Add Float.round function

Float.round allows rounding floats to arbitrary precision, as well
as specifying how midpoint rounding is performed (up/down).

Author: bitwalker

lib/elixir/lib/float.ex

@@ -125,4 +125,29 @@ defmodule Float do
     end
   end
 
+  @doc """
+  Rounds a floating point value to an arbitrary number of fractional digits
+  (between 0 and 15) with an optional midpoint rounding mode (:up or :down, 
+  defaults to :up).
+
+  ## Examples
+
+      iex> Float.round(5.5675, 3)
+      5.568
+      iex> Float.round(5.5675, 3, :down)
+      5.567
+      iex> Float.round(-5.5675, 3)
+      -5.567
+      iex> Float.round(-5.5675, 3, :down)
+      -5.568
+  """
+  @spec round(float, integer, atom | nil) :: float
+  def round(number, precision, midpoint_rounding // :up) when is_float(number) and is_integer(precision) and precision in 0..15 do
+    # Default to :up if anything but :down is provided for midpoint rounding mode
+    case midpoint_rounding do
+      :down -> Kernel.round(Float.floor(number * :math.pow(10, precision))) / :math.pow(10, precision)
+      _     -> Kernel.round(Float.ceil(number * :math.pow(10, precision))) / :math.pow(10, precision)
+    end
+  end
+
 end

lib/elixir/test/elixir/float_test.exs

@@ -57,4 +57,17 @@ defmodule FloatTest do
     assert Float.ceil(-0.32453e-10) === 0
     assert Float.ceil(1.32453e-10) === 1
   end
+
+  test :round do
+    assert Float.round(5.5675, 3) === 5.568
+    assert Float.round(5.5675, 3, :down) === 5.567
+    assert Float.round(5.5, 3) === 5.5
+    assert Float.round(5.5e-10, 10) === 6.0e-10
+    assert Float.round(5.5e-10, 10, :down) === 5.0e-10
+    assert Float.round(5.5e-10, 8) === 1.0e-8
+    assert Float.round(5.5e-10, 8, :down) === 0.0
+    assert Float.round(5.0, 0) === 5.0
+    assert Float.round(-1.3456, 3) === -1.345
+    assert Float.round(-1.3456, 3, :down) === -1.346
+  end
 end