package iterator;

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

/**
 * Aggregate — the collection. Exposes iterators without
 * revealing its internal storage structure.
 */
public class BookShelf {
    private final List<Book> books = new ArrayList<>();

    public void addBook(Book book) { books.add(book); }

    /** Standard forward iterator */
    public BookIterator iterator() {
        return new ForwardIterator();
    }

    /** Filtered iterator — only books from a specific decade */
    public BookIterator iteratorByDecade(int decade) {
        return new DecadeIterator(decade);
    }

    // --- Inner iterator implementations ---

    private class ForwardIterator implements BookIterator {
        private int index = 0;

        @Override public boolean hasNext() { return index < books.size(); }
        @Override public Book    next()    { return books.get(index++); }
    }

    private class DecadeIterator implements BookIterator {
        private final int decade;
        private int index = 0;
        private Book nextBook;

        DecadeIterator(int decade) {
            this.decade = decade;
            advance();
        }

        private void advance() {
            nextBook = null;
            while (index < books.size()) {
                Book b = books.get(index++);
                if (b.getYear() / 10 * 10 == decade) { nextBook = b; break; }
            }
        }

        @Override public boolean hasNext() { return nextBook != null; }
        @Override public Book next() {
            Book b = nextBook; advance(); return b;
        }
    }
}