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QuestDB is high performance SQL time series database

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 *  Copyright (c) 2014-2019 Appsicle
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package io.questdb.griffin.engine.table;

import io.questdb.cairo.sql.RowCursor;
import io.questdb.std.IntLongPriorityQueue;
import io.questdb.std.ObjList;

/**
 * Returns rows from current page frame in table (physical) order:
 * - fetches first record index per cursor into priority queue
 * - then returns record with the smallest index and adds next record
 * from related cursor into queue until all cursors are exhausted.
 */
class HeapRowCursor implements RowCursor {
    private final IntLongPriorityQueue heap;
    private ObjList cursors;

    public HeapRowCursor() {
        this.heap = new IntLongPriorityQueue();
    }

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

    @Override
    public long next() {
        int idx = heap.popIndex();
        RowCursor cursor = cursors.getQuick(idx);
        return cursor.hasNext() ? heap.popAndReplace(idx, cursor.next()) : heap.popValue();
    }

    public void of(ObjList cursors, int activeCursors) {
        this.cursors = cursors;
        this.heap.clear();
        for (int i = 0; i < activeCursors; i++) {
            final RowCursor cursor = cursors.getQuick(i);
            if (cursor.hasNext()) {
                heap.add(i, cursor.next());
            }
        }
    }
}