Speed up tooltip value retrieval

public/app/plugins/panel/graph/graph_tooltip.js

@@ -34,13 +34,22 @@ function ($, core) {
     };
 
     this.findHoverIndexFromData = function(posX, series) {
-      var len = series.data.length;
-      for (var j = 0; j < len; j++) {
-        if (series.data[j][0] > posX) {
-          return Math.max(j - 1,  0);
+      var lower = 0;
+      var upper = series.data.length - 1;
+      var middle;
+      while (true) {
+        if (lower > upper) {
+          return Math.max(upper, 0);
+        }
+        middle = Math.floor((lower + upper) / 2);
+        if (series.data[middle][0] === posX) {
+          return middle;
+        } else if (series.data[middle][0] < posX) {
+          lower = middle + 1;
+        } else { //if (series.data[middle][0] > posX) {
+          upper = middle - 1;
         }
       }
-      return j - 1;
     };
 
     this.renderAndShow = function(absoluteTime, innerHtml, pos, xMode) {

public/app/plugins/panel/graph/specs/tooltip_specs.ts

@@ -40,6 +40,31 @@ function describeSharedTooltip(desc, fn) {
   });
 }
 
+describe("findHoverIndexFromData", function() {
+  var tooltip = new GraphTooltip(elem, dashboard, scope);
+  var series = { data: [[100, 0], [101, 0], [102, 0], [103, 0], [104, 0], [105, 0], [106, 0], [107, 0]] };
+
+  it("should return 0 if posX out of lower bounds", function() {
+    var posX = 99;
+    expect(tooltip.findHoverIndexFromData(posX, series)).to.be(0);
+  });
+
+  it("should return n - 1 if posX out of upper bounds", function() {
+    var posX = 108;
+    expect(tooltip.findHoverIndexFromData(posX, series)).to.be(series.data.length - 1);
+  });
+
+  it("should return i if posX in series", function() {
+    var posX = 104;
+    expect(tooltip.findHoverIndexFromData(posX, series)).to.be(4);
+  });
+
+  it("should return i if posX not in series and i + 1 > posX", function() {
+    var posX = 104.9;
+    expect(tooltip.findHoverIndexFromData(posX, series)).to.be(4);
+  });
+});
+
 describeSharedTooltip("steppedLine false, stack false", function(ctx) {
   ctx.setup(function() {
     ctx.data = [
@@ -168,6 +193,3 @@ describeSharedTooltip("steppedLine false, stack true, individual true", function
     expect(ctx.results[1].value).to.be(2);
   });
 });
-
-
-