class Tins::Unit::UnitParser

def initialize(source, unit, prefixes = nil)
  super source
  if prefixes
    @unit_re    = unit_re(Tins::Unit.prefixes(prefixes), unit)
    @unit_lc_re = @unit_uc_re = nil
  else
    @unit_lc_re = unit_re(Tins::Unit.prefixes(:lc), unit)
    @unit_uc_re = unit_re(Tins::Unit.prefixes(:uc), unit)
    @unit_re    = nil
  end
  @number       = 1.0
end

def parse
  raise NotImplementedError
end

def scan(re)
  re.nil? and return
  super
end

def scan_char(char)
  scan(/#{char}/) or return
end

def scan_number
  scan(NUMBER) or return
  @number *= BigDecimal(self[1])
end

def scan_unit
  case
  when scan(@unit_re)
    prefix = @unit_re.prefixes.find { |pre| pre.name == self[1] } or return
    @number *= prefix.multiplier
  when scan(@unit_lc_re)
    prefix = @unit_lc_re.prefixes.find { |pre| pre.name == self[1] } or return
    @number *= prefix.multiplier
  when scan(@unit_uc_re)
    prefix = @unit_uc_re.prefixes.find { |pre| pre.name == self[1] } or return
    @number *= prefix.multiplier
  end
end

def unit_re(prefixes, unit)
  re = Regexp.new(
    "(#{prefixes.reverse.map { |pre| Regexp.quote(pre.name) } * ?|})(#{unit})"
  )
  re.singleton_class.class_eval do
    define_method(:prefixes) { prefixes }
  end
  re
end