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/*
 * Copyright (c) 2010-2017, KolibriFX AS. Licensed under the Apache License, version 2.0.
 */

package com.kolibrifx.plovercrest.server.streams.folds;

import com.kolibrifx.lancebill.folds.FoldOutputStream;
import com.kolibrifx.lancebill.folds.GenericFold;
import com.kolibrifx.plovercrest.server.internal.folds.FoldCallback;
import com.kolibrifx.plovercrest.server.internal.folds.FoldWriter;

/**
 * {@link FoldCallback} implementation that wraps a {@link FoldWriter} and {@link GenericFold} and
 * implements take-previous behavior when writing to the underlying {@link FoldWriter}.
 */
public class TakePreviousFoldCallbackAdapter implements FoldCallback {
    private final FoldWriter writer;
    private final GenericFold fold;
    private final FoldOutputStream outputStream;
    private boolean hasStarted = false;
    private long previousTimestamp = 0;
    private T previousMessage = null;

    public TakePreviousFoldCallbackAdapter(final long fromTimestamp,
                                           final FoldWriter writer,
                                           final GenericFold fold) {
        this.writer = writer;
        this.fold = fold;
        this.outputStream = new FoldOutputStream() {
            @Override
            public void write(final long timestamp, final T message) {
                // Messages before "fromTimestamp" are normally dropped. But we want "take previous" behavior,
                // so keep track of the last message and trigger it when we receive the first message larger than
                // "fromTimestamp". If we receive a message exactly at "fromTimestamp", the last message is dropped.
                if (timestamp > fromTimestamp) {
                    maybeWritePrevious();
                }
                if (!hasStarted && timestamp < fromTimestamp) {
                    previousMessage = message;
                } else {
                    previousMessage = null;
                    writer.write(timestamp, message);
                    hasStarted = true;
                }
                previousTimestamp = timestamp;
            }
        };
    }

    private void maybeWritePrevious() {
        if (!hasStarted && previousMessage != null) {
            writer.write(previousTimestamp, previousMessage);
            hasStarted = true;
        }
    }

    @Override
    public void onNext(final String tableName, final long timestamp, final U data) {
        fold.onInput(tableName, Math.max(timestamp, previousTimestamp), data, outputStream);
    }

    @Override
    public void onEnd() {
        // This ensures "take previous" behavior for subscribers
        maybeWritePrevious();
    }
}