get_items computes the pagination button information based on

• total number of pages,
• current page,
• sibling count (pages left and right of current page)
• boundary count (pages at start, and at end of the page range)

As this control receives user input, possibly from the internet a reasonable result is computed despite invalid input values and at least one page item is returned always.

• The resulting items list always has 1 + max(0, sibling_count) + max(0, boundary_count) page items
• The resulting items may/will contains ellipsis items only if boundary_count > 0
• The previous item has :enabled? false if page 1 is current
• The next item has :enabled? false if the last page is current

please see the unit tests for examples