a.rb

# -- coding: utf-8

require "rubygems"
require "rspec"

describe do
  def to_num(chars)
    table = ("A".."Z").to_a
    chars = chars.split(//u)
    count = 1
    chars.reverse.inject(0){|r, c|
      r += (table.index(c)+1) * count
      count *= 26
      r
    }
  end

  it "A == 1" do
    to_num("A").should == 1
  end

  it "B == 2" do
    to_num("B").should == 2
  end

  it "Z == 26" do
    to_num("Z").should == 26
  end

  it "AA == 27" do
    to_num("AA").should == 27
  end

  it "XFD == 16384" do
    to_num("XFD").should == 16384
  end
end