class Measured::UnitSystemBuilder
  def initialize
    @units = []
  end

  def unit(unit_name, aliases: [], value: nil)
    @units << build_unit(unit_name, aliases: aliases, value: value)
    nil
  end

  def si_unit(unit_name, aliases: [], value: nil)
    @units += build_si_units(unit_name, aliases: aliases, value: value)
    nil
  end

  def build
    Measured::UnitSystem.new(@units)
  end

  private

  SI_PREFIXES = [
    ["y", "yocto", -24],
    ["z", "zepto", -21],
    ["a", "atto", -18],
    ["f", "femto", -15],
    ["p", "pico", -12],
    ["n", "nano", -9],
    ["μ", "micro", -6],
    ["m", "milli", -3],
    ["c", "centi", -2],
    ["d", "deci", -1],
    ["da", "deca", 1],
    ["h", "hecto", 2],
    ["k", "kilo", 3],
    ["M", "mega", 6],
    ["G", "giga", 9],
    ["T", "tera", 12],
    ["P", "peta", 15],
    ["E", "exa", 18],
    ["Z", "zetta", 21],
    ["Y", "yotta", 24]
  ]

  def build_si_units(name, aliases: [], value: nil)
    si_units = [build_unit(name, aliases: aliases, value: value)]
    SI_PREFIXES.each do |short, long, exp|
      long_names = aliases.map { |suffix| "#{long}#{suffix}" }
      si_units << build_unit("#{short}#{name}", aliases: long_names, value: "#{10 ** exp} #{name}")
    end
    si_units
  end

  def build_unit(name, aliases: [], value: nil)
    unit = Measured::Unit.new(name, aliases: aliases, value: value)
    check_for_duplicate_unit_names!(unit)
    unit
  end

  def check_for_duplicate_unit_names!(unit)
    names = @units.flat_map(&:names)
    if names.any? { |name| unit.names.include?(name) }
      raise Measured::UnitError, "Unit #{unit.name} has already been added."
    end
  end
end