#!/usr/bin/perl
##
# Fetch images for screensaver from the chromecast page.
#
# The images used by the Chromecast screensaver are embedded in a 
# webpage. They could be changed by Google at any time, and - of course
# provide a way for Google to track how many users of the device there
# are. And maybe which individual devices, if it were to supply more
# unique identification in the headers.
#
# Regardless, they're just embedded in a JSON encoded string, which we
# can easily extract and store in a directory.
#

my $url = "https://clients3.google.com/cast/chromecast/home/v/c9541b08";
my $storedir = "pictures";

use LWP::Simple;
use JSON;
use Data::Dumper;

# Fetch page
my $page = get($url);

# Extract the JSON content
my ($json) = ($page =~ /JSON.parse\('(.*?)'\)/);

# Remove the escaped strings
$json =~ s/\\x([a-f0-9]{2})/sprintf "%c", hex($1)/eg;
$json =~ s/\\n/\n/g;

my $data = decode_json($json);

# The structure is pretty simple, although I'm not sure what all the 
# values are:
#    all            array of everything
#       [0]         array of pictures items
#           ...     array of details about the picture
#               [0] URL of the picture
#               [1] Author's name
#               [2] undef
#               [3] undef
#               [4] 120 (I'd guess this is the time to show for?)
#               [5] undef
#               [6] undef
#               [7] undef
#               [8] undef
#               [9] undef
#               [10] undef
#               [11] undef
#               [12] "Photo by " + author's name (no i18n?)
#               [13] undef
#       [1]         array of application id?
#           [0]     uuid
#           [1]     big number ('1408149221009' at the present time)
#       [2]         undef
#       [3]         array of...
#           [0]     the number 1
#       [4]         120 (default display time?)

mkdir $storedir, 0755;

# Extract each of these pictures, in a sorted order by URL, so that we 
# get an unchanging list (hopefully).
my $n = 0;
for my $picture (sort { $a->[0] cmp $b->[0] } @{ $data->[0] })
{
    print "URL: $picture->[0]\n";
    print " by: $picture->[1]\n";
    print "\n";

    # Fetch the picture
    my $picture = get($picture->[0]);
    
    if (!defined $picture)
    {
        die "Could not fetch picture from $picture\n";
    }

    # Where we'll write the file
    my $filename = sprintf "pictures/%03d-%s", $n++, $picture->[1];
    
    # Find the filetype and store the picture based on it.
    # There are better ways of doing this, but I just put this
    # together to catch simple cases. Consider using LWP direct,
    # and getting the media type of the resource, rather than
    # sniffing the file data.   
    my $ext = undef;
    if ($picture =~ /^\x89\x50\x4e\x47/)
    {
        $ext = 'png';
    }
    elsif ($picture =~ /^\xff\xd8/)
    {
        $ext = 'jpg';
    }
    elsif ($picture =~ /^GIF8[79]a/)
    {
        $ext = 'gif';
    }

    # Write out the file
    $filename .= ".$ext";
    open(my $fh, ">", $filename) || die "Could not write $filename\n";
    print $fh $picture;
    close($fh);
}