#pragma once

namespace DX {
	inline void throw_runtime_error(char const* const message)
	{
#ifdef __cplusplus_winrt
		message;
		throw ref new Platform::FailureException();
#else
		throw std::runtime_error(message);
#endif
	}

	// Helper class for animation and simulation timing.
	class StepTimer {
	public:
		StepTimer() :
			m_elapsedTicks(0),
			m_totalTicks(0),
			m_leftOverTicks(0),
			m_frameCount(0),
			m_framesPerSecond(0),
			m_framesThisSecond(0),
			m_qpcSecondCounter(0),
			m_isFixedTimeStep(false),
			m_targetElapsedTicks(TicksPerSecond / 60) {
			if(!QueryPerformanceFrequency(&m_qpcFrequency)) {
				throw_runtime_error("QueryPerformanceFrequency");
			}

			if(!QueryPerformanceCounter(&m_qpcLastTime)) {
				throw_runtime_error("QueryPerformanceCounter");
			}

			// Initialize max delta to 1/10 of a second.
			m_qpcMaxDelta = m_qpcFrequency.QuadPart / 10;
		}

		// Get elapsed time since the previous Update call.
		uint64_t GetElapsedTicks() const { return m_elapsedTicks; }
		double GetElapsedSeconds() const { return TicksToSeconds(m_elapsedTicks); }

		// Get total time since the start of the program.
		uint64_t GetTotalTicks() const { return m_totalTicks; }
		double GetTotalSeconds() const { return TicksToSeconds(m_totalTicks); }

		// Get total number of updates since start of the program.
		uint32_t GetFrameCount() const { return m_frameCount; }

		// Get the current framerate.
		uint32_t GetFramesPerSecond() const { return m_framesPerSecond; }

		// Set whether to use fixed or variable timestep mode.
		void SetFixedTimeStep(bool isFixedTimestep) { m_isFixedTimeStep = isFixedTimestep; }

		// Set how often to call Update when in fixed timestep mode.
		void SetTargetElapsedTicks(uint64_t targetElapsed) { m_targetElapsedTicks = targetElapsed; }
		void SetTargetElapsedSeconds(double targetElapsed) { m_targetElapsedTicks = SecondsToTicks(targetElapsed); }

		// Integer format represents time using 10,000,000 ticks per second.
		static const uint64_t TicksPerSecond = 10000000;

		static double TicksToSeconds(uint64_t ticks) { return static_cast<double>(ticks) / TicksPerSecond; }
		static uint64_t SecondsToTicks(double seconds) { return static_cast<uint64_t>(seconds * TicksPerSecond); }

		// After an intentional timing discontinuity (for instance a blocking IO operation)
		// call this to avoid having the fixed timestep logic attempt a set of catch-up 
		// Update calls.

		void ResetElapsedTime() {
			if(!QueryPerformanceCounter(&m_qpcLastTime)) {
				throw_runtime_error("QueryPerformanceCounter");
			}

			m_leftOverTicks = 0;
			m_framesPerSecond = 0;
			m_framesThisSecond = 0;
			m_qpcSecondCounter = 0;
		}

		// Update timer state, calling the specified Update function the appropriate number of times.
		template<typename TUpdate>
		void Tick(const TUpdate& update) {
			// Query the current time.
			LARGE_INTEGER currentTime;

			if(!QueryPerformanceCounter(&currentTime)) {
				throw_runtime_error("QueryPerformanceCounter");
			}

			uint64_t timeDelta = currentTime.QuadPart - m_qpcLastTime.QuadPart;

			m_qpcLastTime = currentTime;
			m_qpcSecondCounter += timeDelta;

			// Clamp excessively large time deltas (e.g. after paused in the debugger).
			if(timeDelta > m_qpcMaxDelta) {
				timeDelta = m_qpcMaxDelta;
			}

			// Convert QPC units into a canonical tick format. This cannot overflow due to the previous clamp.
			timeDelta *= TicksPerSecond;
			timeDelta /= m_qpcFrequency.QuadPart;

			uint32_t lastFrameCount = m_frameCount;

			if(m_isFixedTimeStep) {
				// Fixed timestep update logic

				// If the app is running very close to the target elapsed time (within 1/4 of a millisecond) just clamp
				// the clock to exactly match the target value. This prevents tiny and irrelevant errors
				// from accumulating over time. Without this clamping, a game that requested a 60 fps
				// fixed update, running with vsync enabled on a 59.94 NTSC display, would eventually
				// accumulate enough tiny errors that it would drop a frame. It is better to just round 
				// small deviations down to zero to leave things running smoothly.

				if(abs(static_cast<int64_t>(timeDelta - m_targetElapsedTicks)) < TicksPerSecond / 4000) {
					timeDelta = m_targetElapsedTicks;
				}

				m_leftOverTicks += timeDelta;

				while(m_leftOverTicks >= m_targetElapsedTicks) {
					m_elapsedTicks = m_targetElapsedTicks;
					m_totalTicks += m_targetElapsedTicks;
					m_leftOverTicks -= m_targetElapsedTicks;
					m_frameCount++;

					update();
				}
			} else {
				// Variable timestep update logic.
				m_elapsedTicks = timeDelta;
				m_totalTicks += timeDelta;
				m_leftOverTicks = 0;
				m_frameCount++;

				update();
			}

			// Track the current framerate.
			if(m_frameCount != lastFrameCount) {
				m_framesThisSecond++;
			}

			if(m_qpcSecondCounter >= static_cast<uint64_t>(m_qpcFrequency.QuadPart)) {
				m_framesPerSecond = m_framesThisSecond;
				m_framesThisSecond = 0;
				m_qpcSecondCounter %= m_qpcFrequency.QuadPart;
			}
		}

	private:
		// Source timing data uses QPC units.
		LARGE_INTEGER m_qpcFrequency;
		LARGE_INTEGER m_qpcLastTime;
		uint64_t m_qpcMaxDelta;

		// Derived timing data uses a canonical tick format.
		uint64_t m_elapsedTicks;
		uint64_t m_totalTicks;
		uint64_t m_leftOverTicks;

		// Members for tracking the framerate.
		uint32_t m_frameCount;
		uint32_t m_framesPerSecond;
		uint32_t m_framesThisSecond;
		uint64_t m_qpcSecondCounter;

		// Members for configuring fixed timestep mode.
		bool m_isFixedTimeStep;
		uint64_t m_targetElapsedTicks;
	};
}
