Render leading span marker with nothing to merge as an empty cell (#245)

A colspan marker (`<`) in the first column has no cell to its left, and a
rowspan marker (`^`) in the first row has no cell above. Such a marker has
nothing to merge into, so it was silently dropped, leaving the row one cell
short. It now renders as an empty cell instead, matching djot-js / carve
parity.

Leftover colspan accumulation after the reverse merge loop is flushed into
empty leading cells. A rowspan marker that finds no originating cell above
becomes an empty cell rather than a dangling marker.

Author: dereuromark

src/Parser/BlockParser.php

@@ -2848,6 +2848,14 @@ protected function tryParseTable(Node $parent, array $lines, int $start): ?int
                 }
             }
 
+            // Leading colspan markers (`<` with no cell to their left) cannot
+            // merge: each becomes an empty cell rather than being dropped
+            // (djot-js / carve parity).
+            while ($colspanAccumulator > 1) {
+                array_unshift($processedCells, ['content' => '', 'attributes' => [], 'colspan' => 1]);
+                $colspanAccumulator--;
+            }
+
             // Parse regular row
             $row = new TableRow(false);
             if ($rowAttributes) {
@@ -2859,11 +2867,51 @@ protected function tryParseTable(Node $parent, array $lines, int $start): ?int
             $rowCellData = [];
             $colPosition = 0;
 
+            // getChildren() hands back a copy-on-write alias of the table's internal child
+            // array. Holding it alive across the appendChild() below would force PHP to copy
+            // the entire array on every row (turning a plain table into O(rows^2)), so it is
+            // released with unset() right before the append - see the note there.
+            $tableChildren = $table->getChildren();
+            $currentRowIndex = count($tableChildren); // Index where current row will be added
+
             foreach ($processedCells as $index => $cellData) {
                 $colspan = $cellData['colspan'];
 
                 // Check for rowspan marker
                 if ($this->tableParser->isRowspanMarker($cellData['content'])) {
+                    // A rowspan marker with no cell above to extend (first row, or
+                    // a column with no origin) cannot merge: it becomes an empty
+                    // cell rather than being dropped (djot-js / carve parity).
+                    $cellAbove = null;
+                    for ($prevRowIdx = $currentRowIndex - 1; $prevRowIdx >= 0; $prevRowIdx--) {
+                        if (!($tableChildren[$prevRowIdx] instanceof TableRow)) {
+                            continue;
+                        }
+                        $cellAbove = $this->findCellAtColumnForRowspan(
+                            $tableChildren,
+                            $prevRowIdx,
+                            $colPosition,
+                            $currentRowIndex,
+                        );
+                        if ($cellAbove !== null) {
+                            break;
+                        }
+                    }
+
+                    if ($cellAbove === null) {
+                        $alignment = $alignments[$index] ?? TableCell::ALIGN_DEFAULT;
+                        $cell = new TableCell(false, $alignment, 1, $colspan);
+                        $row->appendChild($cell);
+                        $rowCellData[] = [
+                            'type' => 'cell',
+                            'cell' => $cell,
+                            'colPosition' => $colPosition,
+                        ];
+                        $colPosition += $colspan;
+
+                        continue;
+                    }
+
                     // Mark this position for rowspan processing
                     $rowCellData[] = [
                         'type' => 'rowspan_marker',
@@ -2894,12 +2942,8 @@ protected function tryParseTable(Node $parent, array $lines, int $start): ?int
 
             // Process rowspan markers - find cells above that should span down
             // We need to track column positions considering rowspan markers in previous rows.
-            // getChildren() hands back a copy-on-write alias of the table's internal child
-            // array. Holding it alive across the appendChild() below would force PHP to copy
-            // the entire array on every row (turning a plain table into O(rows^2)), so it is
-            // released with unset() right before the append - see the note there.
-            $tableChildren = $table->getChildren();
-            $currentRowIndex = count($tableChildren); // Index where current row will be added
+            // $tableChildren / $currentRowIndex were captured before the cell loop above
+            // (the table's child array is unchanged until the appendChild() below).
 
             // Track which cells have already been extended in this row
             // (multiple ^ markers under a colspan should only extend once)

tests/TestCase/TableSpansTest.php

@@ -701,4 +701,64 @@ public function testExplicitMarkersFor2x2Block(): void
         $cells = $dataRow2->getChildren();
         $this->assertCount(1, $cells); // Only "L2" cell
     }
+
+    public function testLeadingColspanMarkerInFirstColumnIsEmptyCell(): void
+    {
+        // A colspan marker `<` in the first column has no cell to its left to
+        // merge into: it must render as an empty cell, not be dropped.
+        $djot = <<<'DJOT'
+| < | a |
+|---|---|
+| b | c |
+DJOT;
+
+        $doc = $this->converter->parse($djot);
+
+        /** @var \Djot\Node\Block\Table $table */
+        $table = $doc->getChildren()[0];
+        $this->assertInstanceOf(Table::class, $table);
+
+        $rows = $table->getChildren();
+
+        /** @var \Djot\Node\Block\TableRow $headerRow */
+        $headerRow = $rows[0];
+        $headerCells = $headerRow->getChildren();
+        $this->assertCount(2, $headerCells);
+
+        /** @var \Djot\Node\Block\TableCell $leadingCell */
+        $leadingCell = $headerCells[0];
+        $this->assertSame(1, $leadingCell->getColspan());
+        $this->assertSame(1, $leadingCell->getRowspan());
+        $this->assertCount(0, $leadingCell->getChildren());
+    }
+
+    public function testLeadingRowspanMarkerInFirstRowIsEmptyCell(): void
+    {
+        // A rowspan marker `^` in the first row has no cell above to merge
+        // into: it must render as an empty cell, not be dropped.
+        $djot = <<<'DJOT'
+| ^ | a |
+|---|---|
+| b | c |
+DJOT;
+
+        $doc = $this->converter->parse($djot);
+
+        /** @var \Djot\Node\Block\Table $table */
+        $table = $doc->getChildren()[0];
+        $this->assertInstanceOf(Table::class, $table);
+
+        $rows = $table->getChildren();
+
+        /** @var \Djot\Node\Block\TableRow $headerRow */
+        $headerRow = $rows[0];
+        $headerCells = $headerRow->getChildren();
+        $this->assertCount(2, $headerCells);
+
+        /** @var \Djot\Node\Block\TableCell $leadingCell */
+        $leadingCell = $headerCells[0];
+        $this->assertSame(1, $leadingCell->getColspan());
+        $this->assertSame(1, $leadingCell->getRowspan());
+        $this->assertCount(0, $leadingCell->getChildren());
+    }
 }