Solution

elixir/roman-numerals/lib/roman_numerals.ex

@@ -1,8 +1,49 @@
 defmodule RomanNumerals do
+  @letters [
+    %{:unitary => "I", :halfway => "V", :next => "X"},
+    %{:unitary => "X", :halfway => "L", :next => "C"},
+    %{:unitary => "C", :halfway => "D", :next => "M"},
+    %{:unitary => "M", :halfway => "" , :next => ""}
+  ]
+
   @doc """
   Convert the number to a roman number.
   """
   @spec numeral(pos_integer) :: String.t()
   def numeral(number) do
+    numeral(number, @letters)
+  end
+
+  defp numeral(0, _letters) do
+    ""
+  end
+
+  defp numeral(number, [current | next]) do
+    next_iteration = Integer.floor_div(number, 10)
+    lower_part = number - (next_iteration * 10)
+    numeral(next_iteration, next) <> letter(lower_part, current)
+  end
+
+  defp letter(single_value, map) do
+    case single_value do
+      0 -> ""
+      1 -> String.duplicate(map[:unitary], 1)
+      2 -> String.duplicate(map[:unitary], 2)
+      3 -> String.duplicate(map[:unitary], 3)
+      4 -> map[:unitary] <> map[:halfway]
+      5 -> map[:halfway]
+      6 -> map[:halfway] <> String.duplicate(map[:unitary], 1)
+      7 -> map[:halfway] <> String.duplicate(map[:unitary], 2)
+      8 -> map[:halfway] <> String.duplicate(map[:unitary], 3)
+      9 -> map[:unitary] <> map[:next]
+
+      # OR...
+      # x when x == 0 -> ""
+      # x when x >= 1 and x <= 3 -> String.duplicate(map[:unitary], x)
+      # x when x == 4 -> map[:unitary] <> map[:halfway]
+      # x when x == 5 -> map[:halfway]
+      # x when x > 5 and x <= 8 -> map[:halfway] <> String.duplicate(map[:unitary], x - 5)
+      # _ -> map[:unitary] <> map[:next]
+    end
   end
 end

elixir/roman-numerals/test/roman_numerals_test.exs

@@ -1,92 +1,78 @@
 defmodule RomanNumeralsTest do
   use ExUnit.Case
 
-  # @tag :pending
   test "1" do
     assert RomanNumerals.numeral(1) == "I"
   end
 
-  @tag :pending
   test "2" do
     assert RomanNumerals.numeral(2) == "II"
   end
 
-  @tag :pending
   test "3" do
     assert RomanNumerals.numeral(3) == "III"
   end
 
-  @tag :pending
   test "4" do
     assert RomanNumerals.numeral(4) == "IV"
   end
 
-  @tag :pending
   test "5" do
     assert RomanNumerals.numeral(5) == "V"
   end
 
-  @tag :pending
   test "6" do
     assert RomanNumerals.numeral(6) == "VI"
   end
 
-  @tag :pending
+  test "7" do
+    assert RomanNumerals.numeral(7) == "VII"
+  end
+
   test "9" do
     assert RomanNumerals.numeral(9) == "IX"
   end
 
-  @tag :pending
   test "27" do
     assert RomanNumerals.numeral(27) == "XXVII"
   end
 
-  @tag :pending
   test "48" do
     assert RomanNumerals.numeral(48) == "XLVIII"
   end
 
-  @tag :pending
   test "59" do
     assert RomanNumerals.numeral(59) == "LIX"
   end
 
-  @tag :pending
   test "93" do
     assert RomanNumerals.numeral(93) == "XCIII"
   end
 
-  @tag :pending
   test "141" do
     assert RomanNumerals.numeral(141) == "CXLI"
   end
 
-  @tag :pending
   test "163" do
     assert RomanNumerals.numeral(163) == "CLXIII"
   end
 
-  @tag :pending
   test "402" do
     assert RomanNumerals.numeral(402) == "CDII"
   end
 
-  @tag :pending
   test "575" do
     assert RomanNumerals.numeral(575) == "DLXXV"
   end
 
-  @tag :pending
   test "911" do
     assert RomanNumerals.numeral(911) == "CMXI"
   end
 
-  @tag :pending
   test "1024" do
     assert RomanNumerals.numeral(1024) == "MXXIV"
   end
 
-  @tag :pending
   test "3000" do
     assert RomanNumerals.numeral(3000) == "MMM"
   end