excel_columns_convertions.rb

module ExcelColumnsConverions

  # convert the column name ("A", "ACB", ...) to an number (1-based)
  def excel_col_number(str)
    offset = 'A'.ord - 1
    str.chars.inject(0){ |x,c| x*26 + c.ord - offset }.to_i
  end

  # convert the column number to a string ("A", "ACB", ...)
  def excel_col_string(number)
    # limit: 16.384
    raise "Number must be less than or equal to 16384" if number>16384
    n = number-1

    d3 = (n-26*27)/(26*26)
    d3 = d3<0 ? -1 : d3

    d2 = (n-26)/26
    d2 = d2<0 ? -1 : d2%26

    d1 = n%26

    arr = ("A".."Z").to_a
    arr_w_space = [""]+arr

    [arr_w_space[d3+1], arr_w_space[d2+1], arr[d1]].join
  end


  def check(max)
    1.upto(max) do |n|
      s = excel_col_string(n)
      nn = excel_col_index(s)
      puts "#{n} -> '#{s}' -> #{nn}" if n!=nn
    end
  end

end

excel_col_numberfunction copied from https://stackoverflow.com/questions/26771129/convert-excel-column-letter-to-integer-in-ruby