Spreadsheet column calculation in Ruby

xlsx_columns.rb

# frozen_string_literal: true

class XlsxColumns
  attr_reader :position
  ALPHABET = ("A".."Z").to_a

  class << self
    def column(position)
      if position > max_position
        raise ArgumentError, "Position must be #{max_position} or lower"
      elsif position <= ALPHABET.length
        letter_from_position(position)
      else
        letter_with_prefix(position)
      end
    end

    private

    def letter_with_prefix(position)
      prefix_index = position / letter_count
      adjusted_position = position % letter_count

      prefix = letter_from_position(prefix_index)
      letter = letter_from_position(adjusted_position)

      prefix + letter
    end

    # It would be possible to make this recursive, but not really necessary.
    def max_position
      letter_count * letter_count
    end

    def letter_from_position(position)
      ALPHABET[position - 1]
    end

    def letter_count
      ALPHABET.length
    end
  end
end

xlsx_columns_spec.rb

# frozen_string_literal: true

require "spec_helper"

require "xlsx_columns"

RSpec.describe XlsxColumns do
  describe "#column" do
    context "when position below 26" do
      it "returns the corresponding letter" do
        expect(described_class.column(3)).to eql("C")
        expect(described_class.column(26)).to eql("Z")
      end
    end

    context "when position is above 26" do
      it "returns a combination of letters" do
        expect(described_class.column(27)).to eql("AA")
        expect(described_class.column(56)).to eql("BD")
        expect(described_class.column(456)).to eql("QN")
        expect(described_class.column(676)).to eql("ZZ")
      end
    end
  end
end