Use :NaN for Decimal encoding (#136)

The Decimal module documents the use of :inf and :NaN but not :qNaN or
:sNaN. It refers to :NaN as being a quiet NaN. Signalling NaNs are
generally expected to trigger an error at the hardware level and then
handled by being converted to a quite NaN if execution should continue.

Author: spanglerco

lib/bson/decimal128.ex

@@ -13,7 +13,6 @@ defmodule BSON.Decimal128 do
   @exponent_bias 6176
   @max_exponent 6111
   @min_exponent -6176
-  @s_nan_mask 0x1 <<< 57
   @significand_mask (0x1 <<< 49) - 1
   @low_mask 0xFFFFFFFFFFFFFFFF
 
@@ -22,14 +21,7 @@ defmodule BSON.Decimal128 do
     combination = high >>> 58 &&& @combination_mask
     two_highest_bits_set = combination >>> 3 == 3
     is_infinity = two_highest_bits_set && combination == @combintation_infinity
-
-    is_nan =
-      case {(two_highest_bits_set && combination) == @combintation_nan, (high &&& @s_nan_mask) == @s_nan_mask} do
-        {true, true} -> :sNan
-        {true, false} -> :qNan
-        _ -> false
-      end
-
+    is_nan = two_highest_bits_set && combination == @combintation_nan
     exponent = exponent(high, two_highest_bits_set)
 
     value(
@@ -56,18 +48,12 @@ defmodule BSON.Decimal128 do
     <<low::little-64, high::little-64>>
   end
 
-  def encode(%Decimal{coef: :qNaN}) do
+  def encode(%Decimal{coef: :NaN}) do
     low = 0
     high = 0x1F <<< 58
     <<low::little-64, high::little-64>>
   end
 
-  def encode(%Decimal{coef: :sNaN}) do
-    low = 0
-    high = 0x3F <<< 57
-    <<low::little-64, high::little-64>>
-  end
-
   def encode(%Decimal{sign: sign, coef: significand, exp: exponent}) when exponent >= @min_exponent and exponent <= @max_exponent do
     biased_exponent = exponent + @exponent_bias
     low = significand &&& @low_mask
@@ -107,12 +93,8 @@ defmodule BSON.Decimal128 do
     %Decimal{coef: :inf}
   end
 
-  defp value(%{is_nan: :qNan}, _, _) do
-    %Decimal{coef: :qNaN}
-  end
-
-  defp value(%{is_nan: :sNan}, _, _) do
-    %Decimal{coef: :sNaN}
+  defp value(%{is_nan: true}, _, _) do
+    %Decimal{coef: :NaN}
   end
 
   defp value(%{two_highest_bits_set: true}, _, _) do

test/bson/decimal128_test.exs

@@ -18,7 +18,7 @@ defmodule BSON.Decimal128Test do
 
   @tag :mongo_3_4
   test "BSON.Decimal128.decode/1" do
-    assert_decimal(@nan_binaries, %Decimal{coef: :qNaN})
+    assert_decimal(@nan_binaries, %Decimal{coef: :NaN})
     assert_decimal(@inf_binaries, %Decimal{coef: :inf})
     assert_decimal(@neg_inf_binaries, %Decimal{sign: -1, coef: :inf})
     assert_decimal(@binaries_0, %Decimal{coef: 0})
@@ -45,8 +45,7 @@ defmodule BSON.Decimal128Test do
 
   @tag :mongo_3_4
   test "BSON.Decimal128.encode/1" do
-    assert_decimal(%Decimal{coef: :qNaN})
-    assert_decimal(%Decimal{coef: :sNaN})
+    assert_decimal(%Decimal{coef: :NaN})
     assert_decimal(%Decimal{sign: -1, coef: :inf})
     assert_decimal(%Decimal{coef: :inf})
     assert_decimal(%Decimal{coef: 0, exp: -611})

test/mongo_test.exs

@@ -537,8 +537,7 @@ defmodule Mongo.Test do
 
     values =
       [
-        %Decimal{coef: :qNaN},
-        %Decimal{coef: :sNaN},
+        %Decimal{coef: :NaN},
         %Decimal{sign: -1, coef: :inf},
         %Decimal{coef: :inf},
         %Decimal{coef: 0, exp: -611},