/*
 * 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.search.suggest.fst;

import java.io.Closeable;
import java.io.IOException;
import java.util.Comparator;

import org.apache.lucene.search.suggest.InMemorySorter;
import org.apache.lucene.util.BytesRef;
import org.apache.lucene.util.BytesRefBuilder;
import org.apache.lucene.util.BytesRefIterator;
import org.apache.lucene.util.IntsRefBuilder;
import org.apache.lucene.util.fst.*;

/**
 * Finite state automata based implementation of "autocomplete" functionality.
 * 
 * <h2>Implementation details</h2>
 * 
 * <p>
 * The construction step in {@link #finalize()} works as follows:
 * <ul>
 * <li>A set of input terms and their buckets is given.</li>
 * <li>All terms in the input are prefixed with a synthetic pseudo-character
 * (code) of the weight bucket the term fell into. For example a term
 * <code>abc</code> with a discretized weight equal '1' would become
 * <code>1abc</code>.</li>
 * <li>The terms are then sorted by their raw value of UTF-8 character values
 * (including the synthetic bucket code in front).</li>
 * <li>A finite state automaton ({@link FST}) is constructed from the input. The
 * root node has arcs labeled with all possible weights. We cache all these
 * arcs, highest-weight first.</li>
 * </ul>
 * 
 * <p>
 * At runtime, in {@link FSTCompletion#lookup(CharSequence, int)}, 
 * the automaton is utilized as follows:
 * <ul>
 * <li>For each possible term weight encoded in the automaton (cached arcs from
 * the root above), starting with the highest one, we descend along the path of
 * the input key. If the key is not a prefix of a sequence in the automaton
 * (path ends prematurely), we exit immediately -- no completions.</li>
 * <li>Otherwise, we have found an internal automaton node that ends the key.
 * <b>The entire subautomaton (all paths) starting from this node form the key's
 * completions.</b> We start the traversal of this subautomaton. Every time we
 * reach a final state (arc), we add a single suggestion to the list of results
 * (the weight of this suggestion is constant and equal to the root path we
 * started from). The tricky part is that because automaton edges are sorted and
 * we scan depth-first, we can terminate the entire procedure as soon as we
 * collect enough suggestions the user requested.</li>
 * <li>In case the number of suggestions collected in the step above is still
 * insufficient, we proceed to the next (smaller) weight leaving the root node
 * and repeat the same algorithm again.</li>
 * </ul>
 * 
 * <h2>Runtime behavior and performance characteristic</h2>
 * 
 * The algorithm described above is optimized for finding suggestions to short
 * prefixes in a top-weights-first order. This is probably the most common use
 * case: it allows presenting suggestions early and sorts them by the global
 * frequency (and then alphabetically).
 * 
 * <p>
 * If there is an exact match in the automaton, it is returned first on the
 * results list (even with by-weight sorting).
 * 
 * <p>
 * Note that the maximum lookup time for <b>any prefix</b> is the time of
 * descending to the subtree, plus traversal of the subtree up to the number of
 * requested suggestions (because they are already presorted by weight on the
 * root level and alphabetically at any node level).
 * 
 * <p>
 * To order alphabetically only (no ordering by priorities), use identical term
 * weights for all terms. Alphabetical suggestions are returned even if
 * non-constant weights are used, but the algorithm for doing this is
 * suboptimal.
 * 
 * <p>
 * "alphabetically" in any of the documentation above indicates UTF-8
 * representation order, nothing else.
 * 
 * <p>
 * <b>NOTE</b>: the FST file format is experimental and subject to suddenly
 * change, requiring you to rebuild the FST suggest index.
 * 
 * @see FSTCompletion
 * @lucene.experimental
 */
public class FSTCompletionBuilder {
  /** 
   * Default number of buckets.
   */
  public static final int DEFAULT_BUCKETS = 10;

  /**
   * The number of separate buckets for weights (discretization). The more
   * buckets, the more fine-grained term weights (priorities) can be assigned.
   * The speed of lookup will not decrease for prefixes which have
   * highly-weighted completions (because these are filled-in first), but will
   * decrease significantly for low-weighted terms (but these should be
   * infrequent, so it is all right).
   * 
   * <p>
   * The number of buckets must be within [1, 255] range.
   */
  private final int buckets;

  /**
   * Finite state automaton encoding all the lookup terms. See class notes for
   * details.
   */
  FST<Object> automaton;

  /**
   * FST construction require re-sorting the input. This is the class that
   * collects all the input entries, their weights and then provides sorted
   * order.
   */
  private final BytesRefSorter sorter;
  
  /**
   * Scratch buffer for {@link #add(BytesRef, int)}.
   */
  private final BytesRefBuilder scratch = new BytesRefBuilder();

  /**
   * Max tail sharing length.
   */
  private final int shareMaxTailLength;

  /**
   * Creates an {@link FSTCompletion} with default options: 10 buckets, exact match
   * promoted to first position and {@link InMemorySorter} with a comparator obtained from
   * {@link Comparator#naturalOrder()}.
   */
  public FSTCompletionBuilder() {
    this(DEFAULT_BUCKETS, new InMemorySorter(Comparator.naturalOrder()), Integer.MAX_VALUE);
  }

  /**
   * Creates an FSTCompletion with the specified options.
   * @param buckets
   *          The number of buckets for weight discretization. Buckets are used
   *          in {@link #add(BytesRef, int)} and must be smaller than the number
   *          given here.
   *          
   * @param sorter
   *          {@link BytesRefSorter} used for re-sorting input for the automaton.
   *          For large inputs, use on-disk sorting implementations. The sorter
   *          is closed automatically in {@link #build()} if it implements
   *          {@link Closeable}.
   *          
   * @param shareMaxTailLength
   *          Max shared suffix sharing length.
   *          
   *          See the description of this parameter in {@link Builder}'s constructor.
   *          In general, for very large inputs you'll want to construct a non-minimal
   *          automaton which will be larger, but the construction will take far less ram.
   *          For minimal automata, set it to {@link Integer#MAX_VALUE}.
   */
  public FSTCompletionBuilder(int buckets, BytesRefSorter sorter, int shareMaxTailLength) {
    if (buckets < 1 || buckets > 255) {
      throw new IllegalArgumentException("Buckets must be >= 1 and <= 255: "
          + buckets);
    }
    
    if (sorter == null) throw new IllegalArgumentException(
        "BytesRefSorter must not be null.");
    
    this.sorter = sorter;
    this.buckets = buckets;
    this.shareMaxTailLength = shareMaxTailLength;
  }

  /**
   * Appends a single suggestion and its weight to the internal buffers.
   * 
   * @param utf8
   *          The suggestion (utf8 representation) to be added. The content is
   *          copied and the object can be reused.
   * @param bucket
   *          The bucket to place this suggestion in. Must be non-negative and
   *          smaller than the number of buckets passed in the constructor.
   *          Higher numbers indicate suggestions that should be presented
   *          before suggestions placed in smaller buckets.
   */
  public void add(BytesRef utf8, int bucket) throws IOException {
    if (bucket < 0 || bucket >= buckets) {
      throw new IllegalArgumentException(
          "Bucket outside of the allowed range [0, " + buckets + "): " + bucket);
    }
    
    scratch.grow(utf8.length + 10);
    scratch.clear();
    scratch.append((byte) bucket);
    scratch.append(utf8);
    sorter.add(scratch.get());
  }

  /**
   * Builds the final automaton from a list of added entries. This method may
   * take a longer while as it needs to build the automaton.
   */
  public FSTCompletion build() throws IOException {
    this.automaton = buildAutomaton(sorter);

    if (sorter instanceof Closeable) {
      ((Closeable) sorter).close();
    }

    return new FSTCompletion(automaton);
  }

  /**
   * Builds the final automaton from a list of entries.
   */
  private FST<Object> buildAutomaton(BytesRefSorter sorter) throws IOException {
    // Build the automaton.
    final Outputs<Object> outputs = NoOutputs.getSingleton();
    final Object empty = outputs.getNoOutput();
    final Builder<Object> builder = new Builder<>(
        FST.INPUT_TYPE.BYTE1, 0, 0, true, true, 
        shareMaxTailLength, outputs, true, 15);
    
    BytesRefBuilder scratch = new BytesRefBuilder();
    BytesRef entry;
    final IntsRefBuilder scratchIntsRef = new IntsRefBuilder();
    int count = 0;
    BytesRefIterator iter = sorter.iterator();
    while((entry = iter.next()) != null) {
      count++;
      if (scratch.get().compareTo(entry) != 0) {
        builder.add(Util.toIntsRef(entry, scratchIntsRef), empty);
        scratch.copyBytes(entry);
      }
    }
    
    return count == 0 ? null : builder.finish();
  }
}