POC of accurate_to_s

accurate_to_s.rb

require "bigdecimal"

class Float
    HEX_FORMAT = /(-?)0x([01])\.?([0-9a-f]*)p([+-][0-9]*)/
    BIG_DECIMAL_2 = BigDecimal(2)

    def accurate_to_s
        bin_str = "%a" % [self]
        mat = HEX_FORMAT.match bin_str
        sgn, msbit, bits, ex = mat[1, 4]
        if msbit == "0" then
            unless bits == "" and ex == "+0" then
                raise "assertion error"
            end
            return "#{sgn}0"
        end
        result = BigDecimal((sgn+msbit+bits).to_i 16)
        ex = ex.to_i - bits.length * 4
        case ex <=> 0
        when 1 then  # ex > 0
            result *= 2**ex
        when -1 then  # ex < 0
            result *= BIG_DECIMAL_2**ex
        end
        return result.to_s
    end
end

class Rational
    BIG_DECIMAL_2 = BigDecimal(2)
    BIG_DECIMAL_5 = BigDecimal(5)

    def accurate_to_s
        deno = self.denominator
        if deno == 1 then
            return self.numerator.to_s
        end
        tmp = deno
        t_2 = 0
        t_5 = 0
        loop {
            q, r = tmp.divmod 2

            break if r != 0

            tmp = q
            t_2 += 1
        }
        loop {
            q, r = tmp.divmod 5

            break if r != 0

            tmp = q
            t_5 += 1
        }
        if tmp != 1 then
            return nil
        end
        result = BigDecimal(self.numerator)
        result *= BIG_DECIMAL_2**-t_2
        result *= BIG_DECIMAL_5**-t_5
        return result.to_s
    end
end

alias BigDecimal_ORIG BigDecimal

def BigDecimal val, *args
    if args == [nil] then
        if val.respond_to? :accurate_to_s then
            result = val.accurate_to_s
            if result then
                return BigDecimal_ORIG(result)
            end
        end
    end
    BigDecimal_ORIG val, *args
end

if $0 == __FILE__
    x = 0.1
    puts BigDecimal(x, nil).to_s
    puts BigDecimal(Rational(x), nil).to_s
end

http://bugs.ruby-lang.org/issues/8295