エクセルのカラム形式文字列(AAなど)を序数に変換

excoltoNum.rb

class String 
  def byte_num(index=0) 
    bytes.to_a[index] 
  end 
end 

def excoltoNum(str) # :nodoc: 
  return 0 if str.empty? 
  raise ArgumentError.new("string includes invalid charactor as column name") unless str =~ /^[a-zA-Z]+$/

  str.upcase.reverse.each_byte.with_index.inject(0) do |sum, (b, i)| 
    sum + (26 ** i) * (b - "A".byte_num).succ
  end
end 

if __FILE__ == $0 
puts excoltoNum("A")  #=> 1
puts excoltoNum("AA") #=> 27
puts excoltoNum("FX") #=> 180

## SPEC:
#describe "#excoltonum" do
#  it "は引数が空文字の時，0を返す" do
#    excoltoNum("").should == 0
#  end
#
#  it "はエクセルカラム形式の文字列を受け取って順数を返す" do
#    excoltoNum("A").should == 1
#    excoltoNum("B").should == 2
#    excoltoNum("Z").should == 26
#    excoltoNum("AA").should == 27
#    excoltoNum("AB").should == 28
#    excoltoNum("FX").should == 180
#    excoltoNum("AAA").should == 703
#  end
#
#  it "は引数が小文字でも正しく順数を返す" do
#    excoltoNum("a").should == 1
#    excoltoNum("b").should == 2
#    excoltoNum("z").should == 26
#    excoltoNum("aa").should == 27
#    excoltoNum("ab").should == 28
#    excoltoNum("fx").should == 180
#    excoltoNum("aaa").should == 703
#  end
#
#  it "は引数がアルファベット以外が含まれている時，例外を投げる" do
#    proc { excoltoNum("日本語") }.should raise_error(ArgumentError)
#    proc { excoltoNum("A-") }.should raise_error(ArgumentError)
#  end
#end
end