Build label diff lookup table

This reduces number of calls to momentjs diff() if there are multiple
datasets sharing the same labels (#3211).

diff --git a/src/scales/scale.time.js b/src/scales/scale.time.js
index ab935a12a0a0ca1f5157c67bc7a8a8ec6dc3bfe1..aac6131eddfbd8ce226611dd11c333ec26f2c643 100755 (executable)
--- a/src/scales/scale.time.js
+++ b/src/scales/scale.time.js
@@ -86,6 +86,13 @@ module.exports = function(Chart) {
 
                        return null;
                },
+               getLabelDiff: function(datasetIndex, index) {
+                       if (typeof this.labelDiffs[datasetIndex] != 'undefined') {
+                               return this.labelDiffs[datasetIndex][index];
+                       }
+
+                       return null;
+               },
                getMomentStartOf: function(tick) {
                        var me = this;
                        if (me.options.time.unit === 'week' && me.options.time.isoWeekday !== false) {
@@ -162,6 +169,46 @@ module.exports = function(Chart) {
                        me.firstTick = (me.firstTick || moment()).clone();
                        me.lastTick = (me.lastTick || moment()).clone();
                },
+               buildLabelDiffs: function() {
+                       var me = this;
+                       me.labelDiffs = [];
+                       var scaleLabelDiffs = [];
+                       // Parse common labels once
+                       if (me.chart.data.labels && me.chart.data.labels.length > 0) {
+                               helpers.each(me.chart.data.labels, function(label) {
+                                       var labelMoment = me.parseTime(label);
+
+                                       if (labelMoment.isValid()) {
+                                               if (me.options.time.round) {
+                                                       labelMoment.startOf(me.options.time.round);
+                                               }
+                                               scaleLabelDiffs.push(labelMoment.diff(me.firstTick, me.tickUnit, true));
+                                       }
+                               }, me);
+                       }
+
+                       helpers.each(me.chart.data.datasets, function(dataset) {
+                               var diffsForDataset = [];
+
+                               if (typeof dataset.data[0] === 'object' && dataset.data[0] !== null) {
+                                       helpers.each(dataset.data, function(value) {
+                                               var labelMoment = me.parseTime(me.getRightValue(value));
+
+                                               if (labelMoment.isValid()) {
+                                                       if (me.options.time.round) {
+                                                               labelMoment.startOf(me.options.time.round);
+                                                       }
+                                                       diffsForDataset.push(labelMoment.diff(me.firstTick, me.tickUnit, true));
+                                               }
+                                       }, me);
+                               } else {
+                                       // We have no labels. Use common ones
+                                       diffsForDataset = scaleLabelDiffs;
+                               }
+
+                               me.labelDiffs.push(diffsForDataset);
+                       }, me);
+               },
                buildTicks: function() {
                        var me = this;
 
@@ -296,6 +343,9 @@ module.exports = function(Chart) {
                        }
 
                        me.ctx.restore();
+
+                       // Build diff table as first tick and tick units are already determined.
+                       me.buildLabelDiffs();
                },
                // Get tooltip label
                getLabelForIndex: function(index, datasetIndex) {
@@ -332,15 +382,22 @@ module.exports = function(Chart) {
                },
                getPixelForValue: function(value, index, datasetIndex) {
                        var me = this;
-                       if (!value || !value.isValid) {
-                               // not already a moment object
-                               value = me.parseTime(me.getRightValue(value));
+                       var offset = null;
+                       if (index !== undefined && datasetIndex !== undefined) {
+                               offset = me.getLabelDiff(datasetIndex, index);
                        }
-                       var labelMoment = value && value.isValid && value.isValid() ? value : me.getLabelMoment(datasetIndex, index);
 
-                       if (labelMoment) {
-                               var offset = labelMoment.diff(me.firstTick, me.tickUnit, true);
+                       if (offset === null) {
+                               if (!value || !value.isValid) {
+                                       // not already a moment object
+                                       value = me.parseTime(me.getRightValue(value));
+                               }
+                               if (value && value.isValid && value.isValid()) {
+                                       offset = value.diff(me.firstTick, me.tickUnit, true);
+                               }
+                       }
 
+                       if (offset !== null) {
                                var decimal = offset !== 0 ? offset / me.scaleSizeInUnits : offset;
 
                                if (me.isHorizontal()) {