Solution for largest series product

largest-series-product/README.md

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+# Largest Series Product
+
+Write a program that, when given a string of digits, can calculate the largest product for a series of consecutive digits of length n.
+
+For example, for the input `'0123456789'`, the largest product for a
+series of 3 digits is 504 (7 * 8 * 9), and the largest product for a
+series of 5 digits is 15120 (5 * 6 * 7 * 8 * 9).
+
+For the input `'73167176531330624919225119674426574742355349194934'`,
+the largest product for a series of 6 digits is 23520.
+
+## Running tests
+
+```bash
+$ elixir bob_test.exs
+```
+
+(Replace `bob_test.exs` with the name of the test file.)
+
+## Source
+
+A variation on Problem 8 at Project Euler [view source](http://projecteuler.net/problem=8)

largest-series-product/largest_series_product.exs

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+defmodule Series do
+
+  @doc """
+  Splits up the given string of numbers into an array of integers.
+  """
+  @spec digits(String.t) :: [non_neg_integer]
+  def digits(number_string) do
+    number_string |> String.codepoints
+                  |> Enum.map(&(elem(Integer.parse(&1), 0)))
+  end
+
+  @doc """
+  Generates sublists of a given size from a given string of numbers.
+  """
+  @spec slices(String.t, non_neg_integer) :: [list(non_neg_integer)]
+  def slices(number_string, size) do
+    guard = String.length(number_string) - size
+
+    list = number_string |> digits |> Enum.with_index
+
+    list |> Enum.map(fn(x) ->
+              if elem(x, 1) <=  guard do
+                [elem(x, 0)] ++ ((1..(size-1)) |> Enum.map(fn (y) ->
+                    elem(Enum.at(list, elem(x, 1) + y), 0)
+                  end))
+              end
+            end)
+         |> Enum.take(guard+1)
+  end
+
+  @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) do
+    if size == 0 do
+      1
+    else
+      number_string |> slices(size)
+                    |> Enum.map(fn(x) ->
+                      x |> Enum.reduce(1, fn(y, acc) -> y * acc end)
+                    end)
+                    |> Enum.max
+    end
+  end
+end

largest-series-product/largest_series_product_test.exs

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+if System.get_env("EXERCISM_TEST_EXAMPLES") do
+  Code.load_file("example.exs")
+else
+  Code.load_file("largest_series_product.exs")
+end
+
+ExUnit.start
+
+defmodule LargestSeriesProductTest do
+  use ExUnit.Case, async: false
+
+  test "digits" do
+    assert Series.digits("0123456789") == [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+  end
+
+  test "same digits reversed" do
+    assert Series.digits("9876543210") == [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
+  end
+
+  test "fewer digits" do
+    assert Series.digits("87654") == [8, 7, 6, 5, 4]
+  end
+
+  test "some other digits" do
+    assert Series.digits("936923468") == [9, 3, 6, 9, 2, 3, 4, 6, 8]
+  end
+
+  test "slices of zero" do
+    assert Series.digits("") == []
+  end
+
+  test "slices of two" do
+    assert Series.slices("01234", 2) == [[0, 1], [1, 2], [2, 3], [3, 4]]
+  end
+
+  test "other slices of two" do
+    assert Series.slices("98273463", 2) == [[9, 8], [8, 2], [2, 7], [7, 3], [3, 4], [4, 6], [6, 3]]
+  end
+
+  test "slices of three" do
+    assert Series.slices("01234", 3) == [[0, 1, 2], [1, 2, 3], [2, 3, 4]]
+  end
+
+  test "other slices of three" do
+    assert Series.slices("982347", 3) == [[9, 8, 2], [8, 2, 3], [2, 3, 4], [3, 4, 7]]
+  end
+
+  test "largest product of 2" do
+    assert Series.largest_product("0123456789", 2) == 72
+  end
+
+  test "largest product of a tiny number" do
+    assert Series.largest_product("12", 2) == 2
+  end
+
+  test "another tiny number" do
+    assert Series.largest_product("19", 2) == 9
+  end
+
+  test "largest product of 2 shuffled" do
+    assert Series.largest_product("576802143", 2) == 48
+  end
+
+  test "largest product of 3" do
+    assert Series.largest_product("0123456789", 3) == 504
+  end
+
+  test "largest product of 3 shuffled" do
+    assert Series.largest_product("1027839564", 3) == 270
+  end
+
+  test "largest product of 5" do
+    assert Series.largest_product("0123456789", 5) == 15120
+  end
+
+  test "some big number" do
+    assert Series.largest_product("73167176531330624919225119674426574742355349194934", 6) == 23520
+  end
+
+  test "some other big number" do
+    assert Series.largest_product("52677741234314237566414902593461595376319419139427", 6) == 28350
+  end
+
+  test "identity" do
+    assert Series.largest_product("", 0) == 1
+  end
+end