added low-level ioLib.available for io_stream's

src/logic/scripting/descriptors_manager.cpp

@@ -28,6 +28,33 @@ namespace scripting {
         }
     }
 
+    int descriptors_manager::available(int descriptor) {
+        if (!is_readable(descriptor))
+            return 0;
+
+        auto* stream = descriptors[descriptor]->in.get();
+
+        auto current_pos = stream->tellg();
+
+        if (current_pos == -1) {
+            return 0;
+        }
+
+        stream->seekg(0, std::ios::end);
+
+        auto end_pos = stream->tellg();
+
+        stream->seekg(current_pos,  std::ios::beg);
+
+        auto remaining = end_pos - current_pos;
+
+        if (remaining > std::numeric_limits<int>::max()) {
+            return std::numeric_limits<int>::max();
+        }
+
+        return static_cast<int>(remaining);
+    }
+
     bool descriptors_manager::has_descriptor(int descriptor) {
         return is_readable(descriptor) || is_writeable(descriptor);
     }

src/logic/scripting/descriptors_manager.hpp

@@ -26,6 +26,8 @@ namespace scripting {
 
         static void flush(int descriptor);
 
+        static int available(int descriptor);
+
         static bool has_descriptor(int descriptor);
 
         static bool is_readable(int descriptor);

src/logic/scripting/lua/libs/libfile.cpp

@@ -397,6 +397,20 @@ static int l_flush_descriptor(lua::State* L) {
     return 0;
 }
 
+static int l_available_descriptor(lua::State* L) {
+    int descriptor = lua::tointeger(L, 1);
+
+    if (!scripting::descriptors_manager::has_descriptor(descriptor)) {
+        throw std::runtime_error("unknown descriptor");
+    }
+
+    if (!scripting::descriptors_manager::is_readable(descriptor)) {
+        throw std::runtime_error("descriptor is not readable");
+    }
+
+    return lua::pushinteger(L, scripting::descriptors_manager::available(descriptor));
+}
+
 static int l_close_descriptor(lua::State* L) {
     int descriptor = lua::tointeger(L, 1);
 
@@ -443,6 +457,7 @@ const luaL_Reg filelib[] = {
     {"__write_descriptor", lua::wrap<l_write_descriptor>},
     {"__seek_descriptor", lua::wrap<l_seek_descriptor>},
     {"__flush_descriptor", lua::wrap<l_flush_descriptor>},
+    {"__available_descriptor", lua::wrap<l_available_descriptor>},
     {"__close_descriptor", lua::wrap<l_close_descriptor>},
     {"__close_all_descriptors", lua::wrap<l_close_all_descriptors>},
     {nullptr, nullptr}