#!/local/bin/perl

# a perl script to return an image or text or existing file on demand

#first get user input data
%user_input = get_form_data();

# depends on the query string, the output is different
if ($query_data{'format'} eq 'text') {
	# return a piece of plain text
	print "Content-type: text/plain", "\n\n";
	
	print "Yellow Crane Tower is an imposing pagoda close to the Yangzi River. ", "\n"; 
	print "Situated at the top of Sheshan (Snake Hill), in Wuchang, the tower ", "\n";
	print "was originally built at a place called Yellow Crane Rock projecting ", "\n";
	print "over the water, hence the name. Over the centuries the tower was ", "\n";
	print "destroyed by fire many times, but its popularity with Wuhan residents ", "\n";
	print "ensured that it was always rebuilt. The current tower was completed ", "\n";
	print "in 1985 and its design was copied from a Qing dynasty (1644-1911) ", "\n";
	print "picture. The tower has 5 stories and rises to 51 meters (168ft). ", "\n";
	print "Covered with yellow glazed tiles and supported with 72 huge pillars, ", "\n";
	print "it has 60 upturned eaves layer upon layer. It is an authentic ", "\n";
	print "reproduction of both the exterior and interior design, with the ", "\n";
	print "exception of the addition of air-conditioning and an elevator.", "\n";
} elsif ($query_data{'format'} eq 'image') {
	# return an image
	$image = 'cranetower.jpg';
	# open the image file in read mode
	if (open (IMAGE, "<" . $image)) {
		# the size of the image in bytes
		$len = (stat ($image))[7];
		print "Content-type: image/jpeg", "\n";
	    print "Content-length: $len", "\n\n";
	}
	print <IMAGE>;
} elsif ($query_data{'format'} eq 'both') {
	# Server Redirection: redirect the server to an existing document
	# the server will return it as if it is the response from the CGI program
	print 'Location: cranetower.html', "\n\n"
} else {
	print "Content-type: text/plain", "\n\n";
	
	print "Sorry! This section is under construction", "\n";
}

sub get_form_data {
	# get the type of method that was used by the 
	# user to issue this request
	$request_method = $ENV{'REQUEST_METHOD'};
	
	# if the GET method was used
	if ($request_method eq "GET") {
		# get the query request from the environment variable
		$query = $ENV{'QUERY_STRING'};
	
	# if the POST method was used
	} elsif ($request_method eq "POST") {
		# get the query request from the standard input
		# first we get the size of the query request (# of chars)
		$query_size = $ENV{'CONTENT_LENGTH'};
		# then we read from STDIN that many number of chars
		read (STDIN, $query, $query_size);
	
	# otherwise, we got an invalid request
	} else {
		print "Content-type: text/html", "\n";
		print "Status: 500 Server Error", "\n\n";
	
		print "<title>Server Error</title>", "\n";
		print "<h1>Server Error</h1>", "\n";
		print "<hr>Unsuported request method<hr>", "\n"; 
		
		exit;
	}
	
	# now we have the query string, let's constrcut a hash table from it
	# where keys are the keys in the query string, values are the corresponding values
	%query_data = ();
	
	# first we separat each key value pairs
	@key_values = split (/&/, $query);
	# for each key value pair ...
	foreach $key_value (@key_values) {
		($key, $value) = split (/=/, $key_value);
		# replace + with a space
		$value =~ tr/+/ /;
	
		# the regular expression matches any hexadecimal value
		# and store it in the variable $1
		# the pack and hex operator convert the value in $1 to an ASCII equivalent
		# e means the second argument as an expression
	        $value =~ s/%([\dA-Fa-f][\dA-Fa-f])/pack ("C", hex ($1))/eg;
	
		# insert the key value pair into the query hash
		# we need to consider multiple values (select element with "multiple" turned on) 
	        # if the key value pair already exist in the hash
		if (defined($query_data{$key})) {
			# we append the new value to the old ones
	        	$query_data{$key} = join (", ", $query_data{$key}, $value);
	        } else {
			# otherwise, just add a new entry in the hash
	        	$query_data{$key} = $value;
	        }
	}
	
	return qurey_data;
}