class GemHadar

def version_log_diff(to_version: 'HEAD', from_version: nil)
  if to_version == 'HEAD'
    if from_version.blank?
      from_version = versions.last
    else
      unless versions.find { |v| v == from_version }
        fail "Could not find #{from_version.inspect}."
      end
    end
    `git log -p #{version_identifier(from_version)}..HEAD`
  else
    unless versions.find { |v| v == to_version }
      fail "Could not find #{to_version.inspect}."
    end
    if from_version.blank?
      from_version = versions.each_cons(2).find do |previous_version, v|
        if v == to_version
          break previous_version
        end
      end
      unless from_version
        fail "Could not find version before #{to_version.inspect}."
      end
    else
      unless versions.find { |v| v == from_version }
        fail "Could not find #{from_version.inspect}."
      end
    end
    `git log -p #{version_identifier(from_version)}..#{version_identifier(to_version)}`
  end
end