fixing issue with double precision floats

Author: mjaric

lib/tds/types.ex

@@ -1011,7 +1011,26 @@ defmodule Tds.Types do
     <<0>>
   end
   def encode_data(@tds_data_type_floatn, value, _) do
-    <<0x04, value::little-float-size(32)>>
+    d_ctx = Decimal.get_context
+    d_ctx = %{d_ctx | precision: 38}
+    Decimal.set_context d_ctx
+    value_list = Decimal.new(value)
+      |> Decimal.abs
+      |> Decimal.to_string(:scientific)
+      |> String.split(".")
+    precision =
+      case value_list do
+        [p,s] ->
+          String.length(p) + String.length(s)
+        [p] ->
+          String.length(p)
+      end
+    if precision <= 7 + 1 do
+      <<0x04, value::little-float-size(32)>>
+    else
+      # up to 15 digits of precision https://docs.microsoft.com/en-us/sql/t-sql/data-types/float-and-real-transact-sql
+      <<0x08, value::little-float-size(64)>>
+    end
   end
 
   @doc """

lib/tds/utils.ex

@@ -85,4 +85,16 @@ defmodule Tds.Utils do
   def pow10(num,pow) when pow < 0 do
     pow10(num/10, pow + 1)
   end
+
+  def pow(_, 0), do: 1
+  def pow(a, 1), do: a
+
+  def pow(a, n) when rem(n, 2) === 0 do
+    tmp = pow(a, div(n, 2))
+    tmp * tmp
+  end
+
+  def pow(a, n) do
+    a * pow(a, n-1)
+  end
 end

test/tds_ex_test.exs

@@ -0,0 +1,40 @@
+defmodule TdsTest do
+  use ExUnit.Case, async: true
+  require Logger
+  import Tds.TestHelper
+
+  @tag timeout: 50000
+
+  setup do
+    opts = Application.fetch_env!(:tds, :opts)
+    {:ok, pid} = Tds.start_link(opts)
+
+    {:ok, [pid: pid]}
+  end
+
+  test "issue 33: Sending Float with more than 9 characters should not fail", context do
+    query("DROP TABLE hades_sealed_cfdis", [])
+    query(
+      """
+      CREATE TABLE hades_sealed_cfdis(
+        [id] int identity(1,1) not null primary key,
+        [total] float(53),
+        [inserted_at] datetime,
+        [updated_at] datetime
+      )
+      """, [])
+    f = fn val ->   
+      res = query("INSERT INTO hades_sealed_cfdis ([total] ,[inserted_at], [updated_at]) VALUES (@1,@2,@3)", 
+                [%Tds.Parameter{name: "@1", value: val, type: :float},
+                  %Tds.Parameter{name: "@2", value: {{2016, 12, 20}, {23, 59, 23, 0}}},
+                  %Tds.Parameter{name: "@3", value: {{2016, 12, 20}, {23, 59, 23, 0}}}]) 
+
+      assert :ok == res
+      assert [[val]] = query("SELECT [total] FROM hades_sealed_cfdis WHERE id in (select max(id) from hades_sealed_cfdis)", [])
+    end
+    Enum.flat_map(1..17, &([1/&1, -1/&1]))
+    |> Enum.each(f)
+    query("DROP TABLE hades_sealed_cfdis", [])
+  end
+
+end