W1/6 - Divisible Sum Pairs (#151)

Penultimate challenge to finalises #117 and closes #121

Author: jamiecurle

w1/README.md

@@ -62,7 +62,7 @@ Count the number of (i, j) index pairs such that i < j and the sum of arr[i] + a
 
 | Python | Elixir | TypeScript | Go | 
 |--------|--------|------------|----|
-|    ✅   |  ➖   |    ➖      |  ➖    |
+|   ✅   |   ✅   |    ➖      |  ➖    |
 
 
 ⸻

w1/divisible_pairs.ex

@@ -0,0 +1,56 @@
+defmodule HackerRank.W1.DivisibleSumPairs do
+  @moduledoc """
+  Divisible Sum Pairs - Challenge 6, Week 1.
+  <https://www.hackerrank.com/challenges/three-month-preparation-kit-divisible-sum-pairs/problem>
+  """
+
+  @doc """
+  Count the number of (i, j) index pairs such that i < j and the sum of arr[i] + arr[j] is divisible by a given integer k.
+
+  ## Parameters
+
+  - _n (integer): The count of items in the supplied array (unused)
+  - k (integer): the integer to be used as the divisor
+  - ar (list[integers]): the array of integers to work on
+
+  Returns:
+  - integer: the number of pairs that are divisible by k
+
+  Example:
+    iex> DivisibleSumPairs.challenge(6, 5, [1, 3, 2, 6, 1, 2])
+    2
+  """
+
+  @spec challenge(integer(), integer(), list(integer())) :: integer()
+  def challenge(_n, k, arr) do
+    # This is a recursy' thing
+    count_pairs(k, arr, %{}, 0)
+  end
+
+  # Tail recursive
+  defp count_pairs(_k, [],_fmap, count), do: count
+  defp count_pairs(k, arr, fmap, count) do
+    # get the value
+    [value | rest] = arr
+
+    # get the mod
+    mod = rem(value, k)
+
+    # get the complement or zero if mod is zero
+    comp = if mod == 0, do: 0, else: k - mod
+
+    # increment count by the frequency of the complement
+    count =
+      case Map.get(fmap, comp) do
+        nil -> count
+        hit -> count + hit
+      end
+
+    # update the frequency map
+    fmap = Map.update(fmap, mod, 1, &(&1 + 1))
+
+    # now go again
+    count_pairs(k, rest, fmap, count)
+
+  end
+end

w1/divisible_pairs_test.exs

@@ -0,0 +1,30 @@
+defmodule HackerRank.W1.DivisibleSumPairsTests do
+  use ExUnit.Case
+  alias HackerRank.Test.Helper
+  alias HackerRank.W1.DivisibleSumPairs
+  doctest HackerRank.W1.DivisibleSumPairs
+
+
+  @pattern "w1/tc/6_*txt"
+
+  test "run the tests" do
+    Helper.test_all(@pattern, &parse_args/1, &parse_expected/1 )
+    |> Enum.each(fn [[n, k, arr], expected] ->
+       assert DivisibleSumPairs.challenge(n, k, arr) == expected
+    end)
+  end
+
+  def parse_args(raw) do
+    # convert raw from a string ints seperated by spaces
+    [[n, k], arr] =
+      raw
+      |> Enum.map(&String.split/1)
+      |> Enum.map(fn args -> Enum.map(args, &String.to_integer/1) end)
+    [n, k, arr]
+  end
+
+  def parse_expected(raw) do
+    Enum.map(raw, &String.to_integer/1) |> List.first
+  end
+
+end