#!/usr/bin/env perl
use 5.016;
use warnings;
use autodie;
use charnames ':full';
use Encode qw(decode);
use JSON::PP;
use Pod::Usage;


if (!@ARGV) {
    pod2usage('Missing arguments.');
}
elsif (grep { /^--?h(elp)?$/ } @ARGV) {
    pod2usage(-verbose => 2);
}


my %COMMIT_ARGS = (
    hash           => '%H',
    author_name    => '%aN',
    author_mail    => '%aE',
    author_date    => '%at',
    committer_name => '%cN',
    committer_mail => '%cE',
    committer_date => '%ct',
    subject        => '%s',
    body           => '%b',
    notes          => '%N',
    signer         => '%GS',
    signer_key     => '%GK',
);

my @COMMIT_KEYS      = sort keys %COMMIT_ARGS;
my @FORMAT_ARGS      = @COMMIT_ARGS{@COMMIT_KEYS};
my $COMMIT_SEPARATOR = "\N{RECORD SEPARATOR}";
my $DIFF_SEPARATOR   = "\N{UNIT SEPARATOR}";

# See the “Internals” in the documentation (run `perldoc parseman`)
my $FORMAT = $COMMIT_SEPARATOR . join('%x00', @FORMAT_ARGS) . $DIFF_SEPARATOR;


sub parse_log {
    my ($log) = @_;
    my @fields = split "\0", $log, -1;
    return map { $COMMIT_KEYS[$_] => $fields[$_] } 0 .. $#fields;
}

sub parse_diff {
    my ($diff) = @_;
    my %changes = (
        touched_files => 0,
        insertions    => 0,
        deletions     => 0,
    );

    for my $diff_line (split "\n", $diff) {
        next unless $diff_line =~ /^(\d+)\s+(\d+)\s+(.+?)\s*$/;
        $changes{insertions} += $1;
        $changes{deletions } += $2;
        $changes{touched_files}++;
    }

    return %changes;
}


my @git_args;

if ($ENV{IDMAN_BEFORE}) {
    push @git_args, "--before=$ENV{IDMAN_BEFORE}";
}


for my $repo_path (@ARGV) {
    open my $fh, '-|', 'git', "--git-dir=$repo_path/.git", 'log', '--all',
                       "--format=$FORMAT", '-M', '-C', '--numstat', @git_args;

    read $fh, $_, length $COMMIT_SEPARATOR; # strip leading separator
    local $/ = "\n$COMMIT_SEPARATOR";

    while (<$fh>) {
        chomp;
        my ($log, $diff) = split $DIFF_SEPARATOR, decode('UTF-8', $_);
        say encode_json({
            repo => $repo_path,
            parse_log($log),
            parse_diff($diff),
        });
    }

    close $fh;
}


__END__

=head1 NAME

parseman - parse commit info from a git directory

=head1 SYNOPSIS

    parseman GIT_REPO_FOLDER...

This will parse all commits from the given git repo(s) and dump the results as
JSON objects on stdout, one per line.

=head1 DESCRIPTION

This script will take a git repository and run C<git log --all> on it,
transforming its output into structured JSON. This output can be piped into
another program.

The output consists of several JSON-encoded objects, one object per line. The
consuming program can just read one line at a time, JSON-decode each one and
then handle its contents.

For example output, just run C<./parseman .> to see the output for this here
repository.

Each JSON object is just a mapping from string keys to string values. Empty
values will be empty strings. The following values are in the object:

=over

=item repo

The path to the repository's local folder. If you want to run further git
commands on it, you might need to append C</.git> to it.

=item hash

The commit's sha-1 hash.

=item author_name

The name of the commit's author.

=item author_mail

The e-mail address of the author. Might not actually be a valid e-mail address,
git doesn't check this.

=item author_date

The date that the commit was authored as a Unix timestamp. Note that this is a
string of digits, not an integer.

=item committer_name

The name of the committer, which may or may not be different from the author.

=item committer_mail

The e-mail address of the committer. Same caveat as for the author applies.

=item committer_date

The date that the commit was committed.

=item subject

The commit message subject line.

=item body

The rest of the commit message.

=item notes

The notes attached to the commit. Basically a message in addition to the
regular commit message.

=item signer

Who signed the commit, if anybody did.

=item signer_key

The signature key of who signed the commit.

=item touched_files

=item insertions

=item deletions

The amount of modified files, inserted lines and deleted lines in the commit,
respectively. Renamed files are taken into account properly, so a rename on
its own counts as a single changed file with zero inserted or deleted lines.

See the C<--find-renames> and C<--find-copies> options in C<git log --help> for
details.

=back

=head1 INTERNALS

Calling C<git log> on every commit would be the easiest implementation for this
script, but that is just prohibitively slow. So instead, we run it only one
single time with a specially formatted output so that we can separate each
commit properly.

The format works as follows:

=over

=item

Each commit is prefixed with a C<RECORD SEPARATOR> character. This separates
the commits from each other.

=item

Each of the fields within a commit (hash, message, author name etc.) is
separated with a C<NUL> character.

=item

After those fields follows a C<UNIT SEPARATOR>, after which git automatically
puts the diff information (changed files and lines).

=back

Parsing the format works by splitting the input by C<RECORD SEPARATOR>s and
splitting each of those records by C<UNIT SEPARATOR>s into a fields portion and
a diff portion. The fields are then simply split by C<NUL>s and the diff is
parsed via regular expression.

=cut