From d37ec9e5b0fffb6d5635b11e67cd9367bb002a5e Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Thu, 5 Oct 2017 18:23:56 -0400
Subject: [PATCH] Attempts to ensure good behaviour if dealt an adjustable
 clock, and `const`s where possible.

---
 Concurrency/BestEffortUpdater.cpp | 25 ++++++++++++++-----------
 1 file changed, 14 insertions(+), 11 deletions(-)

diff --git a/Concurrency/BestEffortUpdater.cpp b/Concurrency/BestEffortUpdater.cpp
index 99f706a1c..551730f0b 100644
--- a/Concurrency/BestEffortUpdater.cpp
+++ b/Concurrency/BestEffortUpdater.cpp
@@ -18,21 +18,24 @@ void BestEffortUpdater::update() {
 		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.
-			std::chrono::time_point<std::chrono::high_resolution_clock> now = std::chrono::high_resolution_clock::now();
-			auto elapsed = now - previous_time_point_;
+			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.
-				int64_t duration = std::chrono::duration_cast<std::chrono::nanoseconds>(elapsed).count();
-				double cycles = ((static_cast<double>(duration) * clock_rate_) / 1e9) + error_;
-				error_ = fmod(cycles, 1.0);
+				// if there is one. Proceed only if the number of cycles is positive, and cap it to the per-second maximum —
+				// it's possible this is an adjustable clock so be ready to swallow unexpected adjustments.
+				const int64_t duration = std::chrono::duration_cast<std::chrono::nanoseconds>(elapsed).count();
+				if(duration > 0) {
+					double cycles = ((static_cast<double>(duration) * clock_rate_) / 1e9) + error_;
+					error_ = fmod(cycles, 1.0);
 
-				if(delegate_) {
-					delegate_->update(this, (int)cycles, has_skipped_);
+					if(delegate_) {
+						delegate_->update(this, static_cast<int>(std::min(cycles, clock_rate_)), has_skipped_);
+					}
+					has_skipped_ = false;
 				}
-				has_skipped_ = false;
 			} else {
 				has_previous_time_point_ = true;
 			}
@@ -51,13 +54,13 @@ void BestEffortUpdater::flush() {
 	async_task_queue_.flush();
 }
 
-void BestEffortUpdater::set_delegate(Delegate *delegate) {
+void BestEffortUpdater::set_delegate(Delegate *const delegate) {
 	async_task_queue_.enqueue([this, delegate]() {
 		delegate_ = delegate;
 	});
 }
 
-void BestEffortUpdater::set_clock_rate(double clock_rate) {
+void BestEffortUpdater::set_clock_rate(const double clock_rate) {
 	async_task_queue_.enqueue([this, clock_rate]() {
 		this->clock_rate_ = clock_rate;
 	});