!$status) {
            &Sprite_error ();
        } else {
            return ($no_matches);
        }

If the status is zero, the Sprite_error subroutine is called to output an error. Otherwise, the number of matches is returned.

    } else {
        $rdb->sql ($rdb_query) || &Sprite_error ();
        $rdb->close ($database);
    }
}

If the user passes a command other than select (in other words, a command that modifies the database), the program executes it and saves the resulting database.

Now, we will look at three very simple subroutines that output the header, the footer, and the "Location:" HTTP header, respectively.

sub print_header
{
    local ($title, $header) = @_;
    print "Content-type: text/html", "\n\n";
        print "<HTML>", "\n";
    print "<HEAD><TITLE>", $title, "</TITLE></HEAD>", "\n";
        print "<BODY>", "\n";
    $header = $title unless ($header);
    print "<H1>", $header, "</H1>", "\n";
    print "<HR>", "\n";
}

The print_header subroutine accepts two arguments: the title and the header. If no header is specified, the title of the document is used as the header.

The next subroutine outputs a plain footer. It is used at the end of forms and displays.

sub print_footer
{
    print "<HR>", "\n";
    print "<ADDRESS>", $webmaster, "</ADDRESS>", "\n";
        print "</BODY></HTML>", "\n";
}

Finally, the Location: header, which we described in Chapter 3, is output by the print_location subroutine after an add, delete, or modify request. By passing a URL in the Location: header, we make the server re-execute the program so that the user sees an initial Calendar page again.

sub print_location
{
    local ($location_URL);
    $location_URL = join ("", $ENV{'SCRIPT_NAME'},                  "?",
                              "browser=", $ENV{'HTTP_USER_AGENT'},  "&",
                              "month=", $current_month_year);
    print "Location: ", $location_URL, "\n\n";
}

This is a very important subroutine, though it may look very simple. The subroutine outputs the Location: HTTP header with a query string that contains the browser name and the specified month and year. The reason we need to supply the browser name is that the HTTP_USER_AGENT environment variable does not get set when there is a URL redirection. When the server gets this script and executes it, it does not set the HTTP_USER_AGENT variable. So this program will not know the user's browser type unless we include the information.

Forms and Displays

In this section you'll find subroutines that figure out what the user has asked for and display the proper output. All searches, additions, and so forth take place here. Usually, a database operation takes place in two steps: one subroutine displays a form, while another accepts input from the form and accesses the database.

Let's start out with display_year_calendar, which displays the full year calendar.

sub display_year_calendar
{
    local (@full_year);
    @full_year = `$cal $current_year`;

If the cal command is specified without a month number, a full year is displayed. The `backtics` execute the command and store the output in the specified variable. Since the variable $current_year can be based on the month field in the query string, it is important to check to see that it does not contain any shell metacharacters. What if some user passed the following query to this program?

http://some.machine/cgi-bin/calendar.pl?action=full&month=11/1995;rm%20-fr%20/

It can be quite dangerous! You might be wondering where we are checking for shell metacharacters. Look back at the beginning of this program, where we made sure that the month and year are decimal numbers.

The output from cal is stored in the @full_year array, one line per element. Now we trim the output.

    @full_year = @full_year[5..$#full_year-3];

The first four and last three lines from the output are discarded, as they contain extra newline characters. The array will contain information in the following format:

                                1995
         Jan                    Feb                    Mar
 S  M Tu  W Th  F  S    S  M Tu  W Th  F  S    S  M Tu  W Th  F  S
 1  2  3  4  5  6  7             1  2  3  4             1  2  3  4
 8  9 10 11 12 13 14    5  6  7  8  9 10 11    5  6  7  8  9 10 11
15 16 17 18 19 20 21   12 13 14 15 16 17 18   12 13 14 15 16 17 18
22 23 24 25 26 27 28   19 20 21 22 23 24 25   19 20 21 22 23 24 25
29 30 31               26 27 28               26 27 28 29 30 31
.
.
.

Let's move on.

    grep (s|(\w{3})|<B>$1</B>|g, @full_year);

This might look like some deep magic. But it is actually quite a simple construct. The grep iterates through each line of the array, and adds the <B>..</B> tags to strings that are three characters long. In this case, the strings correspond to the month names. This one line statement is equivalent to the following:

foreach (@full_year) {
    s|(\w{3})|<B>$1</B>|g;
}

Now, here is the rest of this subroutine, which simply outputs the calendar.

    &print_header ("Calendar for $current_year");
    print "<PRE>", @full_year, "</PRE>", "\n";
    &print_footer ();
}

The following subroutine displays the search form. It is pretty straightforward. The only dynamic information in this form is the query string.

sub display_search_form
{
    local ($search_URL);
    $search_URL = join ("", $ENV{'SCRIPT_NAME'},  "?",
                            "action=search",      "&",
                            "type=execute",       "&",
                            "month=", $current_month_year);

The query string sets the type field to execute, which means that this program will call the search_appointment subroutine to search the database when this form is submitted. The month and year are also set; this information is passed back and forth between all the forms, so that the user can safely view and modify the calendars for months other than the current month.

    &print_header ("Calendar Search");
    print <<End_of_Search_Form;
This form allows you to search the calendar database for certain information. The Keywords and Description fields are searched for the string you enter.
<P>
<FORM ACTION="$search_URL" METHOD="POST"> Enter the string you would like to search for: <P>
<INPUT TYPE="text" NAME="search_string" SIZE=40 MAXLENGTH=40> <P>
Please enter the <B>numerical</B> month and the year in which to search. Leaving these fields empty will default to the current month and year: <P>
<PRE>
Month: <INPUT TYPE="text" NAME="search_month" SIZE=4 MAXLENGTH=4><BR> Year: <INPUT TYPE="text" NAME="search_year" SIZE=4 MAXLENGTH=4> </PRE>
<P>
<INPUT TYPE="submit" VALUE="Search the Calendar!"> <INPUT TYPE="reset" VALUE="Clear the form"> </FORM>
End_of_Search_Form
    &print_footer ();
}

Here is the subroutine that actually performs the search:

sub search_appointment
{
    local ($search_string, $search_month, $search_year, @RESULTS,
           $matches, $loop, $day, $month, $year, $keywords,
           $description, $search_URL, $month_name);
    $search_string = $CALENDAR{'search_string'};
    $search_month = $CALENDAR{'search_month'};
    $search_year = $CALENDAR{'search_year'};

Three variables are declared to hold the form information. We could have used the information from the CALENDAR associative array directly, without declaring these variables. This is done purely for a visual effect; the code looks much neater.

    if ( ($search_month < 1) || ($search_month > 12) ) {
        $CALENDAR{'search_month'} = $search_month = $current_month;
    }

If no month number was specified, or if the month is not in the valid range, it is set to the value stored in $current_month. This value may or may not be the actual month in which the user is running the program. The user changes $current_month by specifying a search for a different month.

    if ($search_year !~ /^\d{2,4}$/) {
        $CALENDAR{'search_year'} = $search_year = $current_year;
    } elsif (length ($search_year) < 4) {
        $CALENDAR{'search_year'} = $search_year += 1900;
    }

If the year is not specified, or if it does not contain at least two digits, it is set to $current_year. And if the length of the year field is less than 4, 1900 is added.

    $search_string =~ s/(\W)/\\$1/g;
    $matches = &open_database (*RESULTS, "select", <<End_of_Select);
select Day, Month, Year, Keywords, Description from $database
where ( (Keywords =~ /$search_string/i) or         
        (Description =~  /$search_string/i) )   and
        (Month       =   $search_month)         and
        (Year        =   $search_year)
End_of_Select

The open_database subroutine is called to search the database for any records that match the specified criteria. The RESULTS array will contain the Day, Month, Year, Keywords, and Description fields for the matched records.

    unless ($matches) {
        &return_error (500, "Calendar Manager",
              "No appointments containing $search_string are found.");
    }

If there are no records that match the search information specified by the user, an error message is output.

    &print_header ("Search Results for: $search_string");
    for ($loop=0; $loop < $matches; $loop++) {
    $RESULTS[$loop] =~ s/([^\w\s\0])/sprintf ("&#%d;", ord ($1))/ge;
        ($day, $month, $year, $keywords, $description) =
                split (/\0/, $RESULTS[$loop], 5);
        $search_URL = join ("", $ENV{'SCRIPT_NAME'},  "?",
                                "action=view",        "&",
                                "date=", $day,        "&",
                                "month=", $month, "/", $year);
        $keywords = "No Keywords Specified!"   unless ($keywords);
        $description = "-- No Description --"  unless ($description);
        $description =~ s/&#60;BR&#62;/<BR>/g;
        $month_name = $month_name[$month - 1];
        print <<End_of_Appointment;
<A HREF="$search_URL">$current_month_name $day, $year</A><BR> <B>$keywords</B><BR>
$description
End_of_Appointment

The for loop iterates through the RESULTS array, and creates a hypertext link with a query string for each appointment. This will allow the user to just click the appointment to get a list of all the appointments for that date. (You may remember that, at the very beginning of this section, we showed how to retrieve appointments for a particular day by passing an action field along with date and month fields).

        print "<HR>" if ($loop < $matches - 1);
    }
    &print_footer ();
}

A horizontal rule is output after each record, except after the last one. This is because the print_footer subroutine outputs a horizontal rule as well.

Now, let's look at the form that is displayed when the "Add New Appointment!" link is selected.

sub display_add_form
{
    local ($add_URL, $date, $message);
    $date = $CALENDAR{'date'};
    $message = join ("", "Adding Appointment for ",
                         $current_month_name, " ", $date, ", ", $current_year);
    $add_URL = join ("", $ENV{'SCRIPT_NAME'},           "?",
                         "action=add",                  "&",
                         "type=execute",                "&",
                         "month=", $current_month_year, "&",
                         "date=", $date);

When the add option is selected by the user, the following query is passed to this program (see the display_all_appointments subroutine):

http://some.machine/cgi-bin/calendar.pl?action=add&type=form&month=11/1995&date=10

Before this subroutine is called, the main program sets the variables $current_month_name and so on.

This information is used to build another query string that will be passed to this program when the form is submitted.

    &print_header ("Add Appointment", $message);
    print <<End_of_Add_Form;
This form allows you to enter an appointment to be stored in the calendar database. 
To make it easier for you to search for specific appointments later on, please use descriptive words to describe an appointment. <P>
<FORM ACTION="$add_URL" METHOD="POST"> Enter a brief message (keywords) describing the appointment: <P>
<INPUT TYPE="text" NAME="add_keywords" SIZE=40 MAXLENGTH=40> <P>
Enter some comments about the appointment: <TEXTAREA ROWS=4 COLS=60 NAME="add_description"></TEXTAREA><P> <P>
<INPUT TYPE="submit" VALUE="Add Appointment!"> <INPUT TYPE="reset" VALUE="Clear Form"> </FORM>
End_of_Add_Form
    &print_footer();
}

The add_appointment subroutine adds a record to the calendar database:

sub add_appointment
{
    local ($time, $date, $keywords, $description);
    $time = time;

The $time variable contains the current time, as the number of seconds since 1970. This is used as a unique identification for the record.

    $date = $CALENDAR{'date'};
    ($keywords = $CALENDAR{'add_keywords'}) =~ s/(['"])/\\$1/g;
    ($description = $CALENDAR{'add_description'}) =~ s/\n/<BR>/g;
    $description =~ s/(['"])/\\$1/g;

All newline characters in the description field are converted to <BR>. This is because of the way the Sprite database stores records. Remember, the database is text-delimited, where each field is delimited by a certain string, and each record is terminated by a newline character.

    &open_database (undef, "insert", <<End_of_Insert);
insert into $database
   (ID, Day, Month, Year, Keywords, Description)
values
   ($time, $date, $current_month, $current_year, '$keywords', '$description')
End_of_Insert

The open_database subroutine is called to insert the record into the database. Notice the quotes around the variables $keywords and $description. These are absolutely necessary since the two variables contain string information.

    &print_location ();
}

The display_delete_form subroutine displays a form that asks for a password before an appointment can be deleted. The delete and modify options are available for each appointment. As a result, when you select one of these options, the identification of that appointment is passed to this script, so that the appropriate information can be retrieved quickly and efficiently.

sub display_delete_form
{
    local ($delete_URL, $id);
    $id = $CALENDAR{'id'};
    $delete_URL = join ("", $ENV{'SCRIPT_NAME'}, "?",
                            "action=delete",     "&",
                            "type=execute",      "&",
                            "id=", $id,          "&",
                            "month=", $current_month_year);

When the user selects the delete option in the calendar, the following query is passed to this script:

http://some.machine/cgi-bin/calendar.pl?action=delete&type=form&month=11/ 1995&id=806421529

This query information is used to construct another query that will be passed to this program when the form is submitted.

    &print_header ("Deleting appointment");
    print <<End_of_Delete_Form;

In order to delete calendar entries, you need to enter a valid identification code (or password):

<HR>
<FORM ACTION="$delete_URL" METHOD="POST">
<INPUT TYPE="password" NAME="code" SIZE=40> <P>
<INPUT TYPE="submit" VALUE="Delete Entry!">
<INPUT TYPE="reset" VALUE="Clear the form"> </FORM>
End_of_Delete_Form
    &print_footer ();
}

The following subroutine checks the password that is entered by the user. If the password is valid, the appointment is deleted, and a server redirect is performed, so that the calendar is displayed.

sub delete_appointment
{
    local ($password, $id);
    $password = $CALENDAR{'code'};
    $id = $CALENDAR{'id'};
    if ($password ne $delete_password) {
        &return_error (500, "Calendar Manager",
                            "The password you entered is not valid!");
    } else {
        &open_database (undef, "delete", <<End_of_Delete);
delete from $database
where (ID = $id)
End_of_Delete
    }
    &print_location ();
}

If the password is valid, the record identified by the unique time is deleted from the database. Otherwise, an error message is output.

The display_modify_form subroutine outputs a form that contains the information about the record to be modified. This information is retrieved from the database with the help of the query information that is passed to this script:

http://some.machine/cgi-bin/calendar.pl?action=modify&type=form&month=11/ 1995&id=806421529

Here is the subroutine:

sub display_modify_form
{
    local ($id, $matches, @RESULTS, $keywords, $description, $modify_URL);
    $id = $CALENDAR{'id'};
    $matches = &open_database (*RESULTS, "select", <<End_of_Select);
select Keywords, Description from $database
where (ID = $id)
End_of_Select
    unless ($matches) {
        &return_error (500, "Calendar Manager",
          "Oops! The appointment that you selected no longer exists!");
    }

The identification number is used to retrieve the Keywords and Description fields from the database. If there are no matches, an error message is output. This will happen only if the Calendar Manager is being used by multiple users, and one of them deletes the record pointed to by the identification number.

    ($keywords, $description) = split (/\0/, shift (@RESULTS), 2);
    $keywords = &escape_html ($keywords);
    $description =~ s/<BR>/\n/g;

The appointment keywords and description are obtained from the results. We call the escape_html subroutine to escape certain characters that have a special significance to the browser, and we also convert the <BR> tags in the description back to newlines, so that the user can modify the description.

    $modify_URL = join ("", $ENV{'SCRIPT_NAME'}, "?",
                            "action=modify",     "&",
                            "type=execute",      "&",
                            "id=", $id,          "&",
                            "month=", $current_month_year);
    &print_header ("Modify Form");
    print <<End_of_Modify_Form;
This form allows you to modify the <B>description</B> field for an existing appointment in the calendar database. <P>
<FORM ACTION="$modify_URL" METHOD="POST"> Enter a brief message (keywords) describing the appointment: <P>
<INPUT TYPE="text" NAME="modify_keywords" SIZE=40        VALUE="$keywords" MAXLENGTH=40>
<P>
Enter some comments about the appointment: <TEXTAREA ROWS=4 COLS=60 NAME="modify_description"> $description
</TEXTAREA><P>
<P>
<INPUT TYPE="submit" VALUE="Modify Appointment!"> <INPUT TYPE="reset" VALUE="Clear Form"> </FORM>
End_of_Modify_Form
    &print_footer ();
}

The form containing the values of the selected appointment is displayed. Only the keywords and description fields can be modified by the user. The escape_html subroutine escapes characters in a specified string to prevent the browser from interpreting them.

sub escape_html
{
    local ($string) = @_;
    local (%html_chars, $html_string);
    %html_chars = ('&', '&',
                   '>', '>',
                   '<', '&lt;',
                   '"', '&quot;');
    $html_string = join ("", keys %html_chars);
    $string =~ s/([$html_string])/$html_chars{$1}/go;
    return ($string);
}

The modify_appointment subroutine modifies the information in the database.

sub modify_appointment
{
    local ($modify_description, $id);
    ($modify_description = $CALENDAR{'modify_description'}) =~ s/(['"])/\\$1/g;
    $id = $CALENDAR{'id'};
    &open_database (undef, "update", <<End_of_Update);
update $database
set Description = ('$modify_description') where (ID = $id)
End_of_Update
    &print_location ();
}

The update SQL command modifies the description for the record in the calendar database. Then a server redirect is performed.

The imagemap display

Now let's change gears and discuss some of the more complicated subroutines, the first one being display_month_calendar. This subroutine either draws a calendar, or interprets the coordinates clicked by the user. Because we're trying to do a lot with this subroutine (and run it in several different situations), don't be surprised to find it rather complicated. There are three things the subroutine can do:

• In the simplest case, this subroutine is called when no coordinate information has been passed to the program. It then creates a calendar covering a one-month display. The output_HTML routine is called to do this (assuming that the user has a graphics browser).

• If coordinate information is passed, the subroutine figures out which date the user clicked and displays the appointments for that date, using the display_all_appointments subroutine.

• Finally, if the user has a non-graphics browser, draw_text_calendar is called to create the one-month display. This display contains hypertext links to simulate the functions that an imagemap performs in the graphics version.

But more subtleties lie in the interaction between the subroutines. In order to generate a calendar for a particular month requested by the user, I have the program invoke itself in a somewhat complex way.

Let me start with our task here: to create an image dynamically. Most CGI programmers create a GIF image, store it in a file, and then create an imagemap based on that temporary file. This is inefficient and involves storing information in temporary files. What I do instead is shown in Figure 11.6.

The program is invoked for the first time, and calls output_HTML. This routine sends the browser some HTML that looks like this:

<A HREF="/cgi-bin/calendar.pl/11/1995">
<IMG SRC="/cgi-bin/calendar.pl?month=11/1995&draw_imagemap" ISMAP></A> 

Embedding an <IMG> tag in an <A> tag is a very common practice--an image with a hypertext link. But in most <IMG> tags, the SRC attribute points to a .gif file. Here, instead, it points back to our program.

So what happens when the browser displays the HTML? It sends a request back to the server for the image, and the server runs this program all over again. (As I said before, the program invokes itself.) This time, an image of a calendar is returned, and the browser happily completes the display.

You may feel that I'm playing games with HTML here, but it's all very legitimate and compatible with the way a web client and server work. And there's no need for temporary files with the resulting delays and cleanup.

Let me explain one more detail before we launch into the code. The decision about whether to display a calendar is determined by a field in the <IMG> tag you saw, the draw_imagemap field. When this field is passed, the program creates an image of a calendar. When the field is not passed, output_HTML is called. So we have to run the program once without draw_imagemap, let it call output_HTML, and have that subroutine run the program again with draw_imagemap set.

Once you understand the basic logic of the program, the display_month_calendar subroutine should be fairly easy to follow.

sub display_month_calendar
{
    local ($nongraphic_browsers, $client_browser, $clicked_point,
           $draw_imagemap, $image_date);
    $nongraphic_browsers = 'Lynx|CERN-LineMode';
    $client_browser = $ENV{'HTTP_USER_AGENT'} || $CALENDAR{'browser'};

We need to know whether the client is using a browser that displays graphics. Normally the name of the browser is passed in the HTTP_USER_AGENT environment variable, but it is not set if a program is executed as a result of server redirection. In that case, we can find out the browser through the query information, where we thoughtfully set a browser field earlier in the program. The line setting $client_browser is equivalent to:

if ($ENV{'HTTP_USER_AGENT'}) {
    $client_browser = $ENV{'HTTP_USER_AGENT'};
} else {
    $client_browser = $CALENDAR{'browser'};
}

The following code checks to see if a graphic browser is being used, and displays output in the appropriate format.

    if ($client_browser =~ /$nongraphic_browsers/) {
        &draw_text_calendar ();

For text browsers, the draw_text_calendar subroutine formats the information from the cal command and displays it.

    } else {
        $clicked_point = $CALENDAR{'clicked_point'};
        $draw_imagemap = $CALENDAR{'draw_imagemap'};

When the program is executed initially, the clicked_point and the draw_imagemap fields are null. As we'll see in a moment, this causes us to execute the output_HTML subroutine.

        if ($clicked_point) {
            $image_date = &get_imagemap_date ();
            &display_all_appointments ($image_date);

If the user clicks on the image, this program stores the coordinates in the variable $CALENDAR{`clicked_point'}. The get_imagemap_date subroutine returns the date corresponding to the clicked region. Finally, the display_all_appointments subroutine displays all the appointments for the selected date.

        } elsif ($draw_imagemap) {
            &draw_graphic_calendar ();

When draw_imagemap is set (because of the complicated sequence of events I explained earlier), the draw_graphic_calendar subroutine is executed and outputs the image of the calendar.

        } else {
            &output_HTML ();
        }
        }
}

In this else block, we know that we are running a graphics browser but that neither $clicked_point nor $draw_imagemap were set. That means we are processing the initial request, and have to call output_HTML to create the first image.

When displaying the current calendar, this program provides two hypertext links (back to this program) that allow the user to view the calendar for a month ahead or for the past month. The next subroutine returns these links.

sub get_next_and_previous
{
    local ($next_month, $next_year, $previous_month, $previous_year,
           $arrow_URL, $next_month_year, $previous_month_year);
    $next_month = $current_month + 1;
    $previous_month = $current_month - 1;
    if ($next_month > 12) {
        $next_month = 1;
        $next_year = $current_year + 1;
    } else {
        $next_year = $current_year;
    }
    if ($previous_month < 1) {
        $previous_month = 12;
        $previous_year = $current_year - 1;
    } else {
        $previous_year = $current_year;
    }

If the month number is either at the low or the high limit, the year is incremented or decremented accordingly.

    $arrow_URL = join ("", $ENV{'SCRIPT_NAME'},  "?",
                           "action=change",      "&",
                           "month=");
    $next_month_year = join ("", $arrow_URL, $next_month, "/", $next_year);
    $previous_month_year = join ("", $arrow_URL,
                                     $previous_month, "/", $previous_year);
    return ($next_month_year, $previous_month_year);
}

The two URLs returned by this subroutine are in the following format (assuming 12/1995 is the selected month):

http://some.machine/cgi-bin/calendar.pl?action=change&month=1/1996

and

http://some.machine/cgi-bin/calendar.pl?action=change&month=11/1995

Now, let's look at the subroutine that is executed initially, which displays the title and header for the document as well as an <IMG> tag that refers back to this script to create a graphic calendar.

sub output_HTML
{
    local ($script, $arrow_URL, $next, $previous, $left, $right);
    $script = $ENV{'SCRIPT_NAME'};
    ($next, $previous) = &get_next_and_previous ();
    $left  = qq|<A HREF="$previous"><IMG SRC="/icons/left.gif"></A>|;
    $right = qq|<A HREF="$next"><IMG SRC="/icons/right.gif"></A>|;
    &print_header
        ("Calendar for $current_month_name $current_year",
         "$left Calendar for $current_month_name $current_year $right");

The two links for the next and previous calendars are embedded in the document's header.

    print <<End_of_HTML;
<A HREF="$script/$current_month_year"> 
<IMG SRC="$script?month=$current_month_year&draw_imagemap" ISMAP></A> 

I described this construct earlier; it creates an imagemap with a hypertext link that runs this script. There are interesting subtleties in both the HREF attribute and the SRC attribute.

The HREF attribute includes the selected month and year (e.g., "11/1995") as path information. That's because we need some way to get this information back to the program when the user clicks on the calendar. The imagemap uses the GET method (so we cannot use the input stream) and passes only the x and y coordinates of the mouse as query information. So the only other option left open to us is to include the month and year as path information.

The SRC attribute, as we said before, causes the whole program to run again. Thanks to the draw_imagemap field, a calendar is drawn.

<HR>
<A HREF="$script?action=full&year=$current_year">Full Year Calendar</A> <BR>
<A HREF="$script?action=search&type=form&month=$current_month_year">Search</A> 
End_of_HTML
    &print_footer ();
}

The main calendar screen contains two links: one to display the full year calendar, and another one to search the database.

Let's look at the subroutine that draws a text calendar. I have no chance to indulge in fancy image manipulation here. Instead, I format the days of the month in rows and provide a hypertext link for each day.

sub draw_text_calendar
{
    local (@calendar, $big_line, $matches, @RESULTS, $header, $first_line,
           $no_spaces, $spaces, $loop, $date, @status, $script, $date_URL,
           $next, $previous);
    @calendar = `$cal $current_month $current_year`;
    shift (@calendar);
    $big_line = join ("", @calendar);

The calendar for the selected month is stored in an array. Here is what the output of the cal command looks like:

   November 1995
 S  M Tu  W Th  F  S
          1  2  3  4
 5  6  7  8  9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25
26 27 28 29 30

The first line of the output is removed, as we do not need it. Then the whole array is joined together to create one large string. This makes it easier to manipulate the information, rather than trying to modify different elements of the array.

    $matches = &open_database (*RESULTS, "select", <<End_of_Select);
select Day from $database
where (Month = $current_month) and
(Year  = $current_year)
End_of_Select

The RESULTS array consists of the Day field for all the appointments in the selected month. This array is used to highlight the appropriate dates on the calendar.

    &print_header ("Calendar for $current_month_name $current_year");
    $big_line =~ s/\b(\w{1,2})\b/$1    /g;
    $big_line =~ s/\n/\n\n/g;

These two statements expand the space between strings that are either one or two characters, and add an extra newline character. The regular expression is illustrated below.

Here is the what the output looks like after these two statements:

 S      M     Tu      W     Th      F      S
          1      2      3      4
 5      6      7      8      9     10     11
12     13     14     15     16     17     18
19     20     21     22     23     24     25
26     27     28     29     30

Because of the leading spaces before the "1," the alignment is off. This can be corrected by taking the difference in length between the line that contains the day names and the first line (without the leading spaces), and adding that number of spaces to align it properly. We do this in the somewhat inelegant code below.

    ($header) = $big_line =~ /( S.*)/;
    $big_line =~ s/ *(1.*)/$1/;
    ($first_line) = $big_line =~ //;
    $no_spaces = length ($header) - length ($first_line);
    $spaces = " " x $no_spaces;
    $big_line =~ s/\b1\b/${spaces}1/;

While the technique I've used here is not a critical part of the program, I'll explain it because it provides an interesting instance of text manipulation. Remember that $big_line contains several lines. Through regular expressions we are extracting two lines: one with names of days of the week in $header, and another with the first line of dates in $first_line. We then compare the lengths of these two lines to make them flush right.

The regular expression /( S.*)/ picks out the cal output's header, which is a line containing a space followed by an S for Sun. This whole line is stored in $header.

In the next two lines of code, we strip all the spaces from the beginning of the first week of the calendar and store the rest of the week in $first_line. The regular expression contains a space followed by an asterisk in order to remove all spaces. The (1.*) and $1 select the date 1 and all the other dates up to the end of the same line. In the next code statement, the // construct means "whatever was matched last in a regular expression." Since the last match was $1, $first_line contains a line of dates starting with 1.

Then, using length commands, we determine how many spaces we need to make the first week flush right with the header. The x command creates the number of spaces we need. Finally we put that number of spaces before the 1 on the first line.

    for ($loop=0; $loop < $matches; $loop++) {
        $date = $RESULTS[$loop];
            unless ($status[$date]) {
                $big_line =~ s|\b$date\b {0,1}|$date\*|;
            $status[$date] = 1;
        }
    }

This loop iterates through the RESULTS array, which we loaded through an SQL select command earlier in this subroutine. Each element of RESULTS is a date on which an appointment has been scheduled. For each of these dates, we search the cal output and add an asterisk ("*").

The substitute command deserves a little examination:

    s|\b$date\b {0,1}|$date\*|

Essentially, we want to replace the space that follows the date with an asterisk (\*). But the date may not be followed by a space. If it's at the end of the line (that is, if it falls on a Saturday) there will be no following space, and we want to just append the asterisk.

The {0,1} construct handles both cases. It means that $date must be followed by zero or one spaces. If there is a space, it's treated as part of the string and stripped off. If there is no space, that's fine too, because $date is still found and the asterisk is appended.

Here is what the output will look like (assuming there are appointments on the 5th, 8th, and 10th):

 S      M     Tu      W     Th      F      S
                      1      2      3      4
 5*     6      7      8*     9     10*    11
12     13     14     15     16     17     18
19     20     21     22     23     24     25
26     27     28     29     30

And that is what the calendar will look like in a text browser. But we still want to provide the same access that a graphic calendar does. The user must be able to select a date and view, add, or modify appointments. So now we turn each date in the calendar into a hypertext link.

    $script = $ENV{'SCRIPT_NAME'};
    $date_URL = join ("", $script,        "?",
                          "action=view",  "&",
                          "month=", $current_month_year);
    $big_line =~ s|\b(\d{1,2})\b|<A HREF="$date_URL&date=$1">$1</A>|g;

Below is the regular expression that we're searching for in the last line of the preceding code. It defines a date as one or two digits surrounded by word boundaries. (Spaces are recognized as word boundaries, and so are the beginnings and ends of lines.) We add <A> and </A> tags around the date. The URL in each A tag includes the name of this script, an action=view tag, the current month, and the particular date chosen.

Let's continue with the subroutine:

    ($next, $previous) = &get_next_and_previous ();
    print <<End_of_Output;
<UL>
<LI><A HREF="$previous">Previous Month!</A></LI> <LI><A HREF="$next">Next Month!</A></LI> </UL>
<PRE>
$big_line
</PRE>
<HR>
<A HREF="$script?action=full&year=$current_year">Full Year Calendar</A> <BR>
<A HREF="$script?action=search&type=form&month=$current_month_year">Search</A> 
End_of_Output
    &print_footer ();
}

Four final links are displayed: two to allow the user to view the last or next month calendar, one to display the full year calendar, and one to search the database for information contained within appointments.

The display_all_appointments subroutine displays all of the appointments for a given date. It is invoked by clicking a region of the graphic calendar or by following a link on the text calendar.

sub display_all_appointments
{
    local ($date) = @_;
    local ($script, $matches, @RESULTS, $loop, $id, $keywords,
           $description, $display_URL);
    $matches = &open_database (*RESULTS, "select", <<End_of_Select);
select ID, Keywords, Description from $database 
where (Month = $current_month) and    
      (Year  = $current_year)  and
      (Day   = $date)
End_of_Select

The SQL statement retrieves the ID, Keywords, and Description for each appointment that falls on the specified date.

    &print_header ("Appointments",
          "Appointments for $current_month_name $date, $current_year");
    $display_URL = join ("", $ENV{'SCRIPT_NAME'},  "?",
                             "type=form",          "&",
                             "month=", $current_month_year);
    if ($matches) {
        for ($loop=0; $loop < $matches; $loop++) {
    $RESULTS[$loop] =~ s/([^\w\s\0])/sprintf ("&#%d;", ord ($1))/ge;
            ($id, $keywords, $description) = split (/\0/, $RESULTS[$loop], 3);
            $description =~ s/&#60;BR&#62;/<BR>/g;
            print <<End_of_Each_Appointment;
Keywords: <B>$keywords</B>
<BR>
Description:
$description
<P>
<A HREF="$display_URL&action=modify&id=$id">Modify!</A> <A HREF="$display_URL&action=delete&id=$id">Delete!</A> 
End_of_Each_Appointment
            print "<HR>", "\n"    if ($loop < $matches - 1);
        }

If there are appointments scheduled for the given date, they are displayed. Each one has two links: one to modify the appointment description, and the other to delete it from the database.

    } else {
        print "There are no appointments scheduled!", "\n";
    }
    print <<End_of_Footer;
<HR>
<A HREF="$display_URL&action=add&date=$date">Add New Appointment!</A> 
End_of_Footer
    &print_footer ();
}

If no appointments are scheduled for the date, a simple error message is displayed. Finally, a link allows the user to add appointments for the specified day.

Graphics

Up to this point, we have not discussed how the graphic calendar is created, or how the coordinates are interpreted on the fly. The next three subroutines are responsible for performing those tasks. The first one we will look at is a valuable subroutine that calculates various aspects of the graphic calendar.

sub graphics_calculations
{
    local (*GIF) = @_;

This subroutine expects a symbolic reference to an associative array as an argument. The purpose of the subroutine is to populate this array with numerous values that aid in implementing a graphic calendar.

    $GIF{'first_day'} = &get_first_day ($current_month, $current_year);

The get_first_day subroutine returns the day number for the first day of the specified month, where Sunday is 0 and Saturday is 6. For example, the routine will return the value 3 for November 1995, which indicates a Wednesday.

    $GIF{'last_day'}  = &get_last_day ($current_month, $current_year);

The get_last_day subroutine returns the number of days in a specified month. It takes leap years into effect.

    $GIF{'no_rows'} = ($GIF{'first_day'} + $GIF{'last_day'}) / 7;
    if ($GIF{'no_rows'} != int ($GIF{'no_rows'})) {
        $GIF{'no_rows'} = int ($GIF{'no_rows'} + 1);
    }

This calculates the number of rows that the calendar will occupy. We simply divide the number of days in this month by the number of days in a week, and round up if part of a week is left.

Now we are going to define some coordinates.

    $GIF{'box_length'} = $GIF{'box_height'} = 100;
    $GIF{'x_offset'}   = $GIF{'y_offset'} = 10;

The box length and height define the rectangular portion for each day in the calendar. You can modify this to a size that suits you. Nearly all calculations are based on this, so a modification in these values will result in a proportionate calendar. The x and y offsets define the offset of the calendar from the left and top edges of the image, respectively.

    $GIF{'large_font_length'} = 8;
    $GIF{'large_font_height'} = 16;
    $GIF{'small_font_length'} = 6;
    $GIF{'small_font_height'} = 12;

These sizes are based on the gdLarge and gdSmall fonts in the gd library.

    $GIF{'x'} = ($GIF{'box_length'} * 7)   +
                ($GIF{'x_offset'} * 2)     +
                $GIF{'large_font_length'};

The length of the image is based primarily on the size of each box length multiplied by the number of days in a week. The offset and the length of the large font size are added to this so the calendar fits nicely within the image.

    $GIF{'y'} = ($GIF{'large_font_height'} * 2)         +
                ($GIF{'no_rows'} * $GIF{'box_height'})  +
                ($GIF{'no_rows'} + 1)                   +
                ($GIF{'y_offset'} * 2)                  +
                $GIF{'large_font_height'};

The height of the image is based on the number of rows multiplied by the box height. Other offsets are added to this because there must be room at the top of the image for the month name and the weekday names.

    $GIF{'start_calendar'} = $GIF{'y_offset'}                +
                             (3 * $GIF{'large_font_height'});

This variable refers to the actual y coordinate where the calendar starts. If you were to subtract this value from the height of the image, the difference would equal the area at the top of the image where the titles (i.e., month name and weekday names) are placed.

    $GIF{'date_x_offset'} = int ($GIF{'box_length'} * 0.80);
    $GIF{'date_y_offset'} = int ($GIF{'box_height'} * 0.05);

These offsets specify the number of pixels from the upper right corner of a box to the day number.

    $GIF{'appt_x_offset'} = $GIF{'appt_y_offset'} = 10;

The appointment x offset refers to the number of pixels from the left edge of the box to the point where the appointment keywords are displayed. And the y offset is the number of pixels from the day number to a point where the appointment keywords are started.

    $GIF{'no_chars'} = int (($GIF{'box_length'}     -
                             $GIF{'appt_x_offset'}) /
                             $GIF{'small_font_length'}) - 1;

This contains the number of 6x12 font characters that will fit horizontally in each box, and is used to truncate appointment keywords.

    $GIF{'no_appts'} = int (($GIF{'box_height'}           -
                             $GIF{'large_font_height'}    -
                             $GIF{'date_y_offset'}        -
                             $GIF{'appt_y_offset'})       /
                             $GIF{'small_font_height'});
}

Finally, this variable specifies the number of appointment keywords that will fit vertically. Then next subroutine, get_imagemap_date, uses some of these constants to determine the exact region (and date) where the user click originated.

sub get_imagemap_date
{
    local (%DATA, $x_click, $y_click, $error_offset, $error,
           $start_y, $end_y, $start_x, $end_x, $horizontal, $vertical,
           $box_number, $clicked_date);
    &graphics_calculations (*DATA);
    ($x_click, $y_click) = split(/,/, $CALENDAR{'clicked_point'}, 2);

We start by calling the subroutine just discussed, graphics_calculations, to initialize coordinates and other important information about the calendar. The variable $CALENDAR{`clicked_point'} is a string containing the x and y coordinates of the click, as transmitted by the browser. The parse_query_and_form_data subroutine at the end of this chapter sets the value for this variable.

    $error_offset = 2;
    $error = $error_offset / 2;
    $start_y = $DATA{'start_calendar'} + $error_offset;
    $end_y = $DATA{'y'} - $DATA{'y_offset'} + $error_offset;
    $start_x = $DATA{'x_offset'} + $error_offset;
    $end_x = $DATA{'x'} - $DATA{'x_offset'} + $error_offset;

The error offset is defined as two pixels. This is introduced to make the clickable area the region just inside the actual calendar.

The $DATA{`start_calendar'} and $DATA{`x_offset'} elements of the array define the x and y coordinates where the actual calendar starts, as I discussed when listing the previous subroutine. We draw lines to create boxes starting at that point. Therefore, the y coordinate does not include the titles and headers at the top of the image.

    if ( ($x_click >= $start_x) && ($x_click <= $end_x) &&
         ($y_click >= $start_y) && ($y_click <= $end_y) ) {

This conditional ensures that a click is inside the calendar. If it is not, we send a status of 204 No Response to the browser.

If the browser can handle this status code, it will produce no response. Otherwise, an error message is displayed.

        $horizontal = int (($x_click - $start_x) /
                            ($DATA{'box_length'} + $error));
        $vertical = int (($y_click - $start_y) /
                        ($DATA{'box_height'} + $error));

The horizontal box number (starting from the left edge) of the user click is determined by the following algorithm:

The vertical box number (starting from the top) that corresponds to the user click can be calculated by the following algorithm:

To continue with the subroutine:

        $box_number = ($vertical * 7) + $horizontal;

The vertical box number is multiplied by seven--since there are seven boxes (i.e., seven days) per row--and added to the horizontal box number to get the raw box number. For instance, the first box in the second row would be considered raw box number 8. However, this will equal the date only if the first day of the month starts on a Sunday. Since we know this will not be true all the time, we have to take into effect what is really the first day of the month.

        $clicked_date = ($box_number - $DATA{'first_day'}) + 1;

The difference between the raw box number and the first day of the month is incremented by one (since the first day of the month returned by the get_first_date subroutine is zero based) to determine the date. We are still not out of trouble, because the calculated date can still be either less than zero, or greater than the last day of the month. How, you may ask? Say that a month has 31 days and the first day falls on Friday. There will be 7 rows, and a total of 42 boxes. If the user clicks in box number 42 (the last box of the last row), the $clicked_date variable above will equal 37, which is invalid. That is the reason for the conditional below:

        if (($clicked_date <= 0) ||
            ($clicked_date > $DATA{'last_day'})) {
            &return_error (204, "No Response", "Browser doesn't support 204");
        } else {
            return ($clicked_date);
        }
    } else {
        &return_error (204, "No Response", "Browser doesn't support 204");
    }
}

If the user clicked in a valid region, the date corresponding to that region is returned.

Now we can look at perhaps the most significant subroutine in this program. It invokes the gd graphics extension to draw the graphic calendar with the appointment keywords in the boxes.

sub draw_graphic_calendar
{
    local (%DATA, $image, $black, $cadet_blue, $red, $yellow,
           $month_title, $month_point, $day_point, $loop, $temp_day,
           $temp_x, $temp_y, $inner, $counter, $matches, %APPTS,
           @appt_list);
    &graphics_calculations (*DATA);
    $image = new GD::Image ($DATA{'x'}, $DATA{'y'});

A new image object is created, based on the dimensions returned by the graphics_calculations subroutine.

    $black      = $image->colorAllocate (0, 0, 0);
    $cadet_blue = $image->colorAllocate (95, 158, 160);
    $red        = $image->colorAllocate (255, 0, 0);
    $yellow     = $image->colorAllocate (255, 255, 0);

Various colors are defined. The background color is black, and the lines between boxes are yellow. All text is drawn in red, except for the dates, which are cadet blue.

    $month_title = join (" ", $current_month_name, $current_year);
    $month_point = ($DATA{'x'} -
                   (length ($month_title) *
                   $DATA{'large_font_length'})) / 2;
    $image->string (gdLargeFont, $month_point, $DATA{'y_offset'},
                    $month_title, $red);

The month title (e.g., "November 1995") is centered in red, with the $month_point variable giving the right amount of space on the left.

    $day_point = (($DATA{'box_length'} + 2) -
                  ($DATA{'large_font_length'} * 3)) / 2;

The $day_point variable centers the weekday string (e.g., "Sun") with respect to a single box.

    for ($loop=0; $loop < 7; $loop++) {
        $temp_day = (split(/,/, $weekday_names))[$loop];
        $temp_x = ($loop * $DATA{'box_length'}) +
                   $DATA{'x_offset'} +
                   $day_point + $loop;
        $image->string ( gdLargeFont,
                         $temp_x,
                         $DATA{'y_offset'} +
                         $DATA{'large_font_height'} + 10,
                         $temp_day,
                         $red );
    }

The for loop draws the seven weekday names (as stored in the $weekday_names global variable) above the first row of boxes.

    for ($loop=0; $loop <= $DATA{'no_rows'}; $loop++) {
        $temp_y = $DATA{'start_calendar'} +
                  ($loop * $DATA{'box_height'}) + $loop;
        $image->line ( $DATA{'x_offset'},
                       $temp_y,
                       $DATA{'x'} - $DATA{'x_offset'} - 1,
                       $temp_y,
                       $yellow );
    }

This loop draws the horizontal yellow lines, in effect separating each box.

    for ($loop=0; $loop <= 7; $loop++) {
        $temp_x = $DATA{'x_offset'} + ($loop * $DATA{'box_length'}) + $loop;
        $image->line ( $temp_x,
                       $DATA{'start_calendar'},
                       $temp_x,
                       $DATA{'y'} - $DATA{'y_offset'} - 1,
                       $yellow );
    }

The for loop draws yellow vertical lines, creating boundaries between the weekdays. We have finished the outline for the calendar; now we have to fill in the blanks with the particular dates and appointments.

    $inner = $DATA{'first_day'};
    $counter = 1;
    $matches = &appointments_for_graphic (*APPTS);

The appointments_for_graphic subroutine returns an associative array of appointment keywords for the selected month (keyed by the date). For example, here is what an array might look like:

$APPTS{'02'} = "See Professor";
$APPTS{'03'} = "ABC Enterprises\0Luncheon Meeting";

This example shows one appointment on the 2nd of this month, and two appointments (separated by a \0 character) on the 3rd.

In several nested loops--one for the rows, one for the days in each row, and one for the appointments on each day--we draw the date for each box and list the appointment keywords in the appropriate boxes.

    for ($outer=0; $outer <= $DATA{'no_rows'}; $outer++) {
        $temp_y = $DATA{'start_calendar'} + $outer +
                  ($outer * $DATA{'box_height'}) +
                  $DATA{'date_y_offset'};

This outermost loop iterates through the rows, based on $DATA{`no_rows'}. The $temp_y variable contains the y coordinate where the date should be drawn for a particular row.

        while (($inner < 7) && ($counter <= $DATA{'last_day'})) {
            $temp_x = $DATA{'x_offset'} +
                      ($inner * $DATA{'box_length'}) +
                      $inner + $DATA{'date_x_offset'};
            $image->string (gdLargeFont, $temp_x, $temp_y,
                            sprintf ("%2d", $counter),
                            $cadet_blue);

This inner loop draws the dates across a row. A while loop was used instead of a for loop because the number of dates across a row may not be seven (in cases when the month does not start on Sunday or does not end on Saturday). The variable $counter keeps track of the actual date that is being output.

            if ($APPTS{$counter}) {
                @appt_list = split (/\0/, $APPTS{$counter});
                for ($loop=0; $loop < $matches; $loop++) {
                    last if ($loop >= $DATA{'no_appts'});

If appointments exist for the date, a for loop is used to iterate through the list. The number of appointments that can fit in a box is governed by $DATA{`no_appts'}; others are ignored. But the user can click on the individual date to see all of them.

                    $image->string (gdSmallFont,
                                    $DATA{'x_offset'} +
                                    ($inner * $DATA{'box_length'} +
                                    $inner +
                                    $DATA{'appt_x_offset'}),
                                    $temp_y +
                                    $DATA{'large_font_height'}+
                                    ($loop * $DATA{'small_font_height'}) +
                                    $DATA{'appt_y_offset'},
                                    pack ("A$DATA{'no_chars'}",
                                    $appt_list[$loop]),
                                    $red);
                }
            }

The keywords for an appointment are displayed in the box. The pack operator truncates the string to fit in the box.

            $inner++;
            $counter++;
        }
            $inner = 0;
    }
    $| = 1;
    print "Content-type: image/gif", "\n";
    print "Pragma: no-cache", "\n\n";
    print $image->gif;
}

Finally, the program turns output buffering off and sends the image to the client for display.

The following subroutine returns an associative array containing the keywords for all the appointments for the selected month.

sub appointments_for_graphic
{
    local (*DATES) = @_;
    local ($matches, @RESULTS, $loop, $day, $keywords);
    $matches = &open_database (*RESULTS, "select", <<End_of_Select);
select Day, Keywords from $database where 
       (Month = $current_month) and
       (Year  = $current_year)
End_of_Select

RESULTS now contains the number of elements indicated by $matches. Each element contains the date for an appointment followed by the keyword list for that appointment, as requested by our select statement. We need to put all the appointments for a given day into one element of our associative array DATES, which we will return to the caller.

    for ($loop=0; $loop < $matches; $loop++) {
        ($day, $keywords) = split (/\0/, $RESULTS[$loop], 2);
        if ($DATES{$day}) {
            $DATES{$day} = join ("\0", $DATES{$day}, $keywords);
        } else {
            $DATES{$day} = $keywords;
        }
    }

When a day in DATES already lists an appointment, we concatenate the next appointment to it with the null string (\0) as separator. When we find an empty day, we do not need to add the null string.

    return ($matches);
}

Finally, a count of the total number of appointments for the month are returned.

The last major subroutine we will discuss parses the form data. It is very similar to the parse_form_data subroutines used up to this point.

sub parse_query_and_form_data
{
    local (*FORM_DATA) = @_;
    local ($request_method, $query_string, $path_info,
           @key_value_pairs, $key_value, $key, $value);
    $request_method = $ENV{'REQUEST_METHOD'};
    $path_info = $ENV{'PATH_INFO'};
    if ($request_method eq "GET") {
        $query_string = $ENV{'QUERY_STRING'};
    } elsif ($request_method eq "POST") {
        read (STDIN, $query_string, $ENV{'CONTENT_LENGTH'});
        if ($ENV{'QUERY_STRING'}) {
            $query_string = join ("&", $query_string, $ENV{'QUERY_STRING'});
        }

If the request method is POST, the information from the input stream and the data in QUERY_STRING are appended to $query_string. We have to do this because our program accepts information in an unusually complex way; some user queries pass both query strings and input streams.

    } else {
        &return_error ("500", "Server Error",
                      "Server uses unsupported method");
    }
    if ($query_string =~ /^\d+,\d+$/) {
        $FORM_DATA{'clicked_point'} = $query_string;
        if ($path_info =~ m|^/(\d+/\d+)$|) {
            $FORM_DATA{'month'} = $1;
        }

If the user clicks on the imagemap, the client sends a query string in the form of two integers ("x,y") to the CGI program. Here, we store the string right into $FORM_DATA{`clicked_point'}, where the get_imagemap_date routine can retrieve it. Previously, we set up our hypertext link so that the month name gets passed as extra path information (see the output_HTML subroutine), and here we store it in $FORM_DATA{`month'}. This value is checked for validity at the top of the program, just to make sure that there are no shell metacharacters.

    } else {
        if ($query_string =~ /draw_imagemap/) {
            $FORM_DATA{'draw_imagemap'} = 1;
        }

The $FORM_DATA{`draw_imagemap'} variable is set if the query contains the string "draw_imagemap". The rest of the code below is common, and we have seen it many times.

        @key_value_pairs = split (/&/, $query_string);
        foreach $key_value (@key_value_pairs) {
            ($key, $value) = split (/=/, $key_value);
            $value =~ tr/+/ /;
            $value =~ s/%([\dA-Fa-f][\dA-Fa-f])/pack ("C", hex ($1))/eg;
            if (defined($FORM_DATA{$key})) {
                $FORM_DATA{$key} = join ("\0", $FORM_DATA{$key}, $value);
            } else {
                $FORM_DATA{$key} = $value;
            }
        }
    }
}

The following subroutine returns the number of days in the specified month. It takes leap years into effect.

sub get_last_day
{
    local ($month, $year) = @_;
    local ($last, @no_of_days);
    @no_of_days = (31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31);
    if ($month == 2) {
        if ( !($year % 4) && ( ($year % 100) || !($year % 400) ) ) {
            $last = 29;
        } else {
            $last = 28;
        }
    } else {
        $last = $no_of_days[$month - 1];
    }
    return ($last);
}

The get_first_day subroutine (algorithm by Malcolm Beattie <mbeattie@black.ox.ac.uk>) returns the day number for the first day of the specified month. For example, if Friday is the first day of the month, this subroutine will return 5. (The value is zero-based, starting with Sunday).

sub get_first_day
{
    local ($month, $year) = @_;
    local ($day, $first, @day_constants);
    $day = 1;
    @day_constants = (0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4);
    if ($month < 3) {
        $year--;
    }
    $first = ($year + int ($year / 4) - int ($year / 100) +
         int ($year/400) + $day_constants [$month - 1] + $day) % 7;
    return ($first);
}