package sagan.support.nav;

import java.util.ArrayList;
import java.util.List;

/**
 * Utility for constructing a list of {@link PageElement}s to be used when rendering
 * pagination controls at the view layer.
 *
 * @see PaginationInfo
 */
class PageElementsBuilder {

    private final long currentPage;
    private final long totalPages;
    private long startPage;
    private long endPage;

    PageElementsBuilder(long currentPage, long totalPages) {
        this.currentPage = currentPage;
        this.totalPages = totalPages;
    }

    public List<PageElement> build() {
        ArrayList<PageElement> elements = new ArrayList<>();

        findStartPage();
        findEndPage();

        if (startPage > 1) {
            addFirstPage(elements);
        }

        addPageElements(elements);

        if (endPage < totalPages) {
            addLastPage(elements);
        }

        return elements;
    }

    private void findStartPage() {
        long previousTwoPages = Math.max((currentPage - 2), 1L);
        startPage = previousTwoPages;
    }

    private void findEndPage() {
        long nextThreePages = currentPage + 3;
        long desiredLastPage = Math.max(6, nextThreePages);
        endPage = Math.min(desiredLastPage, totalPages);
    }

    private void addFirstPage(ArrayList<PageElement> elements) {
        elements.add(new PageElement(1, true, false));
        if (startPage > 2) {
            addEllipsis(elements);
        }
    }

    private void addEllipsis(ArrayList<PageElement> elements) {
        elements.add(new PageElement("...", false, false));
    }

    private void addPageElements(ArrayList<PageElement> elements) {
        for (long n = startPage; n <= endPage; n++) {
            boolean isCurrentPage = n == currentPage;
            boolean isNavigable = !isCurrentPage;
            elements.add(new PageElement(n, isNavigable, isCurrentPage));
        }
    }

    private void addLastPage(ArrayList<PageElement> elements) {
        if (endPage < totalPages - 1) {
            addEllipsis(elements);
        }
        elements.add(new PageElement(totalPages, true, false));
    }
}