edu.byu.nlp.util.Iterators2.java Source code

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Here is the source code for edu.byu.nlp.util.Iterators2.java

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/**
 * Copyright 2012 Brigham Young University
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package edu.byu.nlp.util;

import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;

import com.google.common.base.Preconditions;
import com.google.common.collect.AbstractIterator;
import com.google.common.collect.Iterators;
import com.google.common.collect.Lists;
import com.google.common.collect.Ordering;

/**
 * @author rah67, pfelt
 *
 */
public class Iterators2 {

    private Iterators2() {
    }

    public static <E> List<E> topN(Iterator<? extends E> it, int n, Comparator<? super E> c) {
        return Heaps.largestN(it, n, true, c);
    }

    public static <E extends Comparable<? super E>> E max(Iterator<? extends E> it) {
        return max(it, Ordering.<E>natural());
    }

    public static <E extends Comparable<? super E>> E min(Iterator<? extends E> it) {
        return max(it, Collections.reverseOrder());
    }

    public static <E> E max(Iterator<? extends E> it, Comparator<? super E> comparator) {
        if (!it.hasNext()) {
            return null;
        }

        E max = it.next();
        while (it.hasNext()) {
            E next = it.next();
            if (comparator.compare(next, max) > 0) {
                max = next;
            }
        }
        return max;
    }

    public static <E> Iterator<Enumeration<E>> enumerate(final Iterator<E> iterator) {
        return new Iterator<Enumeration<E>>() {

            private int i = 0;

            @Override
            public boolean hasNext() {
                return iterator.hasNext();
            }

            @Override
            public Enumeration<E> next() {
                return new Enumeration<E>(i++, iterator.next());
            }

            @Override
            public void remove() {
                iterator.remove();
            }

        };
    }

    private static class RepeatItemIterator<E> implements Iterator<E> {

        private final Iterator<E> it;
        private final int n;
        private E next;
        private int curCount;

        public RepeatItemIterator(Iterator<E> it, int n) {
            this.it = it;
            this.n = n;
            this.curCount = n; // allows the hasNext() condition to work correctly for empty it

        }

        /** {@inheritDoc} */
        @Override
        public boolean hasNext() {
            return it.hasNext() || curCount < n;
        }

        /** {@inheritDoc} */
        @Override
        public E next() {
            if (curCount >= n) {
                assert curCount == n;
                next = it.next();
                curCount = 0;
            }
            ++curCount;
            return next;
        }

        /** {@inheritDoc} */
        @Override
        public void remove() {
            throw new UnsupportedOperationException();
        }

    }

    /**
     * Repeats each element of the original iterator n times in order. Currently, does not support removal.
     */
    public static <E> Iterator<E> repeatItems(Iterator<E> it, int n) {
        return new RepeatItemIterator<E>(it, n);
    }

    private static class CycleProvidingIterator<E> implements Iterator<Iterator<E>> {

        private final Iterable<E> it;
        private final int n;
        private int curCount = 0;

        public CycleProvidingIterator(Iterable<E> it, int n) {
            this.it = it;
            this.n = n;
        }

        /** {@inheritDoc} */
        @Override
        public boolean hasNext() {
            return curCount < n;
        }

        /** {@inheritDoc} */
        @Override
        public Iterator<E> next() {
            Preconditions.checkState(curCount < n);
            ++curCount;
            return it.iterator();
        }

        /** {@inheritDoc} */
        @Override
        public void remove() {
            throw new UnsupportedOperationException();
        }
    }

    /**
    * Cycles the instances of it exactly n times in the order of iteration.
    */
    public static <E> Iterator<E> cycle(Iterable<E> it, int n) {
        return Iterators.concat(new CycleProvidingIterator<E>(it, n));
    }

    /**
     * Iterates one after another through all iterators in an iterable. 
     */
    public static <T> Iterator<T> lazyConcatenate(Iterable<Iterator<T>> iterators) {
        final Iterator<Iterator<T>> outerItr = iterators.iterator();

        return new AbstractIterator<T>() {
            Iterator<T> innerItr = null;

            @Override
            protected T computeNext() {
                // find a valid inner iterator
                while (innerItr == null || !innerItr.hasNext()) {
                    if (outerItr == null || !outerItr.hasNext()) {
                        return endOfData();
                    }
                    innerItr = outerItr.next();
                }
                return innerItr.next();
            }
        };
    }

    public static <E> Iterator<E> subInterval(final Iterator<E> it, final int start, final int length) {
        // note: improve this (if necessary) by making it lazy
        List<E> items = Lists.newArrayListWithExpectedSize(length);
        Iterator<Enumeration<E>> enit = enumerate(it);
        while (enit.hasNext() && items.size() < length) {
            Enumeration<E> item = enit.next();
            if (item.getIndex() >= start) {
                items.add(item.getElement());
            }
        }
        return items.iterator();
    }

    public static <E> Iterator<E> firstN(final Iterator<E> iterable, final int n) {
        return subInterval(iterable, 0, n);
    }

}