module UnitConversion

  INTEGER_REGEX = /-?[0-9]+/
  DECIMAL_REGEX = /(?:-?[0-9]+)?\.[0-9]+/
  RATIONAL_REGEX = /-?(?:[0-9]+ )?[0-9]+\/[0-9]+/
  UNIT_REGEX = /[\[\]a-zA-Z][\[\]a-zA-Z\/.\-()0-9]*/
  UNIT_PARSING_REGEX = /^(?<value>#{INTEGER_REGEX}|#{DECIMAL_REGEX}|#{RATIONAL_REGEX})\s+(?<unit>#{UNIT_REGEX})$/

  UNIT_ALIASES = {
      '[cup_us]': %w(cup cups c),
      '[tbs_us]': %w(tablespoon tablespoons tbsp tbs),
      '[tsp_us]': %w(teaspoon teaspoons tsp),
      '[oz_av]': %w(ounce ounces oz),
      '[lb_av]': %w(pound pounds lb lbs),
      '[pt_us]': %w(pint pints),
      '[qt_us]': %w(quart quarts qt),
      '[gal_us]': %w(gallon gallons ga),

      g: %w(gram grams),
      kg: %w(kilograms kilogram),
      ml: %w(milliliter milliliters),
      l: %w(liter liters)
  }

  UNIT_ALIAS_MAP = Hash[UNIT_ALIASES.map { |unit, values| values.map { |v| [v, unit] } }.flatten(1)]

  # map that gives comfortable ranges for a given set of units
  UNIT_RANGES = {
      '[tsp_us]': (0.125...3.0),
      '[tbs_us]': (0.5...4.0),
      '[cup_us]': (0.25...4.0),
      '[qt_us]': (1.0...4.0),
      '[gal_us]': (0.5..9999),

      '[oz_av]': (0..16),
      '[lb_av]': (1.0..9999),

      g: (1.0..750),
      kg: (0.5..9999),

      ml: (1.0..750),
      l: (0.5..9999)
  }

  UNIT_ORDERS = {
      standard_volume: [
          :'[tsp_us]',
          :'[tbs_us]',
          :'[cup_us]',
          :'[qt_us]',
          :'[gal_us]'
      ],

      metric_volume: [
          :ml,
          :l
      ],

      standard_mass: [
          :'[oz_av]',
          :'[lb_av]'
      ],

      metric_mass: [
          :g,
          :kg
      ]
  }

  class UnparseableUnitError < StandardError
  end

  class UnknownUnitError < UnparseableUnitError
  end

  class << self

    def parse(unit_string)
      match = UNIT_PARSING_REGEX.match(unit_string.to_s.strip)

      if match && match[:value].present? && match[:unit].present?
        begin
          value = get_value(match[:value])
          Unitwise(value, normalize_unit_names(match[:unit]))
        rescue Unitwise::ExpressionError => err
          raise UnknownUnitError, err.message
        end
      else
        raise UnparseableUnitError, "'#{unit_string}' does not appear to be a valid measurement of the form <value> <units> (ie '5 cup' or '223 gram/cup')"
      end
    end

    def density?(unit)
      unit.compatible_with? Unitwise(1, 'g/ml')
    end

    def volume?(unit)
      unit.compatible_with? Unitwise(1, 'ml')
    end

    def mass?(unit)
      unit.compatible_with? Unitwise(1, 'g')
    end

    # Returns a Unitwise representation of the density.  Raises an exception if the value is anything other than a
    # valid density
    def get_density(str)
      raise UnknownUnitError, 'No density provided' if str.blank?
      begin
        unit = parse(str)
      rescue UnparseableUnitError => err
        raise UnknownUnitError "Invalid density: #{err.message}"
      end

      raise UnknownUnitError, "Invalid density: #{str} is not a density" unless density?(unit)
      unit
    end

    def normalize_unit_names(unit_description)
      unit_description.downcase.gsub(/[a-z]+/) do |match|
        UNIT_ALIAS_MAP[match] || match
      end
    end

    def get_value(str)
      if str =~ /^#{INTEGER_REGEX}$/
        str.to_i
      elsif str =~ /^#{RATIONAL_REGEX}$/
        parts = str.split(' ')
        if parts.length == 2
          whole = parts.first.to_r
          fractional = parts.last.to_r
          (whole.abs + fractional) * (whole < 0 ? -1.to_r : 1.to_r)
        else
          str.to_r
        end
      elsif str =~ /^#{DECIMAL_REGEX}$/
        str.to_d
      else
        raise UnparseableUnitError, "str (#{str}) is not a valid number"
      end
    end

    def convert(quantity, factor, input_unit, output_unit, density = nil)

      value = get_value(quantity)
      factor = get_value(factor)

      if value.is_a?(BigDecimal) && factor.is_a?(Rational)
        factor = factor.to_d(10)
      end

      converted = value * factor

      input_unit = normalize_unit_names(input_unit) unless input_unit.nil?
      output_unit = normalize_unit_names(output_unit) unless output_unit.nil?

      if input_unit.present? && output_unit.present? && input_unit != output_unit
        in_unit = Unitwise(1, input_unit)
        out_unit = Unitwise(1, output_unit)
        unit = Unitwise(converted, input_unit)

        if volume?(in_unit) && mass?(out_unit)
          unit_density = get_density(density)
          unit = unit * unit_density
        elsif mass?(in_unit) && volume?(out_unit)
          unit_density = get_density(density)
          unit = unit / unit_density
        end

        converted = unit.convert_to(output_unit).value
      end

      quantity_format(converted, value)
    end

    def quantity_format(quantity, original_quantity)
      if original_quantity.is_a?(Rational) || original_quantity.is_a?(Integer)
        rational_val = rationalize(quantity)
        if rational_val.denominator == 1
          rational_val.to_i.to_s
        elsif rational_val.denominator < rational_val.numerator.abs
          whole = rational_val.floor
          fraction = rational_val - whole
          "#{whole} #{fraction}"
        else
          rational_val.to_s
        end
      else
        "%g" % ("%.3f" % quantity)
      end
    end

    def rationalize(value)
      if value.is_a? Integer
        return value
      end

      if value.floor == value
        return value
      end

      useful_denominators = [2, 3, 4, 8, 16]

      if value.is_a?(Rational) && useful_denominators.include?(value.denominator)
        return value
      end

      whole = value.floor
      fraction = value - whole

      approximations = useful_denominators.map do |d|
        best_n = (1...d).each.map { |n| {delta: (fraction - Rational(n, d)).abs, numerator: n} }.sort { |a, b| a[:delta] <=> b[:delta] }.first
        {denominator: d, numerator: best_n[:numerator], delta: best_n[:delta]}
      end

      # Add 0 and 1
      approximations << { denominator: 1, numerator: 0, delta: (fraction - Rational(0,1)).abs }
      approximations << { denominator: 1, numerator: 1, delta: (fraction - Rational(1,1)).abs }

      best = approximations.sort { |a, b| [a[:delta], a[:denominator]] <=> [b[:delta], b[:denominator]] }.first

      Rational((whole * best[:denominator]) + best[:numerator], best[:denominator])
    end

    # Given an awkward measurement such as '18 9/10 oz' or '5/24 cup' or '0.09434 cup',
    # it will round the rational and scale the unit to give a more reasonable, useful measurement, ie
    # 19 oz, 3 Tbsp, 1 1/2 Tbsp
    def auto_unit(quantity, units)
      normalized_unit = normalize_unit_names(units)
      value = initial_value = get_value(quantity)
      type_key, unit_orders = UNIT_ORDERS.detect { |key, orders| orders.include?(normalized_unit.to_sym) }
      new_unit = normalized_unit

      if unit_orders && (unit_range = UNIT_RANGES[normalized_unit.to_sym])

        if value < unit_range.first
          unit_orders = unit_orders.reverse
        end

        idx = unit_orders.index(new_unit.to_sym)

        while !unit_range.include?(value) && idx < (unit_orders.length - 1)
          idx += 1
          next_unit = unit_orders[idx]
          value = Unitwise(value, new_unit).convert_to(next_unit).value.round(4)
          new_unit = next_unit.to_s
          unit_range = UNIT_RANGES[new_unit.to_sym]
        end
      end

      if normalized_unit == new_unit
        new_unit = units
      else
        new_unit = (UNIT_ALIASES[new_unit.to_sym] || []).first || new_unit
      end

      return quantity_format(value, initial_value), new_unit
    end

  end
end