code: function () {
              var prev, page,
                  state = this.state,
                  pages = state.pages;
  
              prev = util.bisectRight(pages, state.scrollLeft, 'x0') - 1;
              page = util.bisectRight(pages, state.scrollLeft + pages[prev].width / 2, 'x0') - 1;
              return 1 + page;
          }
docs: Calculate which page is currently the "focused" page.
  In horizontal mode, this is the page farthest to the left,
  where at least half of the page is showing.
  @returns {int} The current page