algorithmpagination

Algorithm / pseudo-code to create paging links?


Can someome provide code or pseudo-code for how the paging links on StackOverflow are generated?

I keep racking my brain but can't think of a decent way to build the dynamic links that always show the 2 pages around the current, plus the first and last.

Example: 1 ... 5 6 7 ... 593


Solution

  • There are several other answers already, but I'd like to show you the approach I took to solve it: First, let's check out how Stack Overflow handles normal cases and edge cases. Each of my pages displays 10 results, so to find out what it does for 1 page, find a tag that has less than 11 entries: usability works today. We can see nothing is displayed, which makes sense.

    How about 2 pages? Find a tag that has between 11 and 20 entries (emacs works today). We see: "1 2 Next" or "Prev 1 2", depending on which page we're on.

    3 pages? "1 2 3 ... 3 Next", "Prev 1 2 3 Next", and "Prev 1 ... 2 3". Interestingly, we can see that Stack Overflow itself doesn't handle this edge case very well: it should display "1 2 ... 3 Next"

    4 pages? "1 2 3 ... 4 Next", "Prev 1 2 3 ... 4 Next", "Prev 1 ... 2 3 4 Next" and "Prev 1 ... 3 4"

    Finally let's look at the general case, N pages: "1 2 3 ... N Next", "Prev 1 2 3 ... N Next", "Prev 1 ... 2 3 4 ... N Next", "Prev 1 ... 3 4 5 ... N Next", etc.

    Let's generalize based on what we've seen: The algorithm seems to have these traits in common:

    Let's ignore the edge case of a single page and make a good first attempt at the algorithm: (As has been mentioned, the code to actually print out the links would be more complicated. Imagine each place we place a page number, Prev or Next as a function call that will return the correct URL.)

    function printPageLinksFirstTry(num totalPages, num currentPage)
      if ( currentPage > 1 )
        print "Prev"
      print "1"
      print "..."
      print currentPage - 1
      print currentPage
      print currentPage + 1
      print "..."
      print totalPages
      if ( currentPage < totalPages )
        print "Next"
    endFunction
    

    This function works ok, but it doesn't take into account whether we're near the first or last page. Looking at the above examples, we only want to display the ... if the current page is two or more away.

    function printPageLinksHandleCloseToEnds(num totalPages, num currentPage)
      if ( currentPage > 1 )
        print "Prev"
      print "1"
      if ( currentPage > 2 )
        print "..."
      if ( currentPage > 2 )
        print currentPage - 1
      print currentPage
      if ( currentPage < totalPages - 1 )
        print currentPage + 1
      if ( currentPage < totalPages - 1 )
        print "..."
      print totalPages
      if ( currentPage < totalPages )
        print "Next"
    endFunction
    

    As you can see, we have some duplication here. We can go ahead and clean that up for readibility:

    function printPageLinksCleanedUp(num totalPages, num currentPage)
      if ( currentPage > 1 )
        print "Prev"
      print "1"
      if ( currentPage > 2 )
        print "..."
        print currentPage - 1
      print currentPage
      if ( currentPage < totalPages - 1 )
        print currentPage + 1
        print "..."
      print totalPages
      if ( currentPage < totalPages )
        print "Next"
    endFunction
    

    There are only two problems left. First, we don't print out correctly for one page, and secondly, we'll print out "1" twice if we're on the first or last page. Let's clean those both up in one go:

    function printPageLinksFinal(num totalPages, num currentPage)
      if ( totalPages == 1 )
        return
    
      if ( currentPage > 1 )
        print "Prev"
    
      print "1"
    
      if ( currentPage > 2 )
        print "..."
        print currentPage - 1
    
      if ( currentPage != 1 and currentPage != totalPages )
        print currentPage
    
      if ( currentPage < totalPages - 1 )
        print currentPage + 1
        print "..."
    
      print totalPages
    
      if ( currentPage < totalPages )
        print "Next"
    
    endFunction
    

    Actually, I lied: We have one remaining issue. When you have at least 4 pages and are on the first or last page, you get an extra page in your display. Instead of "1 2 ... 10 Next" you get "1 2 3 ... 10 Next". To match what's going on at Stack Overflow exactly, you'll have to check for this situation:

    function printPageLinksFinalReally(num totalPages, num currentPage)
      if ( totalPages == 1 )
        return
    
      if ( currentPage > 1 )
        print "Prev"
    
      print "1"
    
      if ( currentPage > 2 )
        print "..."
        if ( currentPage == totalPages and totalPages > 3 )
          print currentPage - 2
        print currentPage - 1
    
      if ( currentPage != 1 and currentPage != totalPages )
        print currentPage
    
      if ( currentPage < totalPages - 1 )
        print currentPage + 1
        if ( currentPage == 1 and totalPages > 3 )
          print currentPage + 2
        print "..."
    
      print totalPages
    
      if ( currentPage < totalPages )
        print "Next"
    
    endFunction
    

    I hope this helps!