class Base
  attr_accessor :value, :unit

  def initialize(value, unit, valid_units)
    raise TypeError, 'no implicit conversion of String into Integer' unless value.is_a? Numeric
    raise TypeError, 'Invalid Unit Type' unless valid_units.include?(unit.to_s.downcase)
    @value = value
    @unit = unit.downcase
  end

  def ==(other)
    a_to_b = one_unit_a_to_b(unit, other.unit) * value
    b_to_a = one_unit_a_to_b(other.unit, unit) * other.value
    a_to_b == other.value || b_to_a == value
  end

  def ===(other)
    value == other.value && unit == other.unit
  end

  def +(other)
    b_to_a = other.to(unit)
    self.class.new(value + b_to_a.value, unit)
  end

  def -(other)
    b_to_a = other.to(unit)
    self.class.new(value - b_to_a.value, unit)
  end

  def inspect
    "#{@value}#{@unit}"
  end

  protected

  def one_unit_a_to_b(unit_a, unit_b)
    conversion_rate = @conversion_rate_for_one_unit[unit_a][unit_b]
    format('%<value>f', value: conversion_rate).to_f
  end

  def convert_to(c_unit)
    one_unit_a_to_b(unit, c_unit) * value
  end
end