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 * Copyright (C) 2014-2019 Appsicle
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package com.questdb.cairo;

import com.questdb.mp.Job;
import com.questdb.mp.RingQueue;
import com.questdb.mp.SOCountDownLatch;
import com.questdb.mp.Sequence;

class ColumnIndexerJob implements Job {
    private final RingQueue queue;
    private final Sequence sequence;

    public ColumnIndexerJob(CairoWorkScheduler workScheduler) {
        this.queue = workScheduler.getIndexerQueue();
        this.sequence = workScheduler.getIndexerSubSequence();
    }

    @Override
    public boolean run() {
        long cursor = sequence.next();
        if (cursor < 0) {
            return false;
        }

        ColumnIndexerEntry queueItem = queue.get(cursor);
        // copy values and release queue item
        final ColumnIndexer indexer = queueItem.indexer;
        final long lo = queueItem.lo;
        final long hi = queueItem.hi;
        final long indexSequence = queueItem.sequence;
        final SOCountDownLatch latch = queueItem.countDownLatch;
        sequence.done(cursor);

        // On the face of it main thread could have consumed same sequence as
        // child workers. The reason it is undesirable is because all writers
        // share the same queue and main thread end up indexing content for other writers.
        // Using CAS allows main thread to steal only parts of its own job.
        if (indexer.tryLock(indexSequence)) {
            TableWriter.indexAndCountDown(indexer, lo, hi, latch);
            return true;
        }
        // This is hard to test. Condition occurs when main thread successfully steals
        // work from under nose of this worker.
        return false;
    }
}