Published at Aug 06 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.

Execute the tests with:

```
$ mix test
```

In the test suites, all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by
commenting out the relevant `@tag :pending`

with a `#`

symbol.

For example:

```
# @tag :pending
test "shouting" do
assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"
end
```

Or, you can enable all the tests by commenting out the
`ExUnit.configure`

line in the test suite.

```
# ExUnit.configure exclude: :pending, trace: true
```

If you're stuck on something, it may help to look at some of the available resources out there where answers might be found.

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.

```
defmodule LargestSeriesProductTest do
use ExUnit.Case
# @tag :pending
test "largest product of 2" do
assert Series.largest_product("0123456789", 2) == 72
end
@tag :pending
test "largest product of a tiny number" do
assert Series.largest_product("12", 2) == 2
end
@tag :pending
test "another tiny number" do
assert Series.largest_product("19", 2) == 9
end
@tag :pending
test "largest product of 2 shuffled" do
assert Series.largest_product("576802143", 2) == 48
end
@tag :pending
test "largest product of 3" do
assert Series.largest_product("0123456789", 3) == 504
end
@tag :pending
test "largest product of 3 shuffled" do
assert Series.largest_product("1027839564", 3) == 270
end
@tag :pending
test "largest product of 5" do
assert Series.largest_product("0123456789", 5) == 15120
end
@tag :pending
test "some big number" do
assert Series.largest_product("73167176531330624919225119674426574742355349194934", 6) ==
23520
end
@tag :pending
test "some other big number" do
assert Series.largest_product("52677741234314237566414902593461595376319419139427", 6) ==
28350
end
@tag :pending
test "number with all zeroes" do
assert Series.largest_product("0000", 2) == 0
end
@tag :pending
test "number where all products are zero" do
assert Series.largest_product("99099", 3) == 0
end
@tag :pending
test "identity with empty string" do
assert Series.largest_product("", 0) == 1
end
@tag :pending
test "identity with non-empty string" do
assert Series.largest_product("123", 0) == 1
end
@tag :pending
test "raises if span is too large" do
assert_raise ArgumentError, fn ->
Series.largest_product("123", 4)
end
end
@tag :pending
test "raises with empty string but non-zero span size" do
assert_raise ArgumentError, fn ->
Series.largest_product("", 1)
end
end
@tag :pending
test "raises with non-empty string and negative span size" do
assert_raise ArgumentError, fn ->
Series.largest_product("1234", -1)
end
end
end
```

```
ExUnit.start()
ExUnit.configure(exclude: :pending, trace: true)
```

```
defmodule Series do
defguardp span_too_large?(number_string, size)
when size > byte_size(number_string)
defguardp non_zero_size_on_empty_string?(number_string, size)
when size != 0 and number_string == ""
defguardp negative_size_on_non_empty_string?(number_string, size)
when size < 0 and number_string != ""
defguardp invalid?(number_string, size)
when span_too_large?(number_string, size) or
non_zero_size_on_empty_string?(number_string, size) or
negative_size_on_non_empty_string?(number_string, size)
@zero_series_product 1
@doc """
Finds the largest product of a given number of consecutive numbers in a given string of numbers.
"""
@spec largest_product(String.t(), non_neg_integer) :: non_neg_integer
def largest_product(number_string, size) when invalid?(number_string, size) do
raise ArgumentError
end
def largest_product(_number_string, 0), do: @zero_series_product
def largest_product(number_string, size) do
number_string
|> String.graphemes()
|> Enum.map(&String.to_integer/1)
|> Enum.chunk_every(size, 1, :discard)
|> Enum.reduce(0, &compare_products/2)
end
defp compare_products(subseries, acc) do
product = Enum.reduce(subseries, &*/2)
if product > acc, do: product, else: acc
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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