According to rstudio#418, improve dataTables.buttons.min.js, mimickin…

…g with minor adjustments changes from https://github.com/chrisvwn/DT/blob/master/inst/htmlwidgets/lib/datatables-extensions/Buttons/js/dataTables.buttons.min.js, which is inpired by  https://datatables.net/forums/discussion/comment/106434/#Comment_106434

This prepares the dataTables.buttons.min.js to handle multiple row headers. Without further adjustments this change actually create problems for all output functions, which expect a single row header instead of an array.

inst/htmlwidgets/lib/datatables-extensions/Buttons/js/dataTables.buttons.min.js

@@ -342,6 +342,64 @@
         g(a);
         return c
     };
+
+/* ----- BEGIN added Code ----- */
+    getHeaders = function( dt ){
+        var thRows = dt.nTHead.rows;
+        var numRows = thRows.length;
+        var matrix = [];
+
+        // Iterate over each row of the header and add information to matrix.
+        for ( var rowIdx = 0;  rowIdx < numRows;  rowIdx++ ) {
+            var $row = $(thRows[rowIdx]);
+
+            // Iterate over actual columns specified in this row.
+            var $ths = $row.children("th");
+            for ( var colIdx = 0;  colIdx < $ths.length;  colIdx++ )
+            {
+                var $th = $($ths.get(colIdx));
+                var colspan = $th.attr("colspan") || 1;
+                var rowspan = $th.attr("rowspan") || 1;
+                var colCount = 0;
+
+                // ----- add this cell's title to the matrix
+                if (matrix[rowIdx] === undefined) {
+                    matrix[rowIdx] = [];  // create array for this row
+                }
+                // find 1st empty cell
+                for ( var j = 0;  j < (matrix[rowIdx]).length;  j++, colCount++ ) {
+                    if ( matrix[rowIdx][j] === "PLACEHOLDER" ) {
+                        break;
+                    }
+                }
+                var myColCount = colCount;
+                matrix[rowIdx][colCount++] = $th.text();
+
+                // ----- If title cell has colspan, add empty titles for extra cell width.
+                for ( var j = 1;  j < colspan;  j++ ) {
+                    matrix[rowIdx][colCount++] = "";
+                }
+
+                // ----- If title cell has rowspan, add empty titles for extra cell height.
+                for ( var i = 1;  i < rowspan;  i++ ) {
+                    var thisRow = rowIdx+i;
+                    if ( matrix[thisRow] === undefined ) {
+                        matrix[thisRow] = [];
+                    }
+                    // First add placeholder text for any previous columns.                
+                    for ( var j = (matrix[thisRow]).length;  j < myColCount;  j++ ) {
+                        matrix[thisRow][j] = "PLACEHOLDER";
+                    }
+                    for ( var j = 0;  j < colspan;  j++ ) {  // and empty for my columns
+                        matrix[thisRow][myColCount+j] = "";
+                    }
+                }
+            }
+        }
+        return matrix;
+    };
+    /*END added code*/
+
     l.buttonSelector = function(a, b) {
         for (var c = [], e = function(a, b, c) {
                 for (var d, g, f = 0, h = b.length; f < h; f++)
@@ -698,10 +756,14 @@
                     c.decodeEntities && (v.innerHTML = a, a = v.value);
                     return a
                 },
-                a = b.columns(c.columns).indexes().map(function(a) {
+                /*BEGIN ADD*/
+                /*a = b.columns(c.columns).indexes().map(function(a) {
                     var d = b.column(a).header();
                     return c.format.header(d.innerHTML, a, d)
                 }).toArray(),
+                */
+                headerMatrix = getHeaders( b.settings()[0] ),
+                /*END ADD*/
                 g = b.table().footer() ? b.columns(c.columns).indexes().map(function(a) {
                     var d =
                         b.column(a).footer();
@@ -713,12 +775,12 @@
             }, h)).any() && d.extend(h, {
                 selected: !0
             });
-            for (var h = b.rows(c.rows, h).indexes().toArray(), f = b.cells(h, c.columns), h = f.render(c.orthogonal).toArray(), f = f.nodes().toArray(), k = a.length, j = [], l = 0, n = 0, o = 0 < k ? h.length / k : 0; n < o; n++) {
+            for (var h = b.rows(c.rows, h).indexes().toArray(), f = b.cells(h, c.columns), h = f.render(c.orthogonal).toArray(), f = f.nodes().toArray(), k = /*a.length*/headerMatrix[headerMatrix.length - 1].length, j = [], l = 0, n = 0, o = 0 < k ? h.length / k : 0; n < o; n++) {
                 for (var r = [k], s = 0; s < k; s++) r[s] = c.format.body(h[l], n, s, f[l]), l++;
                 j[n] = r
             }
             a = {
-                header: a,
+                header: /*a*/headerMatrix,
                 footer: g,
                 body: j
             };