can toggle between paused and realtime

src/VisualEditor/time.ts

@@ -0,0 +1,61 @@
+
+export type TimeMode = TimePaused | TimeRealTime;
+
+export type TimePaused = {
+  kind: "paused",
+  simtime: number, // the current simulated time
+}
+
+export type TimeRealTime = {
+  kind: "realtime",
+  scale: number, // time scale relative to wall-clock time
+  since: {
+    simtime: number,  // the simulated time at which the time was set to realtime
+    wallclktime: number, // the wall-clock time at which the time was set to realtime
+  }
+}
+
+export function getSimTime(currentMode: TimeMode, wallclktime: number): number {
+  if (currentMode.kind === "paused") {
+    return currentMode.simtime;
+  }
+  else {
+    const elapsedWallclk = wallclktime - currentMode.since.wallclktime;
+    return currentMode.since.simtime + currentMode.scale * elapsedWallclk;
+  }
+}
+
+export function setRealtime(currentMode: TimeMode, scale: number, wallclktime: number): TimeRealTime {
+  if (currentMode.kind === "paused") {
+    return {
+      kind: "realtime",
+      scale,
+      since: {
+        simtime: currentMode.simtime,
+        wallclktime,
+      },
+    };
+  }
+  else {
+    return {
+      kind: "realtime",
+      scale,
+      since: {
+        simtime: getSimTime(currentMode, wallclktime),
+        wallclktime,
+      },
+    };
+  }
+}
+
+export function setPaused(currentMode: TimeMode, wallclktime: number): TimePaused {
+  if (currentMode.kind === "paused") {
+    return currentMode; // no change
+  }
+  else {
+    return {
+      kind: "paused",
+      simtime: getSimTime(currentMode, wallclktime),
+    };
+  }
+}