Published at Jul 13 2018
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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:

```
$ elixir largest_series_product_test.exs
```

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
```

For more detailed information about the Elixir track, please see the help page.

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.

```
if !System.get_env("EXERCISM_TEST_EXAMPLES") do
Code.load_file("largest_series_product.exs", __DIR__)
end
ExUnit.start()
ExUnit.configure(exclude: :pending, trace: true)
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
```

```
defmodule Series do
@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(_ , 0), do: 1
def largest_product(number_string, size) do
if size < 0 || size > String.length(number_string), do: raise ArgumentError
do_largest_product(number_string
|> String.graphemes
|> Enum.map(&String.to_integer/1),
size,
0)
end
# take next N numbers and multiply, rather than divide by the
# oldest and multiply by the next, because there may be zeroes.
defp do_largest_product(list, size, highest) do
cur_numbers = list |> Enum.take(size)
if length(cur_numbers) == size do
do_largest_product(tl(list),
size,
Enum.max([highest,
cur_numbers |> Enum.reduce(&*/2)]))
else
highest
end
end
end
```

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