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
}