仕様

例えば (1, 1) を A1 に、(2, 26) を Z2 に変換する。

require 'minitest/autorun'

class CellAddressTest < Minitest::Test
  def test_to_s
    assert_equal('A1', CellAddress.new(row: 1, column: 1).to_s)
    assert_equal('Z2', CellAddress.new(row: 2, column: 26).to_s)
    assert_equal('AA3', CellAddress.new(row: 3, column: 27).to_s)
    assert_equal('AAA100', CellAddress.new(row: 100, column: 703).to_s)
  end
end

実装

class CellAddress
  ALPHABETS = [*'A'..'Z'].freeze

  attr_reader :row_number, :column_number

  def initialize(row:, column:)
    @row_number = row
    @column_number = column
  end

  def to_s
    "#{column_letter}#{row_number}"
  end

  private

  def column_letter
    return nil unless column_number > 0

    q = column_number
    letters = []

    until q.zero?
      # 商は英語で quotient、余りは remainder というらしい。
      q, r = (q - 1).divmod(ALPHABETS.length)
      letters.push(ALPHABETS[r])
    end

    letters.reverse.join
  end
end