t5de-renderer/adapter/frame_pacer.cpp

#include "frame_pacer.hpp"

#include <algorithm>
#include <mutex>
#include <string>
#include <thread>
#include <vector>

#include "log.hpp"

namespace t5de::frame_pacer {

namespace {

// The client's own rate: its ticks already draw these frames.
constexpr DWORD clientRate = 30;
// The client is left idle at least this share of a frame's time between frames.
constexpr double idleShare = 1.0 / 3.0;

// The user's frame rate, from the registry (unset or unreadable: the client's own).
DWORD read_rate()
{
    DWORD rate = 0;
    DWORD size = sizeof(rate);
    if (RegGetValueW(HKEY_CURRENT_USER, L"Software\\T5DE\\Graphics", L"FrameRate", RRF_RT_REG_DWORD, nullptr, &rate,
                     &size) != ERROR_SUCCESS) {
        return clientRate;
    }
    return rate;
}

double now_seconds()
{
    static const double frequency = [] {
        LARGE_INTEGER value;
        QueryPerformanceFrequency(&value);
        return static_cast<double>(value.QuadPart);
    }();
    LARGE_INTEGER counter;
    QueryPerformanceCounter(&counter);
    return static_cast<double>(counter.QuadPart) / frequency;
}

class Pacer {
public:
    static Pacer& get()
    {
        static Pacer pacer;
        return pacer;
    }

    void add(HWND window)
    {
        const std::scoped_lock lock(mutex_);
        windows_.push_back({.window = window});
        if (!started_) {
            started_ = true;
            std::thread([this] { run(); }).detach(); // runs while the client does
        }
    }

    void remove(HWND window)
    {
        const std::scoped_lock lock(mutex_);
        std::erase_if(windows_, [window](const Window& w) { return w.window == window; });
    }

    void frameDrawn(HWND window, double seconds)
    {
        const std::scoped_lock lock(mutex_);
        for (Window& w : windows_) {
            if (w.window == window) {
                w.pending = false;
                w.frameTime = w.frameTime == 0 ? seconds : w.frameTime * 0.9 + seconds * 0.1;
                w.drawnAt = now_seconds();
            }
        }
    }

private:
    struct Window {
        HWND window;
        bool pending = false;  // a repaint asked for and not yet drawn
        double frameTime = 0;  // seconds a frame takes the client, averaged
        double drawnAt = 0;    // when its last frame was drawn
        double askedAt = 0;    // when a repaint was last asked for
    };

    // Re-reads the setting now and then; above the client's rate, asks each window for repaints at the rate, or as often
    // as leaves the client its idle time.
    void run()
    {
        HANDLE timer = CreateWaitableTimerExW(nullptr, nullptr, CREATE_WAITABLE_TIMER_HIGH_RESOLUTION, TIMER_ALL_ACCESS);
        double checked = 0;
        DWORD rate = clientRate;
        for (;;) {
            const double now = now_seconds();
            if (now - checked >= 0.5) {
                checked = now;
                const DWORD read = read_rate();
                if (read != rate) {
                    rate = read;
                    log::info("frame pacer: frame rate {}", rate == 0 ? std::string("uncapped") : std::to_string(rate));
                }
            }
            if (rate != 0 && rate <= clientRate) {
                Sleep(100);
                continue;
            }
            const double interval = rate == 0 ? 0 : 1.0 / rate;
            {
                const std::scoped_lock lock(mutex_);
                for (Window& w : windows_) {
                    if (w.pending && now - w.askedAt < 0.25) {
                        continue; // not drawn yet (asked again after a while: the client may have skipped it)
                    }
                    // Since the last frame ended: the rest of the interval since it began, and the idle time.
                    if (now - w.drawnAt >= std::max(interval - w.frameTime, w.frameTime * idleShare)) {
                        w.pending = true;
                        w.askedAt = now;
                        InvalidateRect(w.window, nullptr, FALSE);
                    }
                }
            }
            // Wake often enough for the rate (1 ms at most when uncapped).
            const double wait = std::clamp(interval / 4, 0.0005, 0.004);
            LARGE_INTEGER due{.QuadPart = -static_cast<LONGLONG>(wait * 10'000'000)}; // relative, 100 ns units
            if (timer != nullptr && SetWaitableTimer(timer, &due, 0, nullptr, nullptr, FALSE)) {
                WaitForSingleObject(timer, INFINITE);
            } else {
                Sleep(1);
            }
        }
    }

    std::mutex mutex_;
    std::vector<Window> windows_;
    bool started_ = false;
};

} // namespace

void add(HWND window) { Pacer::get().add(window); }

void remove(HWND window) { Pacer::get().remove(window); }

void frameDrawn(HWND window, double seconds) { Pacer::get().frameDrawn(window, seconds); }

} // namespace t5de::frame_pacer