%% This script tries to discover which library can be used 
%% to support curve encryption (if any). Curve encryption is
%% used by the CURVE security model.
%%
%% If the CHUMAK_CURVE_LIB environment variable is set,
%% that determines what is used. Supported values are:
%% - nacerl - this is the minimal variant using tweetnacl.
%%            https://github.com/willemdj/NaCerl
%% - nacl   - this is similar to nacerl, but it depends on libsodium.
%%            https://github.com/tonyg/erlang-nacl
%% - enacl  - this also depends on libsodium, but it also requires 
%%            an Erlang VM that supports dirty schedulers.
%%            https://github.com/jlouis/enacl
%% - none   - no support for the CURVE security model.
%%
%% If the CHUMAK_CURVE_LIB environment variable is _not_ set,
%% nacerl will be used. On Windows an additional check will be 
%% preformed first: if gcc and make are not available, `none` will
%% be assumed. 
%% 
%% Since it is not possible to reliably and completely build any of
%% the options that depend on libsodium from rebar3, only in the `nacerl` 
%% case the library will be fetched and built automatically as a 
%% dependency. In the other cases you wil have to make the library available
%% via other means.
%%
%% The selected option ("nacl", "enacl", "nacerl" or "none") will be 
%% passed as a macro to the compiler. Based on this macro it is 
%% determined which library is used by the code.

%% Some of the ideas used here were found in 
%% https://github.com/klacke/yaws/blob/master/rebar.config.script
%% and 
%% https://github.com/dgud/esdl/blob/master/rebar.config.script

UpdateCfg = fun(Config, _Key, undefined) ->
                    Config;
                (Config, Key, NewVal) ->
                    case lists:keyfind(Key, 1, Config) of
                        {Key, Vals} ->
                            NVals = [NewVal | Vals],
                            lists:keyreplace(Key, 1, Config, {Key, NVals});
                        false ->
                            Config ++ [{Key, [NewVal]}]
                    end
            end,

NaCerlDep = {nacerl, ".*", {git, "https://github.com/willemdj/NaCerl.git", {branch, "master"}}},

IsAvailable =
    fun(Executable) ->
        case os:find_executable(Executable) of
            false ->
                io:format("Building Chumak with curve support on windows"
                          " requires ~p~n", [Executable]),
                false;
            _ ->
                true
        end
    end,

AllAvailable =
    fun(Executables) ->
        lists:all(fun(Bool) -> Bool end, [IsAvailable(E) || E <- Executables])
    end,

%% The only dependency where it makes sense to fecth and build it
%% from here, is NaCerl. The other ones need libsodium and must be installed
%% as a separate step.
DetermineCurveDep = 
    fun() ->
        case os:type() of 
            {win32, _} ->
                case AllAvailable(["gcc", "make"]) of
                    true ->
                        NaCerlDep;
                    false ->
                        io:format("not all required tools are available to build "
                                  "curve security mechanism.~n"),
                        undefined
                end;
            _ ->
                NaCerlDep
        end
    end,

%% If the CHUMAK_CURVE_LIB environment variable is set, that determines 
%% what is used.
CurveLibEnv = os:getenv("CHUMAK_CURVE_LIB"),

CurveDep = case CurveLibEnv of
               false ->
                   undefined;
               Val ->
                   case string:to_lower(Val) of
                       "nacerl" ->
                           NaCerlDep;
                       _ ->
                           DetermineCurveDep()
                   end
           end,

CurveCompilerOption = case CurveDep of
                          NaCerlDep -> 
                              {d, 'CHUMAK_CURVE_LIB_NACERL', true};
                          _ -> 
                              case CurveLibEnv of
                                  false ->
                                      undefined;
                                  Value ->
                                      case string:to_lower(Value) of
                                          "nacerl" ->
                                              {d, 'CHUMAK_CURVE_LIB_NACERL', true};
                                          "nacl" ->
                                              {d, 'CHUMAK_CURVE_LIB_NACL', true};
                                          "enacl" ->
                                              {d, 'CHUMAK_CURVE_LIB_ENACL', true};
                                          "none" ->
                                              undefined
                                      end
                              end
                      end,

Config2 = UpdateCfg(CONFIG, erl_opts, CurveCompilerOption),
UpdateCfg(Config2, deps, CurveDep).