Add utility for profiling iteration constructs

(imported from commit d4e55353475b86ba06da2d776fce2453cd33cc84)

zephyr/static/js/debug.js

@@ -25,3 +25,77 @@ function print_elapsed_time(name, fun) {
     console.log(name + ': ' + (t1 - t0) + ' ms');
     return out;
 }
+
+
+/* An IterationProfiler is used for profiling parts of looping
+ * constructs (like a for loop or $.each).  You mark sections of the
+ * iteration body and the IterationProfiler will sum the costs of those
+ * sections over all iterations.
+ *
+ * Example:
+ *
+ *     var ip = new IterationProfiler();
+ *     $.each(myarray, function (idx, elem) {
+ *         ip.iteration_start();
+ *
+ *         cheap_op(elem);
+ *         ip.section("a");
+ *         expensive_op(elem);
+ *         ip.section("b");
+ *         another_expensive_op(elem);
+ *
+ *         ip.iteration_stop();
+ *     });
+ *     ip.done();
+ *
+ * The console output will look something like:
+ *     _iteration_overhead 0.8950002520577982
+ *     _rest_of_iteration 153.415000159293413
+ *     a 2.361999897402711
+ *     b 132.625999901327305
+ *
+ * The _rest_of_iteration section is the region of the iteration body
+ * after section b.
+ */
+function IterationProfiler() {
+    this.sections = {};
+    this.last_time = window.performance.now();
+}
+
+IterationProfiler.prototype = {
+    iteration_start: function () {
+        this.section('_iteration_overhead');
+    },
+
+    iteration_stop: function () {
+        var now = window.performance.now();
+        var diff = now - this.last_time;
+        if (diff > 1) {
+            if (this.sections._rest_of_iteration === undefined) {
+                this.sections._rest_of_iteration = 0;
+            }
+            this.sections._rest_of_iteration += diff;
+        }
+        this.last_time = now;
+    },
+
+    section: function(label) {
+        var now = window.performance.now();
+        if (this.sections[label] === undefined) {
+            this.sections[label] = 0;
+        }
+        this.sections[label] += (now - this.last_time);
+        this.last_time = now;
+    },
+
+    done: function () {
+        this.section('_iteration_overhead');
+        var prop;
+
+        for (prop in this.sections) {
+            if (this.sections.hasOwnProperty(prop)) {
+                console.log(prop, this.sections[prop]);
+            }
+        }
+    }
+};