Initial commit.

simulation.cpp

@@ -0,0 +1,45 @@
+// -*- C++ -*-
+
+#include <SDL/sdl.h>
+
+#include "simulation.h"
+
+using namespace mbostock;
+
+/*
+ * If for some reason we haven't been able to run the simulation in a long time
+ * (e.g., the computer went to sleep for two hours!), then we don't want to run
+ * the simulation a zillion times to catch up. Instead we just drop some frames
+ * and hope it gets better.
+ */
+static const uint32_t maxSkippedMs = 500;
+
+Simulation::Simulation(uint32_t timeStepMs)
+  : timeStepMs_(timeStepMs), skippedMs_(0), lastTimeMs_(0), paused_(false) {
+}
+
+uint32_t Simulation::elapsedMillis() {
+  uint32_t currentTimeMs = SDL_GetTicks();
+  uint32_t differenceMs = currentTimeMs - lastTimeMs_;
+  lastTimeMs_ = currentTimeMs;
+  return differenceMs;
+}
+
+void Simulation::togglePaused() {
+  paused_ = !paused_;
+}
+
+void Simulation::simulate() {
+  if (paused_ || (lastTimeMs_ == 0)) {
+    lastTimeMs_ = SDL_GetTicks();
+    return;
+  }
+  skippedMs_ += elapsedMillis();
+  if (skippedMs_ > maxSkippedMs) {
+    skippedMs_ = timeStepMs_;
+  }
+  while (skippedMs_ >= timeStepMs_) {
+    step();
+    skippedMs_ -= timeStepMs_;
+  }
+}