Fix compare against 0 (#197)

Co-authored-by: vshev4enko <[email protected]>

lib/decimal.ex

@@ -344,6 +344,11 @@ defmodule Decimal do
 
   def compare(%Decimal{coef: 0}, %Decimal{coef: 0}), do: :eq
 
+  def compare(%Decimal{sign: 1}, %Decimal{coef: 0}), do: :gt
+  def compare(%Decimal{coef: 0}, %Decimal{sign: 1}), do: :lt
+  def compare(%Decimal{sign: -1}, %Decimal{coef: 0}), do: :lt
+  def compare(%Decimal{coef: 0}, %Decimal{sign: -1}), do: :gt
+
   def compare(%Decimal{sign: 1}, %Decimal{sign: -1}), do: :gt
   def compare(%Decimal{sign: -1}, %Decimal{sign: 1}), do: :lt
 
@@ -358,14 +363,9 @@ defmodule Decimal do
           padded_num2 = pad_num(num2, num2.exp - num1.exp)
 
           cond do
-            padded_num1 == padded_num2 ->
-              0
-
-            padded_num1 < padded_num2 ->
-              -num1.sign
-
-            true ->
-              num1.sign
+            padded_num1 == padded_num2 -> 0
+            padded_num1 < padded_num2 -> -num1.sign
+            true -> num1.sign
           end
 
         adjusted_exp1 < adjusted_exp2 ->

mix.exs

@@ -24,7 +24,8 @@ defmodule Decimal.Mixfile do
 
   defp deps() do
     [
-      {:ex_doc, ">= 0.0.0", only: :dev, runtime: false}
+      {:ex_doc, ">= 0.0.0", only: :dev},
+      {:stream_data, "~> 0.5.0", only: :test}
     ]
   end
 

mix.lock

@@ -5,4 +5,5 @@
   "makeup_elixir": {:hex, :makeup_elixir, "0.16.1", "cc9e3ca312f1cfeccc572b37a09980287e243648108384b97ff2b76e505c3555", [:mix], [{:makeup, "~> 1.0", [hex: :makeup, repo: "hexpm", optional: false]}, {:nimble_parsec, "~> 1.2.3 or ~> 1.3", [hex: :nimble_parsec, repo: "hexpm", optional: false]}], "hexpm", "e127a341ad1b209bd80f7bd1620a15693a9908ed780c3b763bccf7d200c767c6"},
   "makeup_erlang": {:hex, :makeup_erlang, "0.1.1", "3fcb7f09eb9d98dc4d208f49cc955a34218fc41ff6b84df7c75b3e6e533cc65f", [:mix], [{:makeup, "~> 1.0", [hex: :makeup, repo: "hexpm", optional: false]}], "hexpm", "174d0809e98a4ef0b3309256cbf97101c6ec01c4ab0b23e926a9e17df2077cbb"},
   "nimble_parsec": {:hex, :nimble_parsec, "1.3.0", "9e18a119d9efc3370a3ef2a937bf0b24c088d9c4bf0ba9d7c3751d49d347d035", [:mix], [], "hexpm", "7977f183127a7cbe9346981e2f480dc04c55ffddaef746bd58debd566070eef8"},
+  "stream_data": {:hex, :stream_data, "0.5.0", "b27641e58941685c75b353577dc602c9d2c12292dd84babf506c2033cd97893e", [:mix], [], "hexpm", "012bd2eec069ada4db3411f9115ccafa38540a3c78c4c0349f151fc761b9e271"},
 }

test/decimal/property_test.exs

@@ -0,0 +1,48 @@
+defmodule Decimal.PropertyTest do
+  use ExUnit.Case, async: true
+  use ExUnitProperties
+
+  describe "compare/2" do
+    test "integer equality" do
+      check all(
+              first <- StreamData.integer(),
+              second <- StreamData.integer()
+            ) do
+        assert Decimal.compare(first, second) == term_compare(first, second)
+      end
+    end
+
+    test "float equality" do
+      check all(
+              first <- StreamData.float(),
+              second <- StreamData.float()
+            ) do
+        assert Decimal.compare(to_dec(first), to_dec(second)) == term_compare(first, second)
+      end
+    end
+
+    test "number equality" do
+      check all(
+              first <- stream_data_number(),
+              second <- stream_data_number()
+            ) do
+        assert Decimal.compare(to_dec(first), to_dec(second)) == term_compare(first, second)
+      end
+    end
+  end
+
+  defp to_dec(float) when is_float(float), do: Decimal.from_float(float)
+  defp to_dec(other), do: Decimal.new(other)
+
+  defp term_compare(first, second) do
+    cond do
+      first < second -> :lt
+      first > second -> :gt
+      true -> :eq
+    end
+  end
+
+  defp stream_data_number() do
+    StreamData.one_of([StreamData.integer(), StreamData.float()])
+  end
+end

test/decimal_test.exs

@@ -240,6 +240,12 @@ defmodule DecimalTest do
     assert Decimal.compare(~d"0", ~d"inf") == :lt
     assert Decimal.compare(~d"0", ~d"-inf") == :gt
 
+    assert Decimal.compare(~d"0.123", ~d"0") == :gt
+    assert Decimal.compare(~d"0.123", ~d"0.122") == :gt
+    assert Decimal.compare(~d"0.123", ~d"0.123") == :eq
+    assert Decimal.compare(~d"0.123", ~d"0.124") == :lt
+    assert Decimal.compare(~d"0.0123", ~d"0.124") == :lt
+
     assert Decimal.compare("Inf", "Inf") == :eq
 
     assert Decimal.compare(~d"5e10000000000", ~d"0") == :gt