limit the number of autocompleted cells in a table

fix markdown-it#1000

lib/rules_block/table.mjs

@@ -2,6 +2,14 @@
 
 import { isSpace } from '../common/utils.mjs'
 
+// Limit the amount of empty autocompleted cells in a table,
+// see https://github.com/markdown-it/markdown-it/issues/1000,
+//
+// Both pulldown-cmark and commonmark-hs limit the number of cells this way to ~200k.
+// We set it to 65k, which can expand user input by a factor of x370
+// (256x256 square is 1.8kB expanded into 650kB).
+const MAX_AUTOCOMPLETED_CELLS = 0x10000
+
 function getLine (state, line) {
   const pos = state.bMarks[line] + state.tShift[line]
   const max = state.eMarks[line]
@@ -157,6 +165,7 @@ export default function table (state, startLine, endLine, silent) {
   state.push('thead_close', 'thead', -1)
 
   let tbodyLines
+  let autocompletedCells = 0
 
   for (nextLine = startLine + 2; nextLine < endLine; nextLine++) {
     if (state.sCount[nextLine] < state.blkIndent) { break }
@@ -177,6 +186,11 @@ export default function table (state, startLine, endLine, silent) {
     if (columns.length && columns[0] === '') columns.shift()
     if (columns.length && columns[columns.length - 1] === '') columns.pop()
 
+    // note: autocomplete count can be negative if user specifies more columns than header,
+    // but that does not affect intended use (which is limiting expansion)
+    autocompletedCells += columnCount - columns.length
+    if (autocompletedCells > MAX_AUTOCOMPLETED_CELLS) { break }
+
     if (nextLine === startLine + 2) {
       const token_tbo = state.push('tbody_open', 'tbody', 1)
       token_tbo.map = tbodyLines = [startLine + 2, 0]