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to Largest Series Product in the Elixir Track

Published at Jul 13 2018 · 0 comments
Instructions
Test suite
Solution

Note:

This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

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.

Running tests

Execute the tests with:

$ elixir largest_series_product_test.exs

Pending tests

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.

Source

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

Submitting Incomplete Solutions

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

largest_series_product_test.exs

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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