refactor: unroll decoding

From 1.5x to 1.8 speedupt

lib/varint/leb128.ex

@@ -88,7 +88,52 @@ defmodule Varint.LEB128 do
       ** (ArgumentError) not a valid LEB128 encoded integer
   """
   @spec decode(binary) :: {non_neg_integer, binary}
-  def decode(b) when is_binary(b), do: do_decode(0, 0, b)
+  def decode(<<0::1, byte0::7, rest::binary>>),
+    do: {byte0, rest}
+
+  def decode(<<1::1, byte1::7, 0::1, byte0::7, rest::binary>>),
+    do: {byte1 <<< 0 ||| byte0 <<< 7, rest}
+
+  def decode(<<1::1, byte2::7, 1::1, byte1::7, 0::1, byte0::7, rest::binary>>),
+    do: {byte2 <<< 0 ||| byte1 <<< 7 ||| byte0 <<< 14, rest}
+
+  def decode(<<1::1, byte3::7, 1::1, byte2::7, 1::1, byte1::7, 0::1, byte0::7, rest::binary>>),
+    do: {byte3 <<< 0 ||| byte2 <<< 7 ||| byte1 <<< 14 ||| byte0 <<< 21, rest}
+
+  def decode(
+        <<1::1, byte4::7, 1::1, byte3::7, 1::1, byte2::7, 1::1, byte1::7, 0::1, byte0::7,
+          rest::binary>>
+      ),
+      do: {byte4 <<< 0 ||| byte3 <<< 7 ||| byte2 <<< 14 ||| byte1 <<< 21 ||| byte0 <<< 28, rest}
+
+  def decode(
+        <<1::1, byte5::7, 1::1, byte4::7, 1::1, byte3::7, 1::1, byte2::7, 1::1, byte1::7, 0::1,
+          byte0::7, rest::binary>>
+      ),
+      do:
+        {byte5 <<< 0 ||| byte4 <<< 7 ||| byte3 <<< 14 ||| byte2 <<< 21 ||| byte1 <<< 28 |||
+           byte0 <<< 35, rest}
+
+  def decode(
+        <<1::1, byte6::7, 1::1, byte5::7, 1::1, byte4::7, 1::1, byte3::7, 1::1, byte2::7, 1::1,
+          byte1::7, 0::1, byte0::7, rest::binary>>
+      ),
+      do:
+        {byte6 <<< 0 ||| byte5 <<< 7 ||| byte4 <<< 14 ||| byte3 <<< 21 ||| byte2 <<< 28 |||
+           byte1 <<< 35 |||
+           byte0 <<< 42, rest}
+
+  def decode(
+        <<1::1, byte7::7, 1::1, byte6::7, 1::1, byte5::7, 1::1, byte4::7, 1::1, byte3::7, 1::1,
+          byte2::7, 1::1, byte1::7, 0::1, byte0::7, rest::binary>>
+      ),
+      do:
+        {byte7 <<< 0 ||| byte6 <<< 7 ||| byte5 <<< 14 ||| byte4 <<< 21 ||| byte3 <<< 28 |||
+           byte2 <<< 35 |||
+           byte1 <<< 42 |||
+           byte0 <<< 49, rest}
+
+  def decode(b), do: do_decode(0, 0, b)
 
   # -- Private
 

test/varint/leb128_test.exs

@@ -1,3 +1,28 @@
+defmodule VarintSimple do
+  import Bitwise
+
+  @spec encode(integer) :: binary()
+  def encode(v) when v < 1 <<< 7, do: <<v>>
+  def encode(v), do: <<1::1, v::7, encode(v >>> 7)::binary>>
+
+  @spec decode(binary) :: {non_neg_integer, binary}
+  def decode(<<0::1, byte0::7, rest::binary>>), do: {byte0, rest}
+  def decode(b), do: do_decode(0, 0, b)
+
+  @spec do_decode(non_neg_integer, non_neg_integer, binary) :: {non_neg_integer, binary}
+  defp do_decode(result, shift, <<0::1, byte::7, rest::binary>>) do
+    {result ||| byte <<< shift, rest}
+  end
+
+  defp do_decode(result, shift, <<1::1, byte::7, rest::binary>>) do
+    do_decode(
+      result ||| byte <<< shift,
+      shift + 7,
+      rest
+    )
+  end
+end
+
 defmodule Varint.LEB128Test do
   import Bitwise
   use ExUnit.Case, async: true
@@ -7,7 +32,14 @@ defmodule Varint.LEB128Test do
 
   property "Unrolled encoding produces the same result as the reference implementation" do
     check all int <- integer(0..(1 <<< 64)) do
-      assert Varint.LEB128.encode(int) == encode_reference(int)
+      assert Varint.LEB128.encode(int) == VarintSimple.encode(int)
+    end
+  end
+
+  property "Unrolled decoding produces the same result as the reference implementation" do
+    check all int <- integer(0..(1 <<< 64)) do
+      encoded = VarintSimple.encode(int)
+      assert Varint.LEB128.decode(encoded) == VarintSimple.decode(encoded)
     end
   end
 
@@ -52,7 +84,4 @@ defmodule Varint.LEB128Test do
   test "Decode raises an error for non-LEB128 encoded data" do
     assert_raise ArgumentError, fn -> Varint.LEB128.decode(<<255>>) end
   end
-
-  defp encode_reference(v) when v < 1 <<< 7, do: <<v>>
-  defp encode_reference(v), do: <<1::1, v::7, encode_reference(v >>> 7)::binary>>
 end