Change NIF build order to prefer local builds

Makefile

@@ -20,8 +20,10 @@ compile:
 	./rebar3 compile
 
 priv/$(nif_lib): src/lib.rs
+ifneq ($(ECRECOVER_DISABLE_NIF_BUILD), true)
 	cargo build --release
 	cp target/release/$(nif_lib_src) $@
+endif
 
 clean:
 	rm -f priv/$(nif_lib) target/release/$(nif_lib_src)

README.md

@@ -1,15 +1,29 @@
 # ecrecover
 FFI (NIF) export of Ethereum's ecrecover for use from Erlang.
 
-### prerequisite:
+### Prerequisites
-- a checked out copy of my fork of parity-ethereum (https://github.com/johnsnewby/parity-ethereum) checked out into this directory this is (i.e. it will be referenced as `./parity-ethereum`)
+The NIF is written in Rust, therefore the following additional build
+dependencies are needed:
 
-### to compile:
+- Rust
-`cargo build`
+- Cargo
+- Cmake
+
+### Build
+Execute:
+```
+make
+```
+
+To disable the local build of the NIF library, e.g. to use a prebuilt binary,
+use the following command:
+```
+ECRECOVER_DISABLE_NIF_BUILD=true make
+```
 
 ## Erlang integration
 
-The shared library uses NIF. Use the erlang file `src/ecrecover.erl` to use this:
+The shared library uses NIF. Use the Erlang file `src/ecrecover.erl` to use this:
 
 ```
 c("src/ecrecover").

src/ecrecover.erl

@@ -11,8 +11,14 @@
 %% NIF API
 
 load() ->
-    SoName = filename:join(priv(), atom_to_list(?MODULE)),
+    % Prefer locally built NIF (good for testing and exotic platforms) over
-    ok = erlang:load_nif(SoName, 0).
+    % prebuilt binaries.
+    case load_local_nif() of
+        ok ->
+            ok;
+        {error, _} ->
+            load_prebuilt_nif()
+    end.
 
 not_loaded(Line) ->
     erlang:nif_error({error, {not_loaded, [{module, ?MODULE}, {line, Line}]}}).
@@ -35,14 +41,20 @@ recover(<<_:32/binary>> = Hash, <<_:65/binary>> = Sig) ->
 %%=============================================================================
 %% Internal Functions
 
-priv()->
+load_local_nif() ->
+    EbinDir = filename:dirname(code:which(?MODULE)),
+    AppDir = filename:dirname(EbinDir),
+    PrivDir = filename:join(AppDir, "priv"),
+    SoName = filename:join(PrivDir, atom_to_list(?MODULE)),
+    erlang:load_nif(SoName, 0).
+
+load_prebuilt_nif() ->
     case code:priv_dir(ecrecoverprebuilt) of
         {error, _} ->
-          EbinDir = filename:dirname(code:which(?MODULE)),
+            {error, prebuilt_priv_dir_not_found};
-          AppPath = filename:dirname(EbinDir),
+        PrivDir ->
-          filename:join(AppPath, "priv");
+            SoName = filename:join(PrivDir, atom_to_list(?MODULE)),
-      Path ->
+            erlang:load_nif(SoName, 0)
-          Path
     end.
 
 recover_(_Input) ->