Add hexadecimal exercise

Author: bernardoamc

config.json

@@ -29,6 +29,7 @@
     "queen-attack",
     "saddle-points",
     "triangle",
+    "hexadecimal",
     "scrabble-score",
     "crypto-square",
     "diamond",

exercises/hexadecimal/example.exs

@@ -0,0 +1,45 @@
+defmodule Hexadecimal do
+  @base 16
+  @hex_table String.split("0123456789abcdef", "", trim: true)
+               |> Enum.with_index
+               |> Enum.into(%{})
+
+  @doc """
+    Accept a string representing a hexadecimal value and return the
+    corresponding decimal value.
+    It returns the integer 0 if the hexadecimal is invalid.
+    Otherwise returns an integer representing the decimal value.
+
+    ## Examples
+
+      iex> Hexadecimal.to_decimal("invalid")
+      0
+
+      iex> Hexadecimal.to_decimal("af")
+      175
+
+  """
+
+  @spec to_decimal(binary) :: integer
+  def to_decimal(hex) do
+    if invalid?(hex) do
+      0
+    else
+      hex
+        |> String.downcase
+        |> String.reverse
+        |> String.split("", trim: true)
+        |> hex_to_int(0, 0)
+    end
+  end
+
+  defp invalid?(hex) do
+    String.match?(hex, ~r/[^0-9a-fA-F]/)
+  end
+
+  defp hex_to_int([], acc, _), do: round(acc)
+  defp hex_to_int([char|hex], acc, power) do
+    acc = acc + Map.get(@hex_table, char) * :math.pow(@base, power)
+    hex_to_int(hex, acc, power + 1)
+  end
+end

exercises/hexadecimal/hexadecimal.exs

@@ -0,0 +1,22 @@
+defmodule Hexadecimal do
+  @doc """
+    Accept a string representing a hexadecimal value and returns the
+    corresponding decimal value.
+    It returns the integer 0 if the hexadecimal is invalid.
+    Otherwise returns an integer representing the decimal value.
+
+    ## Examples
+
+      iex> Hexadecimal.to_decimal("invalid")
+      0
+
+      iex> Hexadecimal.to_decimal("af")
+      175
+
+  """
+
+  @spec to_decimal(binary) :: integer
+  def to_decimal(hex) do
+
+  end
+end

exercises/hexadecimal/hexadecimal_test.exs

@@ -0,0 +1,64 @@
+if !System.get_env("EXERCISM_TEST_EXAMPLES") do
+  Code.load_file("hexadecimal.exs")
+end
+
+ExUnit.start
+ExUnit.configure exclude: :pending, trace: true
+
+defmodule ChangeTest do
+  use ExUnit.Case
+
+  test "returns 1 when hex is 1" do
+    assert Hexadecimal.to_decimal("1") ==  1
+  end
+
+  @tag :pending
+  test "returns 12 when hex is c" do
+    assert Hexadecimal.to_decimal("c") == 12
+  end
+
+  @tag :pending
+  test "hexadecimal is case insensitive" do
+    assert Hexadecimal.to_decimal("C") == 12
+  end
+
+  @tag :pending
+  test "returns 16 when hex is 10" do
+    assert Hexadecimal.to_decimal("10") == 16
+  end
+
+  @tag :pending
+  test "returns 175 when hex is af" do
+    assert Hexadecimal.to_decimal("af") == 175
+  end
+
+  @tag :pending
+  test "returns 256 when hex is 100" do
+    assert Hexadecimal.to_decimal("100") == 256
+  end
+
+  @tag :pending
+  test "returns 105_166 when hex is 19ace" do
+    assert Hexadecimal.to_decimal("19ace") == 105_166
+  end
+
+  @tag :pending
+  test "returns 0 when hex is invalid" do
+    assert Hexadecimal.to_decimal("carrot") == 0
+  end
+
+  @tag :pending
+  test "returns 0 when hex represents black" do
+    assert Hexadecimal.to_decimal("000000") == 0
+  end
+
+  @tag :pending
+  test "returns 16_777_215 when hex represents white" do
+    assert Hexadecimal.to_decimal("ffffff") == 16_777_215
+  end
+
+  @tag :pending
+  test "returns 16_776_960 when hex represents yellow" do
+    assert Hexadecimal.to_decimal("ffff00") == 16_776_960
+  end
+end