Previous link url is always the first page when using offset-based pagination in facade

[ydaniju]

Describe the bug
We are using hapi fhir as a facade. Pagination of search endpoints is performed by using SimpleBundleProvider which generated the right links for the current and next pages. However, once we get to page 3 (and above), the previous link always points to page 1 instead of page 2 (currentPage - 1).

To Reproduce
Steps to reproduce the behavior:

1. Create a Resource Provider search endpoint eg PatientResourceProvider

// List<IBaseResource> resourceList -> could be empty for reproducing this issue
SimpleBundleProvider bundleProvider = new SimpleBundleProvider(resourceList);

/**
* meta is a DTO for the backend service providing search data
* Offset and counterPerPage are from @Count and @Offset respectively
* Could be hardcoded to reproduce this issue
* /

bundleProvider.setCurrentPageOffset(meta.getOffset());
bundleProvider.setCurrentPageSize(meta.getCountPerPage());
bundleProvider.setSize(meta.getTotalCount()); // you can use a fixed size of 10

return bundleProvider;

2. Set currentPageSize = 1, currentPageOffset = 3, totalCount (ie size) = 10 corresponding to ...?_count=1&_offset=3
3. Make a request to the search endpoint of the resource provider
4. You will get

  "link": [
    {
      "relation": "self",
      "url": "...?_count=1&_offset=3" // correct
    },
    {
      "relation": "next",
      "url": "...?_count=1&_offset=4" // correct
    },
    {
      "relation": "previous",
      "url": "...?_count=1&_offset=0" // wrong
    }

Expected behavior
The previous link url should look like "url": "...?_count=1&_offset=2" ie _offset should be current page offset minus count per page
Screenshots
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Environment (please complete the following information):

• HAPI FHIR Version: 6.2.5
• OS: N/A
• Browser: N/A

Additional context
If this is a bug, we could raise a fix for it.