Returns the highest product from a string of numbers for the given series length (defaulted to 3). For example, the string "123456789" with a series length of 3 returns 504 (7 * 8 * 9). If I adjust the series length to 4, it returns 3024 (6 * 7 * 8 * 9).

largest_series_product.rb

def largest_series_product(number, span_length=3)
  if number.match(/\D/)
    raise ArgumentError.new("Numbers only")
  elsif number.length < span_length
    raise ArgumentError.new("The number must have at least #{span_length} digits")
  end
  product_finder(series_splitter(number, span_length)).max
end

def series_splitter(number, span_length)
  ranges = []
  (number.length - span_length + 1).times do |i|
    ranges << number[i, span_length]
  end 
  ranges
end

def product_finder(spans)
  spans.map do |span|
    span.split("").inject(1) do |product, n|
      product * n.to_i
    end
  end
end

largest_series_product_spec.rb

require 'rspec'
require 'largest_series_product'

describe 'largest_series_product' do
  it 'returns the product of the digits of a three digit number string' do
    largest_series_product('256').should eq 60
  end

  it 'rejects any number shorter than 3' do
    expect { largest_series_product('25') }.to raise_error
  end

  it 'rejects any string that contains non-numbers' do
    expect { largest_series_product('foo525') }.to raise_error
    expect { largest_series_product('123 123') }.to raise_error
    expect { largest_series_product('132-4433%') }.to raise_error
  end

  it 'returns the highest series product' do
    largest_series_product("12345").should eq 60
  end

  it 'can take an optional argument of span width' do
    largest_series_product("12342", 2).should eq 12
  end

  it 'can handle a large string of numbers' do
    largest_series_product('73167176531330624919225119674426574742355349194934', 6).should eq 23520
  end
end

describe 'series_splitter' do
  it 'splits a number string into a 3 digit series' do
    series_splitter("12345", 3).should eq ["123", "234", "345"]
  end
end

describe 'product_finder' do
  it 'returns an array of digit products' do
    product_finder(["123", "234", "345"]).should eq [6, 24, 60]
  end
end