Feature #8850

Updated by Nobuyoshi Nakada over 1 year ago

On Ruby 2.1.0, decimal literal is introduced.
It generates Rational but it cannot easily convert to decimal string.
You know, Rational#to_f is related to this but
* Float is not exact number
** 0.123456789123456789r.to_f.to_s #=> "0.12345678912345678"
* it can't handle recursive decimal
** (151/13r).to_f.to_s #=> "11.615384615384615"

* the method name
** to_decimal
** to_decimal_string
** to_s(format: :decimal)
** extend sprintf
* how does it express recursive decimal
** (151/13r).to_decimal_string #=> "11.615384..."
** (151/13r).to_decimal_string #=> "11.(615384)"

Example implementation is following.
Its result is
** 0.123456789123456789r.to_f.to_s #=> "0.123456789123456789"
** (151/13r).to_f.to_s #=> "11.(615384)"

```
class Rational
def to_decimal_string(base=10)
n = numerator
d = denominator
r, n = n.divmod d
str = r.to_s(base)
return str if n == 0
h = {}
str << '.'
n *= base
str.size.upto(Float::INFINITY) do |i|
r, n = n.divmod d
if n == 0
str << r.to_s(base)
break
elsif h.key? n
str[h[n], 0] = '('
str << ')'
break
else
str << r.to_s(base)
h[n] = i
n *= base
end
end
str
end
end
```

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