The bijective base-26 system

bb26.rb

def naive(n)
  s = ?A
  (n-1).times { s.succ! }
  s
end

def digits(n, base=10)
  ds = []
  while n > 0
    case n.divmod(base)
    in [n, mod]
      ds << mod
    end
  end
  ds
end

AZ = [*?A..?Z].freeze

def to_bb26(n)
  digits(n, 26).map { AZ[_1-1] }.reverse.join
end