require 'colored'
require 'grit'

class GitUp
  def initialize(args)
    @repo   = Grit::Repo.new(File.expand_path("."))
    @head   = @repo.head
  end

  def run
    system "git fetch"

    with_stash do
      returning_to_current_branch do
        col_width = @repo.branches.map { |b| b.name.length }.max + 1

        @repo.branches.each do |branch|
          next unless remote = remote_for_branch(branch)

          print branch.name.ljust(col_width)

          if remote.commit.sha == branch.commit.sha
            puts "up to date".green
            next
          end

          base = merge_base(branch.name, remote.name)

          if base == remote.commit.sha
            puts "ahead of upstream".green
            next
          end

          if base == branch.commit.sha
            puts "fast-forwarding...".yellow
          else
            puts "rebasing...".yellow
          end

          checkout(branch.name)
          rebase(remote)
        end
      end
    end
  rescue GitError => e
    puts e.message.red
    puts "Here's what Git said:".red
    puts e.output
  end

  def remote_for_branch(branch)
    remote_name = @repo.config["branch.#{branch.name}.remote"] || "origin"
    @repo.remotes.find { |r| r.name == "#{remote_name}/#{branch.name}" }
  end

  def with_stash
    stashed = false

    status = @repo.status
    change_count = status.added.length + status.changed.length + status.deleted.length

    if change_count > 0
      puts "stashing #{change_count} changes".magenta
      @repo.git.stash
      stashed = true
    end

    yield

    if stashed
      puts "unstashing".magenta
      @repo.git.stash({}, "apply")
    end
  end

  def returning_to_current_branch
    unless @repo.head.respond_to?(:name)
      puts "You're not currently on a branch. I'm exiting in case you're in the middle of something.".red
      return
    end

    branch_name = @repo.head.name

    yield

    unless on_branch?(branch_name)
      puts "returning to #{branch_name}".magenta
      checkout(branch_name)
    end
  end

  def checkout(branch_name)
    output = @repo.git.checkout({}, branch_name)

    unless on_branch?(branch_name)
      raise GitError.new("Failed to checkout #{branch_name}", output)
    end
  end

  def rebase(target_branch)
    current_branch = @repo.head

    output, err = @repo.git.sh("#{Grit::Git.git_binary} rebase #{target_branch.name}")

    unless on_branch?(current_branch.name) and is_fast_forward?(current_branch, target_branch)
      raise GitError.new("Failed to rebase #{current_branch.name} onto #{target_branch.name}", output+err)
    end
  end

  def is_fast_forward?(a, b)
    merge_base(a.name, b.name) == b.commit.sha
  end

  def merge_base(a, b)
    @repo.git.send("merge-base", {}, a, b).strip
  end

  def on_branch?(branch_name=nil)
    @repo.head.respond_to?(:name) and @repo.head.name == branch_name
  end

  class GitError < StandardError
    attr_reader :output

    def initialize(message, output)
      super(message)
      @output = output
    end
  end
end