Memory Experiment in Xlsx Writer

phpstan-baseline.neon

@@ -5250,11 +5250,6 @@ parameters:
 			count: 1
 			path: src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php
 
-		-
-			message: "#^Offset int\\<1, max\\> on array\\<string, non\\-empty\\-array\\<int, string\\>\\> in isset\\(\\) does not exist\\.$#"
-			count: 2
-			path: src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php
-
 		-
 			message: "#^Parameter \\#2 \\$content of method XMLWriter\\:\\:writeElement\\(\\) expects string\\|null, int\\|string given\\.$#"
 			count: 1

src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php

@@ -1145,11 +1145,15 @@ class Worksheet extends WriterPart
         // Highest row number
         $highestRow = $worksheet->getHighestRow();
 
-        // Loop through cells
+        // Loop through cells building a comma-separated list of the columns in each row
+        // This is a trade-off between the memory usage that is required for a full array of columns,
+        //      and execution speed
+        /** @var array<int, string> $cellsByRow */
         $cellsByRow = [];
         foreach ($worksheet->getCoordinates() as $coordinate) {
-            $cellAddress = Coordinate::coordinateFromString($coordinate);
-            $cellsByRow[$cellAddress[1]][] = $coordinate;
+            [$column, $row] = Coordinate::coordinateFromString($coordinate);
+            $cellsByRow[$row] = $cellsByRow[$row] ?? '';
+            $cellsByRow[$row] .= "{$column},";
         }
 
         $currentRow = 0;
@@ -1195,9 +1199,12 @@ class Worksheet extends WriterPart
 
                 // Write cells
                 if (isset($cellsByRow[$currentRow])) {
-                    foreach ($cellsByRow[$currentRow] as $cellAddress) {
+                    // We have a comma-separated list of column names (with a trailing entry); split to an array
+                    $columnsInRow = explode(',', $cellsByRow[$currentRow]);
+                    array_pop($columnsInRow);
+                    foreach ($columnsInRow as $column) {
                         // Write cell
-                        $this->writeCell($objWriter, $worksheet, $cellAddress, $aFlippedStringTable);
+                        $this->writeCell($objWriter, $worksheet, "{$column}{$currentRow}", $aFlippedStringTable);
                     }
                 }