Function buildHeaderGroups

packages/table-core/src/core/headers.ts:484–639

(
  allColumns: Column<TData, unknown>[],
  columnsToGroup: Column<TData, unknown>[],
  table: Table<TData>,
  headerFamily?: 'center' | 'left' | 'right',
)

}

export function buildHeaderGroups<TData extends RowData>(
  allColumns: Column<TData, unknown>[],
  columnsToGroup: Column<TData, unknown>[],
  table: Table<TData>,
  headerFamily?: 'center' | 'left' | 'right',
) {
  // Find the max depth of the columns:
  // build the leaf column row
  // build each buffer row going up
  //    placeholder for non-existent level
  //    real column for existing level

  let maxDepth = 0

  const findMaxDepth = (columns: Column<TData, unknown>[], depth = 1) => {
    maxDepth = Math.max(maxDepth, depth)

    columns
      .filter((column) => column.getIsVisible())
      .forEach((column) => {
        if (column.columns?.length) {
          findMaxDepth(column.columns, depth + 1)
        }
      }, 0)
  }

  findMaxDepth(allColumns)

  let headerGroups: HeaderGroup<TData>[] = []

  const createHeaderGroup = (
    headersToGroup: Header<TData, unknown>[],
    depth: number,
  ) => {
    // The header group we are creating
    const headerGroup: HeaderGroup<TData> = {
      depth,
      id: [headerFamily, `${depth}`].filter(Boolean).join('_'),
      headers: [],
    }

    // The parent columns we're going to scan next
    const pendingParentHeaders: Header<TData, unknown>[] = []

    // Scan each column for parents
    headersToGroup.forEach((headerToGroup) => {
      // What is the latest (last) parent column?

      const latestPendingParentHeader = [...pendingParentHeaders].reverse()[0]

      const isLeafHeader = headerToGroup.column.depth === headerGroup.depth

      let column: Column<TData, unknown>
      let isPlaceholder = false

      if (isLeafHeader && headerToGroup.column.parent) {
        // The parent header is new
        column = headerToGroup.column.parent