How to convert a Zip9 or Zip4 to a Zip5 if you're missing leading zeros

require 'minitest/autorun'

describe 'zip5 disambiguation' do
  {
    999501234  => '99950',
  '99950-1234' => '99950',
     57531234  => '05753',
   '5753-1234' => '05753',
  '05753-1234' => '05753',
      5011234  => '00501',
    99950      => '99950',
     5753      => '05753',
     1000      => '01000',
      501      => '00501',
  }.each do |input, expected|
    it "parses #{input.inspect} as #{expected.inspect}" do
      zip5(input).must_equal expected
    end
    # here we also test 57531234.0, etc.
    if input.is_a?(Integer)
      input_f = input.to_f
      it "parses #{input_f.inspect} as #{expected.inspect}" do
        zip5(input_f).must_equal expected
      end
    end
  end
end

def zip5(input)
  input = input.to_i.to_s.delete('-')
  zip5 = case input.length
  when 9
    input[0,5]
  when 8
    '0' + input[0,4]
  when 7
    '00' + input[0,3]
  when 5
    input
  when 4
    '0' + input
  when 3
    '00' + input
  else
    raise "can't derive zip5 from #{input.inspect}"
  end
  unless (501..99950).include?(zip5.to_i)
    $stderr.puts "warning: looks like a bad zip5 (expected 00500..99950): #{zip5}"
  end
  zip5
end

Run options: --seed 32751

## Running:

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