Start working on the Bridge class

Author: vendethiel

src/cpp/lib/testml/bridge.hpp

@@ -0,0 +1,71 @@
+#pragma once
+
+#include <tuple>
+#include <utility>
+#include <unordered_map>
+#include <memory>
+#include <stdexcept>
+#include "../../ext/nlohmann/json.hpp"
+
+namespace testml {
+  
+  namespace details {
+
+    using json = nlohmann::json;
+
+    // we need this details class so that we can have a non-templated value
+    struct FnHolder {
+      virtual json call(std::vector<json> const&) = 0;
+    };
+
+    template<typename Ret, typename... Arg>
+    class FnHolderImpl : public FnHolder {
+      using Fn = Ret(*)(Arg...);
+      Fn _fn;
+
+      // type of the N-th argument that the stored function takes
+      template<std::size_t>
+      using ArgType = std::tuple_element<I, std::tuple<Arg...>>;
+
+      // uncook each argument to its expected type, and call the function
+      template<std::size_t... I>
+      Ret call_impl(std::vector<json> const& args, std::index_sequence<I...>) {
+        return _fn(uncook<ArgType<I>>(args[I]), ...);
+      }
+
+    public:
+      FnHolderImpl(Fn fn) : _fn{std::move(fn)} {
+      }
+
+      // check arity and call the function using our little helper, before wrapping it back to json
+      json call(std::vector<json> const& args) override {
+        if (args.size() != sizeof...(Arg)) {
+          throw new std::runtime_error("Bridge method call with wrong arity, expected " + sizeof...(Arg) + ", got " + args.size() + ".");
+        }
+
+        return cook(_call(args, std::make_index_sequence<sizeof...(Arg)>{}));
+      }
+
+    };
+
+  }
+
+  class Bridge {
+    std::unordered_map<std::string, std::unique_ptr<details::FnHolder>> _fns;
+
+  public:
+    template<typename Fn>
+    void register(std::string const& name, Fn&& fn) {
+      _fns[name] = std:make_unique<details::FnHolderImpl>(std::move(fn));
+    }
+
+    json call(std::string const& name, std::vector<json> const& args) override {
+      auto it = _fns.find(name);
+      if (it == _fns.end()) {
+        throw new std::runtime_error("Bridge method not found: " + name + ".");
+      }
+      return it->call(args);
+    }
+  };
+
+}