#include <fstream>

#include <git2cpp/initializer.h>
#include <git2cpp/repo.h>

namespace
{
    struct visitor_t
    {
        void operator()(git::RevWalker & walker)
        {
            walker.sort(git::revwalker::sorting::topological);
            walker.simplify_first_parent();

            while (git::Commit commit = walker.next())
            {
                output_commit(commit);

                for (size_t i = 0; i != commit.parents_num(); ++i)
                {
                    output_hash(commit.id()) << " -> ";
                    output_hash(commit.parent_id(i)) << "\n";
                }

                for (size_t i = 1; i < commit.parents_num(); ++i)
                {
                    auto branch_walker = repo_.rev_walker();
                    branch_walker.push(commit.parent_id(i));
                    branch_walker.hide(commit.merge_base(0, i));
                    (*this)(branch_walker);
                }
            }
        }

        visitor_t(git::Repository const & repo, std::ostream & out)
            : repo_(repo)
            , out_(out)
        {
        }

    private:
        std::ostream & output_hash(git_oid const & id) const
        {
            out_ << "C" << git::id_to_str(id, 6);
            return out_;
        }

        void output_commit(git::Commit const & commit) const
        {
            output_hash(commit.id()) << " [label=\"" << commit.summary() << "\"];"
                                     << "\n";
        }

    private:
        git::Repository const & repo_;
        std::ostream & out_;
    };

    void visit(git::Repository const & repo, std::ostream & out)
    {
        visitor_t visitor(repo, out);

        git::RevWalker walker = repo.rev_walker();
        walker.push_head();
        visitor(walker);
    }
}

int main(int argc, char * argv[])
{
    auto_git_initializer;
    git::Repository repo(argc >= 2 ? argv[1] : ".");
    std::ofstream out(argc >= 3 ? argv[2] : "commit-graph.dot");

    out << "digraph {\n";
    visit(repo, out);
    out << "}\n";
}