Published at Jul 29 2019
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Instructions

Test suite

Solution

Given a string of digits, calculate the largest product for a contiguous substring of digits of length n.

For example, for the input `'1027839564'`

, the largest product for a
series of 3 digits is 270 (9 * 5 * 6), and the largest product for a
series of 5 digits is 7560 (7 * 8 * 3 * 9 * 5).

Note that these series are only required to occupy *adjacent positions*
in the input; the digits need not be *numerically consecutive*.

For the input `'73167176531330624919225119674426574742355349194934'`

,
the largest product for a series of 6 digits is 23520.

For installation and learning resources, refer to the Ruby resources page.

For running the tests provided, you will need the Minitest gem. Open a terminal window and run the following command to install minitest:

```
gem install minitest
```

If you would like color output, you can `require 'minitest/pride'`

in
the test file, or note the alternative instruction, below, for running
the test file.

Run the tests from the exercise directory using the following command:

```
ruby largest_series_product_test.rb
```

To include color from the command line:

```
ruby -r minitest/pride largest_series_product_test.rb
```

A variation on Problem 8 at Project Euler http://projecteuler.net/problem=8

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

```
require 'minitest/autorun'
require_relative 'largest_series_product'
# Common test data version: 1.2.0 85da7a5
class LargestSeriesProductTest < Minitest::Test
def test_finds_the_largest_product_if_span_equals_length
# skip
assert_equal 18, Series.new('29').largest_product(2)
end
def test_can_find_the_largest_product_of_2_with_numbers_in_order
skip
assert_equal 72, Series.new('0123456789').largest_product(2)
end
def test_can_find_the_largest_product_of_2
skip
assert_equal 48, Series.new('576802143').largest_product(2)
end
def test_can_find_the_largest_product_of_3_with_numbers_in_order
skip
assert_equal 504, Series.new('0123456789').largest_product(3)
end
def test_can_find_the_largest_product_of_3
skip
assert_equal 270, Series.new('1027839564').largest_product(3)
end
def test_can_find_the_largest_product_of_5_with_numbers_in_order
skip
assert_equal 15120, Series.new('0123456789').largest_product(5)
end
def test_can_get_the_largest_product_of_a_big_number
skip
assert_equal 23520, Series.new('73167176531330624919225119674426574742355349194934').largest_product(6)
end
def test_reports_zero_if_the_only_digits_are_zero
skip
assert_equal 0, Series.new('0000').largest_product(2)
end
def test_reports_zero_if_all_spans_include_zero
skip
assert_equal 0, Series.new('99099').largest_product(3)
end
def test_rejects_span_longer_than_string_length
skip
assert_raises(ArgumentError) do
Series.new('123').largest_product(4)
end
end
def test_reports_1_for_empty_string_and_empty_product_0_span
skip
assert_equal 1, Series.new('').largest_product(0)
end
def test_reports_1_for_nonempty_string_and_empty_product_0_span
skip
assert_equal 1, Series.new('123').largest_product(0)
end
def test_rejects_empty_string_and_nonzero_span
skip
assert_raises(ArgumentError) do
Series.new('').largest_product(1)
end
end
def test_rejects_invalid_character_in_digits
skip
assert_raises(ArgumentError) do
Series.new('1234a5').largest_product(2)
end
end
def test_rejects_negative_span
skip
assert_raises(ArgumentError) do
Series.new('12345').largest_product(-1)
end
end
end
```

```
class Series
attr_reader :input, :series
def initialize(input)
@input = input
@series = get_series
raise ArgumentError, "Invalid numbers" unless valid?
end
def largest_product(n)
raise ArgumentError unless valid_cons?(n)
return 1 if n == 0
series.each_cons(n).inject(0) do |result, cons|
product = cons.inject(:*)
result = product > result ? product : result
end
end
private
def get_series
input.scan(/[\d]/).map(&:to_i)
end
def valid?
series.size == input.length
end
def valid_cons?(n)
n <= series.size
end
end
```

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

- What compromises have been made?
- Are there new concepts here that you could read more about to improve your understanding?

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