Converts 15 character Salesforce IDs to 18 character Salesforce IDs (adds checksum)

sf_id_converter.rb

CHECKSUM_CHARS = ((?A..?Z).to_a + (0..5).to_a).join

# String -> String
def id_with_checksum(sfid)
  # protection from bad people who misuse things
  return sfid if sfid.length == 18
  raise ArgumentError.new("sfid must be 15 characters") unless sfid.length == 15

  # The meat.
  suffix = sfid.chars \ # String -> [String]
               .each_slice(5) \ # [String] -> [[String]]
               .map { |chars| chars.reverse.map { |c| ("A".."Z").include?(c) ? "1" : "0" }.join.to_i(2) } \ # [[String]] -> [Int]
               .map { |i| CHECKSUM_CHARS[i] } \ # [Int] -> [String]
               .join # [String] -> String
  sfid + suffix
end

The fact that you had to write this in the first place makes Salseforce a bowl of fail. Neat though!