Add precision to Float.ceil/1 & Float.floor/1

Author: Johnny Winn

lib/elixir/lib/float.ex

@@ -98,54 +98,67 @@ defmodule Float do
 
   @doc """
   Rounds a float to the largest integer less than or equal to `num`.
+  Floor also accepts a precision to round a floating point value down
+  to an arbitrary number of fractional digits (between 0 and 15).
 
   ## Examples
 
-      iex> Float.floor(34)
-      34
-
       iex> Float.floor(34.25)
-      34
+      34.0
 
       iex> Float.floor(-56.5)
-      -57
+      -57.0
+
+      iex> Float.floor(34.253, 2)
+      34.25
 
   """
-  @spec floor(float | integer) :: integer
-  def floor(num) when is_integer(num), do: num
+  @spec floor(float) :: float
   def floor(num) when is_float(num) do
     truncated = :erlang.trunc(num)
     case :erlang.abs(num - truncated) do
-      x when x > 0 and num < 0 -> truncated - 1
-      _ -> truncated
+      x when x > 0 and num < 0 -> truncated - 1.0
+      _ -> truncated + 0.0
     end
   end
 
+  @spec floor(float, integer) :: float
+  def floor(num, precision) when is_float(num) do
+    calculate_precision(num, -0.5, precision) |> round(precision)
+  end
+
   @doc """
   Rounds a float to the largest integer greater than or equal to `num`.
+  Ceil also accepts a precision to round a floating point value down to
+  an arbitrary number of fractional digits (between 0 and 15).
 
   ## Examples
 
-      iex> Float.ceil(34)
-      34
-
       iex> Float.ceil(34.25)
-      35
+      35.0
 
       iex> Float.ceil(-56.5)
-      -56
+      -56.0
+
+      iex> Float.ceil(34.253, 2)
+      34.26
 
   """
-  @spec ceil(float | integer) :: integer
-  def ceil(num) when is_integer(num), do: num
+
+  @spec ceil(float) :: float
   def ceil(num) when is_float(num) do
     truncated = :erlang.trunc(num)
     case :erlang.abs(num - truncated) do
-      x when x > 0 and num > 0 -> truncated + 1
-      _ -> truncated
+      x when x > 0 and num > 0 -> truncated + 1.0
+      _ -> truncated + 0.0
     end
   end
 
+  @spec ceil(float, integer) :: float
+  def ceil(num, precision) when is_float(num) do
+    calculate_precision(num, 0.5, precision) |> round(precision)
+  end
+
   @doc """
   Rounds a floating point value to an arbitrary number of fractional digits
   (between 0 and 15).
@@ -247,6 +260,10 @@ defmodule Float do
     :erlang.float_to_binary(float, expand_compact(options))
   end
 
+  defp calculate_precision(num, variance, precision) do
+    num + (variance / :math.pow(10, precision))
+  end
+
   defp expand_compact([{:compact, false}|t]), do: expand_compact(t)
   defp expand_compact([{:compact, true}|t]),  do: [:compact|expand_compact(t)]
   defp expand_compact([h|t]),                 do: [h|expand_compact(t)]

lib/elixir/test/elixir/float_test.exs

@@ -28,35 +28,41 @@ defmodule FloatTest do
   end
 
   test :floor do
-    assert Float.floor(12) === 12
-    assert Float.floor(-12) === -12
-    assert Float.floor(12.524235) === 12
-    assert Float.floor(-12.5) === -13
-    assert Float.floor(-12.524235) === -13
-    assert Float.floor(7.5e3) === 7500
-    assert Float.floor(7.5432e3) === 7543
-    assert Float.floor(7.5e-3) === 0
-    assert Float.floor(-12.32453e4) === -123246
-    assert Float.floor(-12.32453e-10) === -1
-    assert Float.floor(0.32453e-10) === 0
-    assert Float.floor(-0.32453e-10) === -1
-    assert Float.floor(1.32453e-10) === 0
+    assert Float.floor(12.524235) === 12.0
+    assert Float.floor(-12.5) === -13.0
+    assert Float.floor(-12.524235) === -13.0
+    assert Float.floor(7.5e3) === 7500.0
+    assert Float.floor(7.5432e3) === 7543.0
+    assert Float.floor(7.5e-3) === 0.0
+    assert Float.floor(-12.32453e4) === -123246.0
+    assert Float.floor(-12.32453e-10) === -1.0
+    assert Float.floor(0.32453e-10) === 0.0
+    assert Float.floor(-0.32453e-10) === -1.0
+    assert Float.floor(1.32453e-10) === 0.0
+  end
+
+  test :floor_with_precision do
+    assert Float.floor(12.524235, 2) === 12.52
+    assert Float.floor(-12.524235, 3) === -12.525
   end
 
   test :ceil do
-    assert Float.ceil(12) === 12
-    assert Float.ceil(-12) === -12
-    assert Float.ceil(12.524235) === 13
-    assert Float.ceil(-12.5) === -12
-    assert Float.ceil(-12.524235) === -12
-    assert Float.ceil(7.5e3) === 7500
-    assert Float.ceil(7.5432e3) === 7544
-    assert Float.ceil(7.5e-3) === 1
-    assert Float.ceil(-12.32453e4) === -123245
-    assert Float.ceil(-12.32453e-10) === 0
-    assert Float.ceil(0.32453e-10) === 1
-    assert Float.ceil(-0.32453e-10) === 0
-    assert Float.ceil(1.32453e-10) === 1
+    assert Float.ceil(12.524235) === 13.0
+    assert Float.ceil(-12.5) === -12.0
+    assert Float.ceil(-12.524235) === -12.0
+    assert Float.ceil(7.5e3) === 7500.0
+    assert Float.ceil(7.5432e3) === 7544.0
+    assert Float.ceil(7.5e-3) === 1.0
+    assert Float.ceil(-12.32453e4) === -123245.0
+    assert Float.ceil(-12.32453e-10) === 0.0
+    assert Float.ceil(0.32453e-10) === 1.0
+    assert Float.ceil(-0.32453e-10) === 0.0
+    assert Float.ceil(1.32453e-10) === 1.0
+  end
+
+  test :ceil_with_precision do
+    assert Float.ceil(12.524235, 2) === 12.53
+    assert Float.ceil(-12.524235, 3) === -12.524
   end
 
   test :round do