Extract number of active authors in a git repository at the time of each commit

git-authors-active.pl

#!/usr/bin/perl
my @a = `git log --use-mailmap --reverse --pretty=fuller --no-color --date=short --decorate=full  | egrep "^(Author|CommitDate):"`;

use Time::Piece;
sub delta {
    my ($from, $to) = @_;

    my $f = Time::Piece->strptime($from, "%Y-%m-%d");
    my $t = Time::Piece->strptime($to, "%Y-%m-%d");

    my $diff = $t - $f;
    return int($diff->days);
}

my $auth;
for(@a) {
    chomp;
    my $line = $_;
    if(/^Author: *([^\<]*) \</) {
        ($auth)=($1);
        $person{$auth}++;
    }
    elsif(/^CommitDate: (.*)/) {
        my $d = $1;

        $date{$auth}=$d;
        my @active12;
        my @active9;
        my @active6;
        my @active3;
        my @activew;
        foreach my $p (keys %person) {
            my $de = delta($date{$p}, $d);

            push @active12, $p if($de < 120);
            push @active9, $p if($de < 90);
            push @active6, $p if($de < 60);
            push @active3, $p if($de < 30);
            push @activew, $p if($de < 7);
        }
        if(delta("2010-01-01", $d) < 0) {
            # counted, but don't output data from before 2010
            next;
        }

        #printf "$d (%d): %s\n", $#active+1, join(", ", sort @active);
        printf "$d;%d;%d;%d;%d;%d\n",
            $#active12+1,
            $#active9+1,
            $#active6+1,
            $#active3+1,
            $#activew+1;
    }
}