Converts a binary (or trinary) number string to decimal. There's only one character difference between the two functions (besides the exceptions).

binary.rb

def binary(binary_string)
  if !(binary_string.is_a?(String)) || binary_string.match(/[^0,1]/)
    raise TypeError.new("Input must be only 0s and 1s") 
  end

  total = 0
  binary_string.reverse.split("").each_with_index do |bit, index|
    total += bit.to_i * 2 ** index
  end
  total
end

def trinary(trinary_string)
  if !(trinary_string.is_a?(String)) || trinary_string.match(/[^0,1,2]/)
    raise TypeError.new("Input must be only 0s and 1s and 2s") 
  end

  total = 0
  trinary_string.reverse.split("").each_with_index do |bit, index|
    total += bit.to_i * 3 ** index
  end
  total
end

binary_spec.rb

require 'rspec'
require 'binary'

describe 'binary' do 
  it 'converts 0 in binary to 0' do
    binary("0").should eq 0
  end

  it 'converts longer binary bits' do
    binary("101").should eq 5
  end

  it 'raises an exception if it gets a non-binary number' do
    expect { binary("12") }.to raise_exception
  end
end

describe 'trinary' do
  it 'converts trinary numbers to decimal' do
    trinary("1021").should be 34
  end
end