gistfile1.txt

for page in pages ?? []
{
    guard let pageNumber = page.pageNumber
        else { continue }
    
    if let array = pageLookup[pageNumber]
    {
        pageLookup[pageNumber] = array + [page]
    }
    else
    {
        pageLookup[pageNumber] = [page]
    }
}

let result = pageLookup.sort { $0.0 < $1.0 }.map  { (pageNumber, pageArray) -> [Page] in
                        return pageArray.sort { $0.roundNumber < $1.roundNumber && $0.variantNumber < $1.variantNumber }
}

And, because we can - not because it's a good idea :) - the whole thing as a one-liner:

let result = Array(Set(pages.flatMap({ $0.pageNumber }))).sort(<).map({ (pageNumber: Int) in pages.filter({ (page: Page) in page.pageNumber == pageNumber }).sort({ (a: Page, b: Page) in a.roundNumber < b.roundNumber && a.variantNumber < b.variantNumber }) })