/*
 * 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.lucene.util.packed;


import static org.apache.lucene.util.packed.MonotonicBlockPackedReader.expected;

import java.io.IOException;

import org.apache.lucene.store.DataOutput;
import org.apache.lucene.util.BitUtil;

A writer for large monotonically increasing sequences of positive longs.

The sequence is divided into fixed-size blocks and for each block, values
are modeled after a linear function f: x → A × x + B. The block
encodes deltas from the expected values computed from this function using as
few bits as possible.

Format:

<BLock>BlockCount
BlockCount: ⌈ ValueCount / BlockSize ⌉
Block: <Header, (Ints)>
Header: <B, A, BitsPerValue>
B: the B from f: x → A × x + B using a zig-zag encoded vLong 
A: the A from f: x → A × x + B encoded using Float.floatToIntBits(float) on 4 bytes 
BitsPerValue: a variable-length int 
Ints: if BitsPerValue is 0, then there is nothing to read and all values perfectly match the result of the function. Otherwise, these are the packed deltas from the expected value (computed from the function) using exaclty BitsPerValue bits per value. 
See Also:
MonotonicBlockPackedReader
@lucene.internal
/**
 * A writer for large monotonically increasing sequences of positive longs.
 * <p>
 * The sequence is divided into fixed-size blocks and for each block, values
 * are modeled after a linear function f: x &rarr; A &times; x + B. The block
 * encodes deltas from the expected values computed from this function using as
 * few bits as possible.
 * <p>
 * Format:
 * <ul>
 * <li>&lt;BLock&gt;<sup>BlockCount</sup>
 * <li>BlockCount: &lceil; ValueCount / BlockSize &rceil;
 * <li>Block: &lt;Header, (Ints)&gt;
 * <li>Header: &lt;B, A, BitsPerValue&gt;
 * <li>B: the B from f: x &rarr; A &times; x + B using a
 *     {@link BitUtil#zigZagEncode(long) zig-zag encoded}
 *     {@link DataOutput#writeVLong(long) vLong}
 * <li>A: the A from f: x &rarr; A &times; x + B encoded using
 *     {@link Float#floatToIntBits(float)} on
 *     {@link DataOutput#writeInt(int) 4 bytes}
 * <li>BitsPerValue: a {@link DataOutput#writeVInt(int) variable-length int}
 * <li>Ints: if BitsPerValue is <tt>0</tt>, then there is nothing to read and
 *     all values perfectly match the result of the function. Otherwise, these
 *     are the {@link PackedInts packed} deltas from the expected value
 *     (computed from the function) using exaclty BitsPerValue bits per value.
 * </ul>
 * @see MonotonicBlockPackedReader
 * @lucene.internal
 */
public final class MonotonicBlockPackedWriter extends AbstractBlockPackedWriter {

  
Sole constructor.
Params:
blockSize – the number of values of a single block, must be a power of 2/**
   * Sole constructor.
   * @param blockSize the number of values of a single block, must be a power of 2
   */
  public MonotonicBlockPackedWriter(DataOutput out, int blockSize) {
    super(out, blockSize);
  }

  @Override
  public void add(long l) throws IOException {
    assert l >= 0;
    super.add(l);
  }

  protected void flush() throws IOException {
    assert off > 0;

    final float avg = off == 1 ? 0f : (float) (values[off - 1] - values[0]) / (off - 1);
    long min = values[0];
    // adjust min so that all deltas will be positive
    for (int i = 1; i < off; ++i) {
      final long actual = values[i];
      final long expected = expected(min, avg, i);
      if (expected > actual) {
        min -= (expected - actual);
      }
    }

    long maxDelta = 0;
    for (int i = 0; i < off; ++i) {
      values[i] = values[i] - expected(min, avg, i);
      maxDelta = Math.max(maxDelta, values[i]);
    }

    out.writeZLong(min);
    out.writeInt(Float.floatToIntBits(avg));
    if (maxDelta == 0) {
      out.writeVInt(0);
    } else {
      final int bitsRequired = PackedInts.bitsRequired(maxDelta);
      out.writeVInt(bitsRequired);
      writeValues(bitsRequired);
    }

    off = 0;
  }

}