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/*
 * 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.expressions;

import org.apache.lucene.expressions.js.JavascriptCompiler;
import org.apache.lucene.search.DoubleValues;
import org.apache.lucene.search.DoubleValuesSource;
import org.apache.lucene.search.Rescorer;
import org.apache.lucene.search.SortField;

/**
 * Base class that computes the value of an expression for a document.
 *
 * Example that sorts based on an expression:
 *
 * 
 *   // compile an expression:
 *   Expression expr = JavascriptCompiler.compile("sqrt(_score) + ln(popularity)");
 *
 *   // SimpleBindings just maps variables to DoubleValuesSource instances
 *   SimpleBindings bindings = new SimpleBindings();
 *   bindings.add("_score", DoubleValuesSource.SCORES);
 *   bindings.add("popularity", DoubleValuesSource.fromIntField("popularity"));
 *
 *   // create a sort field and sort by it (reverse order)
 *   Sort sort = new Sort(expr.getSortField(bindings, true));
 *   Query query = new TermQuery(new Term("body", "contents"));
 *   searcher.search(query, 10, sort);
 * 
 *
 * Example that modifies the scores produced by the query:
 *
 * 
 *   // compile an expression:
 *   Expression expr = JavascriptCompiler.compile("sqrt(_score) + ln(popularity)");
 *
 *   // SimpleBindings just maps variables to DoubleValuesSource instances
 *   SimpleBindings bindings = new SimpleBindings();
 *   bindings.add("_score", DoubleValuesSource.SCORES);
 *   bindings.add("popularity", DoubleValuesSource.fromIntField("popularity"));
 *
 *   // create a query that matches based on body:contents but
 *   // scores using expr
 *   Query query = new FunctionScoreQuery(
 *       new TermQuery(new Term("body", "contents")),
 *       expr.getDoubleValuesSource(bindings));
 *   searcher.search(query, 10);
 * 
 *
 * @see JavascriptCompiler#compile
 * @lucene.experimental
 */
public abstract class Expression {

  /** The original source text */
  public final String sourceText;

  /** Named variables referred to by this expression */
  public final String[] variables;

  /**
   * Creates a new {@code Expression}.
   *
   * @param sourceText Source text for the expression: e.g. {@code ln(popularity)}
   * @param variables Names of external variables referred to by the expression
   */
  protected Expression(String sourceText, String[] variables) {
    this.sourceText = sourceText;
    this.variables = variables;
  }

  /**
   * Evaluates the expression for the current document.
   *
   * @param functionValues {@link DoubleValues} for each element of {@link #variables}.
   * @return The computed value of the expression for the given document.
   */
  public abstract double evaluate(DoubleValues[] functionValues);

  /**
   * Get a DoubleValuesSource which can compute the value of this expression in the context of the
   * given bindings.
   *
   * @param bindings Bindings to use for external values in this expression
   * @return A DoubleValuesSource which will evaluate this expression when used
   */
  public DoubleValuesSource getDoubleValuesSource(Bindings bindings) {
    return new ExpressionValueSource(bindings, this);
  }

  /** Get a sort field which can be used to rank documents by this expression. */
  public SortField getSortField(Bindings bindings, boolean reverse) {
    return getDoubleValuesSource(bindings).getSortField(reverse);
  }

  /** Get a {@link Rescorer}, to rescore first-pass hits using this expression. */
  public Rescorer getRescorer(Bindings bindings) {
    return new ExpressionRescorer(this, bindings);
  }
}