Paginator.java

package nc.unc.trec.progression.batch;

import java.util.Optional;
import java.util.function.Function;

import org.springframework.data.domain.Page;
import org.springframework.data.domain.PageRequest;
import org.springframework.data.domain.Pageable;

import lombok.RequiredArgsConstructor;
import lombok.extern.slf4j.Slf4j;

/**
 * Utility class for browsing Spring's {@link Page} and fetch the next one when is required. 
 */
@Slf4j
@RequiredArgsConstructor
public class Paginator<T> {
    
    private final Function<Pageable, Page<T>> nextPageFct;
    private final int pageSize;
    
    private int currentIndex;
    private Page<T> currentPage;
    
    public Optional<T> next() {
        if (currentIndex % pageSize == 0) {
            if (currentPage != null && currentPage.isLast()) {
                // last page => don't fetch the next one
                return Optional.empty();
            }
            currentPage = readNextPage();
            currentIndex = 0;
        }
        if (currentIndex < currentPage.getNumberOfElements()) {
            T next = currentPage.getContent().get(currentIndex);
            log.debug("Next {} : {}", currentIndex, next);
            currentIndex++;
            return Optional.of(next);
        } else {
            log.debug("Finish");
            return Optional.empty();
        }
    }
    
    private Page<T> readNextPage() {
        Pageable nextPageable = Optional.ofNullable(currentPage)
                .map(last -> last.nextPageable())
                .orElseGet(() -> PageRequest.of(0, pageSize));
        log.info("Reading next page {}", nextPageable.getPageNumber() + 1);
        Page<T> nextPage = nextPageFct.apply(nextPageable);
        log.debug("Page {}/{}, last={}, items={}", nextPage.getNumber() + 1, nextPage.getTotalPages(), nextPage.isLast(), nextPage.getNumberOfElements());
        return nextPage;
    }
}