Speed up changes from script to scattered cells in google sheets - google-apps-script

Question

I have a sheet that shows the results of calculations based on other backing sheets. Changes are made manually, but the "summary" sheet is only formulae. These changes appear in scattered cells, non-contiguous for the most part.
I want to highlight which cells have changed in the summary sheet after a manual change in the backing sheets. For that, I'm using a second summary sheet which starts as a copy of the main one.
The final ingredient is a script that runs after edits. It traverses the summary range and compares values to the second copy. Any differences get highlighted in the main summary and copied back to the second summary.
This process does work but is quite slow, I think due to the updates. Pseudo-code:
var src = summary.getRange(...)
var dst = copy.getRange(...)
var src_cell;
var dst_cell;
src.setBackground('white'); // Bulk reset of changes
for (row = 1; row < src.getNumRows(); row++) {
for (col = 1; col < src.getNumColumns(); col++) {
src_cell = src.getCell(row, col);
dst_cell = src.getCell(row, col);
if (src_cell.getDisplayValue() != dst_cell.getDisplayValue()) {
dst_cell.setValue(src_cell.getDisplayValue());
src_cell.setBackground('gray');
}
}
}
I think there is no way to bulk-update scattered ranges, which seems a straightforward solution.
I'm looking for ways to speed up this process, either in the script or by using some other strategy.

Answer 1

Per official "best practices," you should batch-read associated cell data rather than repeatedly read and possibly write values. This statement does assume that setting values in dst does not influence values for future reads.
Thus, the simplest change is to use Range#getDisplayValues on src and dst:
...
src.setBackground("white");
var srcValues = src.getDisplayValues();
var dstValues = dst.getDisplayValues();
srcValues.forEach(function (srcRow, r) {
var dstRow = dstValues[r];
srcRow.forEach(function (value, c) {
if (value !== dstRow[c]) {
dst.getCell(r + 1, c + 1).setValue(value);
src.getCell(r + 1, c + 1).setBackground("gray");
}
});
});
An additional optimization is to use the RangeList class to batch the changes. To create a RangeList, you need an array of cell / range notations, which can use R1C1- or A1-style addressing. R1C1 is simplest to compute.
...
var dstChanges = [];
var srcChanges = [];
...
if (value !== dstRow[c]) {
dstChanges.push({row: r + 1, col: c + 1, newValue: value});
srcChanges.push({row: r + 1, col: c + 1});
}
...
if (srcChanges.length > 0) {
var srcRow = src.getRow();
var dstRow = dst.getRow();
var srcCol = src.getColumn();
var dstCol = dst.getColumn();
copy.getRangeList(dstChanges.map(function (obj) {
return "R" + (obj.row + dstRow) + "C" + (obj.col + dstCol);
}).getRanges().forEach(function (rg, i) {
rg.setValue(dstChanges[i].newValue);
});
summary.getRangeList(srcChanges.map(function (obj) {
return "R" + (obj.row + srcRow) + "C" + (obj.col + srcCol);
}).setBackground("gray");
}
...
Other Refs
Array#forEach
Array#map
Array#push