CLK/Concurrency/BestEffortUpdater.cpp

//
//  BestEffortUpdater.cpp
//  Clock Signal
//
//  Created by Thomas Harte on 04/10/2017.
//  Copyright 2017 Thomas Harte. All rights reserved.
//

#include "BestEffortUpdater.hpp"

#include <cmath>

using namespace Concurrency;

BestEffortUpdater::BestEffortUpdater() {
	// ATOMIC_FLAG_INIT isn't necessarily safe to use, so establish default state by other means.
	update_is_ongoing_.clear();
}

BestEffortUpdater::~BestEffortUpdater() {
	// Don't allow further deconstruction until the task queue is stopped.
	flush();
}

void BestEffortUpdater::update() {
	// Perform an update only if one is not currently ongoing.
	if(!update_is_ongoing_.test_and_set()) {
		async_task_queue_.enqueue([this]() {
			// Get time now using the highest-resolution clock provided by the implementation, and determine
			// the duration since the last time this section was entered.
			const std::chrono::time_point<std::chrono::high_resolution_clock> now = std::chrono::high_resolution_clock::now();
			const auto elapsed = now - previous_time_point_;
			previous_time_point_ = now;

			if(has_previous_time_point_) {
				// If the duration is valid, convert it to integer cycles, maintaining a rolling error and call the delegate
				// if there is one. Proceed only if the number of cycles is positive, and cap it to the per-second maximum as
				// it's possible this is an adjustable clock so be ready to swallow unexpected adjustments.
				const int64_t integer_duration = std::chrono::duration_cast<std::chrono::nanoseconds>(elapsed).count();
				if(integer_duration > 0) {
					if(delegate_) {
						// Cap running at 1/5th of a second, to avoid doing a huge amount of work after any
						// brief system interruption.
						const double duration = std::min(static_cast<double>(integer_duration) / 1e9, 0.2);
						delegate_->update(this, duration, has_skipped_);
					}
					has_skipped_ = false;
				}
			} else {
				has_previous_time_point_ = true;
			}

			// Allow furthers updates to occur.
			update_is_ongoing_.clear();
		});
	} else {
		async_task_queue_.enqueue([this]() {
			has_skipped_ = true;
		});
	}
}

void BestEffortUpdater::flush() {
	async_task_queue_.flush();
}

void BestEffortUpdater::set_delegate(Delegate *const delegate) {
	async_task_queue_.enqueue([this, delegate]() {
		delegate_ = delegate;
	});
}