Add CheatEngine and support for Gateway cheats (#4406)

* Add CheatEngine; Add support for Gateway cheats; Add Cheat UI

* fix a potential crash on some systems

* fix substr with negative length

* Add Joker to the NonOp comp handling

* Fixup JokerOp

* minor fixup in patchop; add todo for nested loops

* Add comment for PadState member variable in HID

* fix: stol to stoul in parsing cheat file

* fix misplaced parsing of values; fix patchop code

* add missing break

* Make read_func and write_func a template parameter

src/core/cheats/cheats.cpp

@@ -0,0 +1,58 @@
+// Copyright 2018 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#include <functional>
+#include <fmt/format.h>
+#include "core/cheats/cheats.h"
+#include "core/cheats/gateway_cheat.h"
+#include "core/core.h"
+#include "core/core_timing.h"
+#include "core/hle/kernel/process.h"
+
+namespace Cheats {
+
+constexpr u64 run_interval_ticks = BASE_CLOCK_RATE_ARM11 / 60;
+
+CheatEngine::CheatEngine(Core::System& system_) : system(system_) {
+    LoadCheatFile();
+    event = system.CoreTiming().RegisterEvent(
+        "CheatCore::run_event",
+        [this](u64 thread_id, s64 cycle_late) { RunCallback(thread_id, cycle_late); });
+    system.CoreTiming().ScheduleEvent(run_interval_ticks, event);
+}
+
+CheatEngine::~CheatEngine() {
+    system.CoreTiming().UnscheduleEvent(event, 0);
+}
+
+const std::vector<std::unique_ptr<CheatBase>>& CheatEngine::GetCheats() const {
+    return cheats_list;
+}
+
+void CheatEngine::LoadCheatFile() {
+    const std::string cheat_dir = FileUtil::GetUserPath(FileUtil::UserPath::CheatsDir);
+    const std::string filepath = fmt::format(
+        "{}{:016X}.txt", cheat_dir, system.Kernel().GetCurrentProcess()->codeset->program_id);
+
+    if (!FileUtil::IsDirectory(cheat_dir)) {
+        FileUtil::CreateDir(cheat_dir);
+    }
+
+    if (!FileUtil::Exists(filepath))
+        return;
+
+    auto gateway_cheats = GatewayCheat::LoadFile(filepath);
+    std::move(gateway_cheats.begin(), gateway_cheats.end(), std::back_inserter(cheats_list));
+}
+
+void CheatEngine::RunCallback([[maybe_unused]] u64 userdata, int cycles_late) {
+    for (auto& cheat : cheats_list) {
+        if (cheat->IsEnabled()) {
+            cheat->Execute(system);
+        }
+    }
+    system.CoreTiming().ScheduleEvent(run_interval_ticks - cycles_late, event);
+}
+
+} // namespace Cheats