Parslet で漢数字を Int にするやつ

kansuji.rb

require 'parslet'

module Kansuji
  NUMBERS = %w{一 二 三 四 五 六 七 八 九}
  DIGITS  = %w{十 百 千}
  UNITS   = %w{万 億 兆 京 垓 𥝱 穣 溝 澗 正 載 極 恒河沙 阿僧祇 那由他 不可思議 無量大数}

  def self.parser
    @parser ||= Parser.new
  end

  def self.transform
    @transform ||= Transform.new
  end

  def self.parse(text)
    transform.apply(
      parser.parse(
        text
      )
    )
  end

  class Parser < Parslet::Parser
    root(:expressions)

    rule(:expressions) {
      (
        expression >> unit.maybe >> expressions.maybe |
        expression
      ).as(:expressions)
    }

    rule(:expression) {
      (
        number.maybe >> digit >> expression.maybe |
        number
      ).as(:expression)
    }

    rule(:number) {
      alternative_str(NUMBERS).as(:number)
    }

    rule(:digit) {
      alternative_str(DIGITS).as(:digit)
    }

    rule(:unit) {
      alternative_str(UNITS).as(:unit)
    }

    def alternative_str(values)
      values.map { |value|
        str(value)
      }.reduce { |alternatives, parslet|
        alternatives | parslet
      }
    end
  end

  class Transform < Parslet::Transform
    rule(expressions: simple(:expressions)) {
      expressions
    }

    rule(expression: simple(:expression)) {
      expression
    }

    rule(number: simple(:number)) {
      Number.new(number).to_i
    }

    rule(digit: simple(:digit)) {
      Digit.new(digit).to_i
    }

    rule(number: simple(:number), digit: simple(:digit)) {
      Number.new(number).to_i * Digit.new(digit).to_i
    }

    rule(digit: simple(:digit), expression: simple(:expression)) {
      Digit.new(digit).to_i + expression
    }

    rule(number: simple(:number), digit: simple(:digit), expression: simple(:expression)) {
      Number.new(number).to_i * Digit.new(digit).to_i + expression
    }

    rule(expression: simple(:expression), unit: simple(:unit), expressions: simple(:expressions)) {
      expression * Unit.new(unit).to_i + expressions
    }
  end

  class Number < Struct.new(:number)
    def to_i
      NUMBERS.index(number) + 1
    end
  end

  class Digit < Struct.new(:digit)
    def to_i
      10 ** (DIGITS.index(digit) + 1)
    end
  end

  class Unit < Struct.new(:unit)
    def to_i
      10_000 ** (UNITS.index(unit) + 1)
    end
  end
end

参考: 漢数字を数値に変換する(rparsec版) - terazzoの日記