#!/usr/bin/env perl
#-------------------------------------------------------------------------------
# (C) British Crown Copyright 2006-16 Met Office.
#
# This file is part of FCM, tools for managing and building source code.
#
# FCM is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# FCM is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with FCM. If not, see <http://www.gnu.org/licenses/>.
#-------------------------------------------------------------------------------

use strict;
use warnings;

use Getopt::Long qw{GetOptions};
use Pod::Usage qw{pod2usage};

my $RE_SVN_EMPTY_FILE = qr{\.svn/empty-file}msx;

my %S = (
    'LABEL'    => "--- %s\n+++ %s",
    'SKIP_ADD' => "Skipping since file has been added (or old file is empty)",
    'SKIP_DEL' => "Skipping since file has been deleted (or new file is empty)",
    'SKIP_BIN' => "Skipping binary file",
);
my %LABELS_HANDLER_FOR = (
    'tkdiff' => sub {map {('-L', $_)} @_},
    'xxdiff' => sub {('--title1', $_[0], '--title2', $_[1])},
    'meld'   => sub {map {('-L', $_)} @_},
);

if (!caller()) {
    # svn diff expects:
    # 0 - no diff
    # 1 - diff
    # other return code - fatal
    exit main(@ARGV);
}

sub main {
    local(@ARGV) = @_;
    my %option;
    my $rc = GetOptions(\%option, 'u', 'L=s@');
    if (!$rc || @ARGV != 2 || grep {!-f $_} @ARGV) {
        pod2usage(1);
    }
    my ($old, $new) = @ARGV;
    ( $old =~ $RE_SVN_EMPTY_FILE || -z $old ? message('SKIP_ADD')
    : $new =~ $RE_SVN_EMPTY_FILE || -z $new ? message('SKIP_DEL')
    : -B $new                               ? message('SKIP_BIN')
    :                                         command(\%option, @ARGV)
    );
}

sub command {
    my ($option_hash_ref, $old, $new) = @_;
    my @labels;
    if ($option_hash_ref->{'L'} && @{$option_hash_ref->{'L'}} >= 2) {
        @labels = @{$option_hash_ref->{'L'}};
        message('LABEL', @labels);
    }
    my $diff_command
        = exists($ENV{FCM_GRAPHIC_DIFF}) ? $ENV{FCM_GRAPHIC_DIFF} : 'xxdiff';
    if (!$diff_command) {
        return;
    }
    my @command = (
        $diff_command,
        (   @labels && exists($LABELS_HANDLER_FOR{$diff_command})
            ? $LABELS_HANDLER_FOR{$diff_command}->(@labels) : ()
        ),
        $old, $new,
    );
    system(@command);
}

sub message {
    my $format = shift();
    printf($S{$format} . "\n\n", @_);
    1;
}

__END__

=head1 NAME

fcm_graphic_diff

=head1 SYNOPSIS

    fcm_graphic_diff [-u] [-L OLD_LABEL] [-L NEW_LABEL] OLD NEW

=head1 DESCRIPTION

Invokes L<xxdiff|xxdiff> (or the command specified in the FCM_GRAPHIC_DIFF
environment variable) to compare the OLD and NEW files, where possible.

If either file does not exist or is empty, or if the NEW file is a binary, the
command will only print a diagnostic message.

The -u option is not used, and is for compatibility with the L<svn diff|svn>
command only.

If OLD_LABEL and NEW_LABEL are set, they are printed in the format:

    ---- OLD_LABEL
    ++++ NEW_LABEL

The command makes use of the labels when the diff command is either
L<xxdiff|xxdiff>, L<tkdiff|tkdiff>, or L<meld|meld>:

    xxdiff --title1 OLD_LABEL --title2 NEW_LABEL OLD NEW
    tkdiff -L OLD_LABEL -L NEW_LABEL OLD NEW
    meld   -L OLD_LABEL -L NEW_LABEL OLD NEW

The command returns 0 if the files are the same or 1 if the files differ. All
other return codes should be regarded as fatal errors.

=head1 COPYRIGHT

(C) Crown copyright Met Office. All rights reserved.

=cut
