Fix a couple minor bugs in particle code

- Copy effect state to particle uniforms before each script, so changes
  from spawn or update are visible.
- Guard path binding against out of range access
- New effect that actually stresses both of these conditions

Change-Id: Ice6112793099e515438af8bb863e9e1bf03d08b1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/249125
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>

modules/particles/src/SkParticleBinding.cpp

@@ -100,11 +100,12 @@ public:
     void call(int index, float* arguments, float* outReturn) override {
         SkScalar len = fPath->fTotalLength * arguments[0];
         int idx = 0;
-        while (idx < fPath->fContours.count() && len > fPath->fContours[idx]->length()) {
+        while (idx < fPath->fContours.count() - 1 && len > fPath->fContours[idx]->length()) {
             len -= fPath->fContours[idx++]->length();
         }
         SkVector localXAxis;
-        if (!fPath->fContours[idx]->getPosTan(len, (SkPoint*)outReturn, &localXAxis)) {
+        if (idx >= fPath->fContours.count() ||
+            !fPath->fContours[idx]->getPosTan(len, (SkPoint*)outReturn, &localXAxis)) {
             outReturn[0] = outReturn[1] = 0.0f;
             localXAxis = { 1, 0 };
         }

modules/particles/src/SkParticleEffect.cpp

@@ -278,6 +278,7 @@ void SkParticleEffect::runParticleScript(double now, const char* entry, int star
                 value->setRandom(randomBase);
                 value->setEffect(this);
             }
+            memcpy(&fParticleUniforms[1], &fState.fAge, sizeof(EffectState));
             SkAssertResult(byteCode->runStriped(fun, count, args, SkParticles::kNumChannels,
                                                 nullptr, 0,
                                                 fParticleUniforms.data(),
@@ -321,7 +322,6 @@ void SkParticleEffect::advanceTime(double now) {
     SkASSERT(!this->particleCode() || this->particleCode()->getUniformLocation("effect.age") == 1);
     fEffectUniforms[0] = deltaTime;
     fParticleUniforms[0] = deltaTime;
-    memcpy(&fParticleUniforms[1], &fState.fAge, sizeof(EffectState));
 
     // Is this the first update after calling start()?
     // Run 'effectSpawn' to set initial emitter properties.

resources/particles/writing.json

@@ -0,0 +1,43 @@
+{
+   "MaxCount": 4000,
+   "Drawable": {
+      "Type": "SkCircleDrawable",
+      "Radius": 2
+   },
+   "EffectCode": [
+      "void effectSpawn(inout Effect effect) {",
+      "  effect.lifetime = 4;",
+      "}",
+      "",
+      "void effectUpdate(inout Effect effect) {",
+      "  effect.color.r = 0;",
+      "  effect.color.g = 1 - effect.age;",
+      "  effect.color.b = effect.age;",
+      "",
+      "  effect.rate = effect.age < 0.75 ? 800 : 0;",
+      "}",
+      ""
+   ],
+   "Code": [
+      "uniform float2 mouse_pos;",
+      "",
+      "void spawn(inout Particle p) {",
+      "  p.lifetime = 4;",
+      "  p.pos = mouse_pos;",
+      "  p.frame = effect.age / 0.75 + mix(-0.05, 0.05, rand);",
+      "}",
+      "",
+      "void update(inout Particle p) {",
+      "  p.pos = mix(p.pos, text(p.frame).xy, 0.05);",
+      "}",
+      ""
+   ],
+   "Bindings": [
+      {
+         "Type": "SkTextBinding",
+         "Name": "text",
+         "Text": "HELLO WORLD",
+         "FontSize": 96
+      }
+   ]
+}