Swift Array extension which creates a new 2d array, rotated 90 degrees clockwise from the original

Array+Rotated.swift

extension Array where Element: Collection {
    typealias InnerElement = Element.Element

    /// Rotates a 2d array clockwise such that the nth column becomes the nth row. Rows which have
    /// fewer elements than the longest row will be padded with nil to make a full, rectangular
    /// matrix prior to rotation
    ///
    /// - Returns: A rectangular matrix where the contents are rotated 90 degrees clockwise. Nil is
    ///            used as padding for rows/columns with fewer eleemnts than the max.
    func rotatedClockwise() -> [[InnerElement?]] {
        guard let numRows: Int = map({ $0.count }).max(), numRows > 0 else { return [[]] }

        return map { row -> [InnerElement?] in
            let row = row as! [InnerElement?] // Force casting is safe because Element === [InnerElement]
            return row + [InnerElement?].init(repeating: nil, count: numRows - row.count)
        }.reduce([[InnerElement?]].init(repeating: [], count: numRows)) { (matrix, row) in
            row.enumerated().reduce(matrix) { (matrix, col) in
                var matrix = matrix
                matrix[col.offset].append(col.element)
                return matrix
            }
        }.map {
            $0.reversed()
        }
    }
}