feat(documents): add title section movement helpers

frontend/utils/documentTitleSections.js

@@ -0,0 +1,41 @@
+const titleLevel = row => Math.min(Math.max(Number(row.titleLevel) || 1, 1), 3)
+
+// A section includes every row up to the next title at the same or a higher level.
+const sectionEnd = (rows, start) => {
+  const level = titleLevel(rows[start])
+  let end = start + 1
+  while (end < rows.length && (rows[end].mode !== 'title' || titleLevel(rows[end]) > level)) end += 1
+  return end
+}
+
+export const getTitleSectionMove = (rows, rowId, direction) => {
+  if (!['up', 'down'].includes(direction)) return null
+  const start = rows.findIndex(row => row.id === rowId)
+  if (start < 0 || rows[start].mode !== 'title') return null
+
+  const level = titleLevel(rows[start])
+  const end = sectionEnd(rows, start)
+  if (direction === 'down') {
+    // A title at a higher level ends the parent section: never cross it.
+    if (end === rows.length || titleLevel(rows[end]) !== level) return null
+    return { start, end, target: sectionEnd(rows, end) }
+  }
+
+  for (let previous = start - 1; previous >= 0; previous -= 1) {
+    if (rows[previous].mode !== 'title' || titleLevel(rows[previous]) > level) continue
+    if (titleLevel(rows[previous]) < level) return null
+    return { start, end, target: previous }
+  }
+  return null
+}
+
+export const moveTitleSection = (rows, rowId, direction) => {
+  const move = getTitleSectionMove(rows, rowId, direction)
+  if (!move) return rows
+
+  const { start, end, target } = move
+  const result = rows.slice()
+  const section = result.splice(start, end - start)
+  result.splice(target > start ? target - section.length : target, 0, ...section)
+  return result
+}