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package com.algorithmia.data;

import com.algorithmia.APIException;

import java.util.NoSuchElementException;
import java.lang.UnsupportedOperationException;
import java.util.Iterator;
import java.util.List;


public abstract class AbstractDataIterator implements Iterator {
    protected DataDirectory dir;
    protected String marker;
    private int offset;
    private List children;
    private boolean loadedFirstPage;

    protected AbstractDataIterator(DataDirectory dir) {
        this.dir = dir;
    }

    public boolean hasNext() {
        attemptToLoadFirstPage();

        return
            (children != null && offset < children.size()) || // We have data in memory
            (children != null && offset >= children.size() && marker != null); // There is another page to fetch
    }

    public T next() throws NoSuchElementException {
        attemptToLoadFirstPage();

        if(children == null) {
            throw new NoSuchElementException();
        }

        if(marker != null && offset >= children.size()) {
            try {
                loadNextPage();
            } catch(APIException ex) {
                throw new NoSuchElementException(ex.getMessage());
            }
        }

        if(offset < children.size()) {
            offset++;
            return newDataObjectInstance(dir.trimmedPath + "/" + children.get(offset-1));

        } else {
            throw new NoSuchElementException();
        }
    }

    private void attemptToLoadFirstPage() {
        if (!loadedFirstPage) {
            loadedFirstPage = true;
            try {
                loadNextPage();
            } catch(APIException ex) {
                throw new NoSuchElementException(ex.getMessage());
            }
        }
    }

    final protected void setChildrenAndMarker(List newChildren, String marker) {
        if (children != null && offset < children.size())
            throw new IllegalStateException("Skipping elements");

        children = newChildren;
        offset = 0;
        this.marker = marker;
    }

    /*
     * @throws UnsupportedOperationException as remove is not supported for DataFileIterators
     */
    public void remove() {
        throw new UnsupportedOperationException();
    }

    abstract protected void loadNextPage() throws APIException;

    // Because this can be statically checked, as opposed to doing something like:
    //   storedClass.getDeclaredConstructor(String.class).newInstance(path)
    abstract protected T newDataObjectInstance(String dataUri);
}