[ruby-core:108461] [Ruby master Bug#18762] Add an Array#undigits that compliments Integer#digits

Issue #18762 has been reported by shan (Shannon Skipper).

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Bug #18762: Add an Array#undigits that compliments Integer#digits
https://bugs.ruby-lang.org/issues/18762

* Author: shan (Shannon Skipper)
* Status: Open
* Priority: Normal
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
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I've found Integer#digits convenient and useful but several times have needed to go from the place-value notation back to an Integer after manipulation and wished there as a complimentary Array#undigits.

``` ruby
class Array
  def undigits(base = 10)
    each_with_index.sum do |digit, exponent|
      digit * base**exponent
    end
  end
end

42.digits.undigits
#=> 42

42.digits(16).undigits(16)
#=> 42
```

Below is my stab at a Ruby implementation with behavior mirroring Integer#digits.
``` ruby
class Array
  def undigits(base = 10)
    base_int = base.to_int
    raise TypeError, "wrong argument type #{base_int.class} (expected Integer)" unless base_int.is_a?(Integer)
    raise ArgumentError, 'negative radix' if base_int.negative?
    raise ArgumentError, "invalid radix #{base_int}" if base_int < 2

    each_with_index.sum do |digit, exponent|
      raise MathDomainError, 'out of domain' if digit.negative?

      digit * base_int**exponent
    end
  end
end
```




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