/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF 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.apache.beam.sdk.transforms.splittabledofn;

import static org.apache.beam.vendor.guava.v20_0.com.google.common.base.Preconditions.checkArgument;
import static org.apache.beam.vendor.guava.v20_0.com.google.common.base.Preconditions.checkNotNull;
import static org.apache.beam.vendor.guava.v20_0.com.google.common.base.Preconditions.checkState;

import java.math.BigDecimal;
import javax.annotation.Nullable;
import org.apache.beam.sdk.io.range.OffsetRange;
import org.apache.beam.vendor.guava.v20_0.com.google.common.base.MoreObjects;

/**
 * A {@link RestrictionTracker} for claiming offsets in an {@link OffsetRange} in a monotonically
 * increasing fashion.
 */
public class OffsetRangeTracker extends RestrictionTracker<OffsetRange, Long>
    implements Backlogs.HasBacklog {
  private OffsetRange range;
  @Nullable private Long lastClaimedOffset = null;
  @Nullable private Long lastAttemptedOffset = null;

  public OffsetRangeTracker(OffsetRange range) {
    this.range = checkNotNull(range);
  }

  @Override
  public synchronized OffsetRange currentRestriction() {
    return range;
  }

  @Override
  public synchronized OffsetRange checkpoint() {
    checkState(
        lastClaimedOffset != null, "Can't checkpoint before any offset was successfully claimed");
    OffsetRange res = new OffsetRange(lastClaimedOffset + 1, range.getTo());
    this.range = new OffsetRange(range.getFrom(), lastClaimedOffset + 1);
    return res;
  }

  /**
   * Attempts to claim the given offset.
   *
   * <p>Must be larger than the last successfully claimed offset.
   *
   * @return {@code true} if the offset was successfully claimed, {@code false} if it is outside the
   *     current {@link OffsetRange} of this tracker (in that case this operation is a no-op).
   */
  @Override
  protected synchronized boolean tryClaimImpl(Long i) {
    checkArgument(
        lastAttemptedOffset == null || i > lastAttemptedOffset,
        "Trying to claim offset %s while last attempted was %s",
        i,
        lastAttemptedOffset);
    checkArgument(
        i >= range.getFrom(), "Trying to claim offset %s before start of the range %s", i, range);
    lastAttemptedOffset = i;
    // No respective checkArgument for i < range.to() - it's ok to try claiming offsets beyond it.
    if (i >= range.getTo()) {
      return false;
    }
    lastClaimedOffset = i;
    return true;
  }

  @Override
  public synchronized void checkDone() throws IllegalStateException {
    checkState(
        lastAttemptedOffset >= range.getTo() - 1,
        "Last attempted offset was %s in range %s, claiming work in [%s, %s) was not attempted",
        lastAttemptedOffset,
        range,
        lastAttemptedOffset + 1,
        range.getTo());
  }

  @Override
  public String toString() {
    return MoreObjects.toStringHelper(this)
        .add("range", range)
        .add("lastClaimedOffset", lastClaimedOffset)
        .add("lastAttemptedOffset", lastAttemptedOffset)
        .toString();
  }

  @Override
  public synchronized Backlog getBacklog() {
    // If we have never attempted an offset, we return the length of the entire range.
    if (lastAttemptedOffset == null) {
      return Backlog.of(BigDecimal.valueOf(range.getTo() - range.getFrom()));
    }

    // Otherwise we return the length from where we are to where we are attempting to get to
    // with a minimum of zero in case we have claimed beyond the end of the range.
    return Backlog.of(BigDecimal.valueOf(Math.max(range.getTo() - lastAttemptedOffset, 0)));
  }
}
