Transducers <3 Iterators

iter.clj

(import clojure.lang.RT)

(defn iterator [xfn iter]
  (XFIterator. xfn (RT/iter iter))) ;; clojure 1.7.0-alpha3 for RT/iter

XFIterator.java

import clojure.lang.RT;
import clojure.lang.IFn;
import clojure.lang.AFn;

import java.util.Iterator;
import java.util.ArrayList;
import java.util.NoSuchElementException;

public class XFIterator implements Iterator {

    static IFn append = new AFn(){
            public Object invoke(Object result){
                return result;
            }
            public Object invoke(Object result, Object input){
                XFIterator i = (XFIterator) result;
                i.buf.add(input);
                return i;
            }
        };

    private final IFn xform;
    private Iterator iter;

    private final ArrayList buf;
    private int pos = 0;

    public XFIterator (IFn xform, Iterator iter) {
        this.iter = iter;
        this.buf = new ArrayList();
        this.xform = (IFn) xform.invoke(append);
    }

    public boolean hasNext() {
        if (remaining())
            return true;
        pull();
        return remaining();
    }

    private boolean remaining() {
        if (pos < buf.size())
            return true;
        return false;
    }

    private Object bufNext() {
        Object ret = buf.get(pos);
        pos++;
        if (pos == buf.size()) {
            buf.clear();
            pos = 0;
        }
        return ret;
    }

    public Object next() {
        if (remaining()) {
            return bufNext();
        }
        pull();
        if (remaining()) {
            return bufNext();
        }
        throw new NoSuchElementException();
    }

    private void pull() {
        if (iter == null)
            return;

        while (buf.size() == 0) {
            if(iter.hasNext()) {
                if(RT.isReduced(xform.invoke(this, iter.next()))) {
                    iter = null;
                    xform.invoke(this);
                    return;
                }
            } else {
                iter = null;
                xform.invoke(this);
                return;
            }
        }
    }
}