clock: Ensure that now() is monotonic

internal/clock/clock.go

@@ -112,6 +112,8 @@ func updateFPSAndTPS(now int64, count int) {
 
 const UncappedTPS = -1
 
+var previousNow int64
+
 // Update updates the inner clock state and returns an integer value
 // indicating how many times the game should update based on given tps.
 // tps represents TPS (ticks per second).
@@ -124,6 +126,10 @@ func Update(tps int) int {
 	defer m.Unlock()
 
 	n := now()
+	if n < previousNow {
+		panic("clock: now() must be monotonic but returned older time than before: perhaps overflowing?")
+	}
+
 	c := 0
 	if tps == UncappedTPS {
 		c = 1
@@ -131,5 +137,7 @@ func Update(tps int) int {
 		c = calcCountFromTPS(int64(tps), n)
 	}
 	updateFPSAndTPS(n, c)
+
+	previousNow = n
 	return c
 }

internal/clock/now.go

@@ -22,5 +22,7 @@ import (
 )
 
 func now() int64 {
+	// time.Now() is monotonic:
+	// https://golang.org/pkg/time/#hdr-Monotonic_Clocks
 	return time.Now().UnixNano()
 }

internal/clock/now_js.go

@@ -23,5 +23,8 @@ import (
 
 func now() int64 {
 	// time.Now() is not reliable until GopherJS supports performance.now().
+	//
+	// performance.now is monotonic:
+	// https://developers.google.com/web/updates/2012/08/When-milliseconds-are-not-enough-performance-now#monotonic_time
 	return int64(js.Global().Get("performance").Call("now").Float() * float64(time.Millisecond))
 }