Not for review - Rebase of crrev.com/62833003

LayoutTests/fast/workers/resources/snap.js

+(function(scope) {
+  'use strict';
+
+  var tokens = null;
+  var scroll = null;
+  var previous = 0;
+  var consecutiveSnappableFrames = 0;
+
+  function getClosestSnapPoint(oldscroll, delta) {
+      var diff = oldscroll % delta;
+      if (diff >= delta / 2) {
+          return oldscroll + (delta - diff);
+      }
+
+      return oldscroll - diff;
+  }
+
+  function getVelocity(distance) {
+      if (distance === 0)
+          return 0;
+      var sign = distance > 0 ? 1 : -1;
+
+      return distance / 5;
+      //return Math.log(distance + 1);
+      //return sign * (Math.log(Math.abs(distance)+0.0025)+2);
+  }
+
+  function shouldSnap(previous, current) {
+      var pixelThreshold = 0;
+      //var pixelThreshold = 10;
+      var frameThreshold = 10;
+
+      if (Math.abs(current - previous) <= pixelThreshold) {
+        consecutiveSnappableFrames++;
+      } else {
+        consecutiveSnappableFrames = 0;
+      }
+
+      return consecutiveSnappableFrames >= frameThreshold;
+  }
+
+  // An extremely cheesy animation.
+  function tick(context) {
+      var seconds = context.timestamp / 1000.0;
+      var oldscroll = context.getScalar(scroll);
+      var newscrollposition = oldscroll;
+
+      if (shouldSnap(previous, oldscroll)) {
+          console.log('good');
+          var snap = getClosestSnapPoint(oldscroll, 400);
+          var distance = snap - oldscroll;
+          var velocity = getVelocity(distance);
+          newscrollposition += velocity;
+          console.log("snap " + snap + ", distance " + distance + ", velocity " + velocity);
+
+          context.setScalar(scroll, newscrollposition);
+      } else {
+        console.log('bad ' + previous + ' ' + oldscroll);
+      }
+
+      previous = newscrollposition;
+      scope.teleportMessage(context, tick);
+  }
+
+  scope.onmessage = function(e) {
+    tokens = e.data;
+    scroll = tokens[0];
+    var context = new TeleportContext(tokens);
+    scope.teleportMessage(context, tick);
+  };
+
+})(self);