/*
 * Licensed to Elasticsearch under one or more contributor
 * license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright
 * ownership. Elasticsearch licenses this file to you 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 org.elasticsearch.common.lucene.docset;

import org.apache.lucene.search.DocIdSet;
import org.apache.lucene.search.DocIdSetIterator;
import org.apache.lucene.util.Bits;

import java.io.IOException;

/**
 *
 */
public class OrDocIdSet extends DocIdSet {

    private final DocIdSet[] sets;

    public OrDocIdSet(DocIdSet[] sets) {
        this.sets = sets;
    }

    @Override
    public boolean isCacheable() {
        for (DocIdSet set : sets) {
            if (!set.isCacheable()) {
                return false;
            }
        }
        return true;
    }

    @Override
    public Bits bits() throws IOException {
        Bits[] bits = new Bits[sets.length];
        for (int i = 0; i < sets.length; i++) {
            bits[i] = sets[i].bits();
            if (bits[i] == null) {
                return null;
            }
        }
        return new OrBits(bits);
    }

    @Override
    public DocIdSetIterator iterator() throws IOException {
        return new IteratorBasedIterator(sets);
    }

    static class OrBits implements Bits {
        private final Bits[] bits;

        OrBits(Bits[] bits) {
            this.bits = bits;
        }

        @Override
        public boolean get(int index) {
            for (Bits bit : bits) {
                if (bit.get(index)) {
                    return true;
                }
            }
            return false;
        }

        @Override
        public int length() {
            return bits[0].length();
        }
    }

    static class IteratorBasedIterator extends DocIdSetIterator {

        final class Item {
            public final DocIdSetIterator iter;
            public int doc;

            public Item(DocIdSetIterator iter) {
                this.iter = iter;
                this.doc = -1;
            }
        }

        private int _curDoc;
        private final Item[] _heap;
        private int _size;
        private final long cost;

        IteratorBasedIterator(DocIdSet[] sets) throws IOException {
            _curDoc = -1;
            _heap = new Item[sets.length];
            _size = 0;
            long cost = 0;
            for (DocIdSet set : sets) {
                DocIdSetIterator iterator = set.iterator();
                if (iterator != null) {
                    _heap[_size++] = new Item(iterator);
                    cost += iterator.cost();
                }
            }
            this.cost = cost;
            if (_size == 0) _curDoc = DocIdSetIterator.NO_MORE_DOCS;
        }

        @Override
        public final int docID() {
            return _curDoc;
        }

        @Override
        public final int nextDoc() throws IOException {
            if (_curDoc == DocIdSetIterator.NO_MORE_DOCS) return DocIdSetIterator.NO_MORE_DOCS;

            Item top = _heap[0];
            while (true) {
                DocIdSetIterator topIter = top.iter;
                int docid;
                if ((docid = topIter.nextDoc()) != DocIdSetIterator.NO_MORE_DOCS) {
                    top.doc = docid;
                    heapAdjust();
                } else {
                    heapRemoveRoot();
                    if (_size == 0) return (_curDoc = DocIdSetIterator.NO_MORE_DOCS);
                }
                top = _heap[0];
                int topDoc = top.doc;
                if (topDoc > _curDoc) {
                    return (_curDoc = topDoc);
                }
            }
        }

        @Override
        public final int advance(int target) throws IOException {
            if (_curDoc == DocIdSetIterator.NO_MORE_DOCS) return DocIdSetIterator.NO_MORE_DOCS;

            if (target <= _curDoc) target = _curDoc + 1;

            Item top = _heap[0];
            while (true) {
                DocIdSetIterator topIter = top.iter;
                int docid;
                if ((docid = topIter.advance(target)) != DocIdSetIterator.NO_MORE_DOCS) {
                    top.doc = docid;
                    heapAdjust();
                } else {
                    heapRemoveRoot();
                    if (_size == 0) return (_curDoc = DocIdSetIterator.NO_MORE_DOCS);
                }
                top = _heap[0];
                int topDoc = top.doc;
                if (topDoc >= target) {
                    return (_curDoc = topDoc);
                }
            }
        }

// Organize subScorers into a min heap with scorers generating the earlest document on top.
        /*
        private final void heapify() {
            int size = _size;
            for (int i=(size>>1)-1; i>=0; i--)
                heapAdjust(i);
        }
        */
        /* The subtree of subScorers at root is a min heap except possibly for its root element.
        * Bubble the root down as required to make the subtree a heap.
        */

        private final void heapAdjust() {
            final Item[] heap = _heap;
            final Item top = heap[0];
            final int doc = top.doc;
            final int size = _size;
            int i = 0;

            while (true) {
                int lchild = (i << 1) + 1;
                if (lchild >= size) break;

                Item left = heap[lchild];
                int ldoc = left.doc;

                int rchild = lchild + 1;
                if (rchild < size) {
                    Item right = heap[rchild];
                    int rdoc = right.doc;

                    if (rdoc <= ldoc) {
                        if (doc <= rdoc) break;

                        heap[i] = right;
                        i = rchild;
                        continue;
                    }
                }

                if (doc <= ldoc) break;

                heap[i] = left;
                i = lchild;
            }
            heap[i] = top;
        }

        // Remove the root Scorer from subScorers and re-establish it as a heap

        private void heapRemoveRoot() {
            _size--;
            if (_size > 0) {
                Item tmp = _heap[0];
                _heap[0] = _heap[_size];
                _heap[_size] = tmp; // keep the finished iterator at the end for debugging
                heapAdjust();
            }
        }

        @Override
        public long cost() {
            return cost;
        }

    }
}
