[performance] faster major tick calculation (#6307)

* [performance] faster major tick calculation

* Address review comments

* Replace isNaN

diff --git a/src/scales/scale.time.js b/src/scales/scale.time.js
index 1be680ef5ed8edbaad3787830c5a792279606906..10dd50e4f0227e21275b7b72592128f7e42cd8de 100644 (file)
--- a/src/scales/scale.time.js
+++ b/src/scales/scale.time.js
@@ -404,21 +404,40 @@ function computeOffsets(table, ticks, min, max, options) {
        return {start: start, end: end};
 }
 
+function setMajorTicks(scale, ticks, map, majorUnit) {
+       var adapter = scale._adapter;
+       var first = +adapter.startOf(ticks[0].value, majorUnit);
+       var last = ticks[ticks.length - 1].value;
+       var major, index;
+
+       for (major = first; major <= last; major = +adapter.add(major, 1, majorUnit)) {
+               index = map[major];
+               if (index >= 0) {
+                       ticks[index].major = true;
+               }
+       }
+       return ticks;
+}
+
 function ticksFromTimestamps(scale, values, majorUnit) {
        var ticks = [];
-       var i, ilen, value, major;
+       var map = {};
+       var ilen = values.length;
+       var i, value;
 
-       for (i = 0, ilen = values.length; i < ilen; ++i) {
+       for (i = 0; i < ilen; ++i) {
                value = values[i];
-               major = majorUnit ? value === +scale._adapter.startOf(value, majorUnit) : false;
+               map[value] = i;
 
                ticks.push({
                        value: value,
-                       major: major
+                       major: false
                });
        }
 
-       return ticks;
+       // We set the major ticks separately from the above loop because calling startOf for every tick
+       // is expensive when there is a large number of ticks
+       return (ilen === 0 || !majorUnit) ? ticks : setMajorTicks(scale, ticks, map, majorUnit);
 }
 
 var defaultConfig = {