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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 java.io.IOException;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.Objects;
import org.apache.lucene.index.LeafReaderContext;
import org.apache.lucene.search.DoubleValues;
import org.apache.lucene.search.DoubleValuesSource;
import org.apache.lucene.search.Explanation;
import org.apache.lucene.search.IndexSearcher;

/**
 * A {@link DoubleValuesSource} which evaluates a {@link Expression} given the context of an {@link
 * Bindings}.
 */
class ExpressionValueSource extends DoubleValuesSource {
  final DoubleValuesSource[] variables;
  final Expression expression;
  final boolean needsScores;

  ExpressionValueSource(Bindings bindings, Expression expression) {
    if (bindings == null) throw new NullPointerException();
    this.expression = Objects.requireNonNull(expression);
    variables = new DoubleValuesSource[expression.variables.length];
    boolean needsScores = false;
    for (int i = 0; i < variables.length; i++) {
      DoubleValuesSource source = bindings.getDoubleValuesSource(expression.variables[i]);
      if (source == null) {
        throw new RuntimeException(
            "Internal error. Variable (" + expression.variables[i] + ") does not exist.");
      }
      needsScores |= source.needsScores();
      variables[i] = source;
    }
    this.needsScores = needsScores;
  }

  ExpressionValueSource(
      DoubleValuesSource[] variables, Expression expression, boolean needsScores) {
    this.variables = variables;
    this.expression = expression;
    this.needsScores = needsScores;
  }

  @Override
  public DoubleValues getValues(LeafReaderContext readerContext, DoubleValues scores)
      throws IOException {
    Map valuesCache = new HashMap<>();
    DoubleValues[] externalValues = new DoubleValues[expression.variables.length];

    for (int i = 0; i < variables.length; ++i) {
      String externalName = expression.variables[i];
      DoubleValues values = valuesCache.get(externalName);
      if (values == null) {
        values = variables[i].getValues(readerContext, scores);
        if (values == null) {
          throw new RuntimeException(
              "Unrecognized variable ("
                  + externalName
                  + ") referenced in expression ("
                  + expression.sourceText
                  + ").");
        }
        valuesCache.put(externalName, values);
      }
      externalValues[i] = zeroWhenUnpositioned(values);
    }

    return new ExpressionFunctionValues(expression, externalValues);
  }

  /**
   * Create a wrapper around all the expression arguments to do two things:
   *
   * 
   *   Default to 0 for any argument that doesn't have a value for a given doc (i.e.,
   *       #advanceExact returns false)
   *   
Be as lazy as possible about actually advancing to the given doc until the argument value
   *       is actually needed by the expression. For a given doc, some arguments may not actually be
   *       needed, e.g., because of condition short-circuiting ((true || X) doesn't
   *       need to evaluate X) or ternary branching (true ? X : Y doesn't
   *       need to evaluate Y).
   * 
   */
  static DoubleValues zeroWhenUnpositioned(DoubleValues in) {
    return new DoubleValues() {

      int currentDoc = -1;
      double value;
      boolean computed = false;

      @Override
      public double doubleValue() throws IOException {
        if (computed == false) {
          value = in.advanceExact(currentDoc) ? in.doubleValue() : 0;
          computed = true;
        }
        return value;
      }

      @Override
      public boolean advanceExact(int doc) {
        if (currentDoc == doc) {
          return true;
        }
        currentDoc = doc;
        computed = false;
        return true;
      }
    };
  }

  @Override
  public String toString() {
    return "expr(" + expression.sourceText + ")";
  }

  @Override
  public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + ((expression == null) ? 0 : expression.sourceText.hashCode());
    result = prime * result + (needsScores ? 1231 : 1237);
    result = prime * result + Arrays.hashCode(variables);
    return result;
  }

  @Override
  public boolean equals(Object obj) {
    if (this == obj) {
      return true;
    }
    if (obj == null) {
      return false;
    }
    if (getClass() != obj.getClass()) {
      return false;
    }
    ExpressionValueSource other = (ExpressionValueSource) obj;
    if (expression == null) {
      if (other.expression != null) {
        return false;
      }
    } else if (!expression.sourceText.equals(other.expression.sourceText)) {
      return false;
    }
    if (needsScores != other.needsScores) {
      return false;
    }
    if (!Arrays.equals(variables, other.variables)) {
      return false;
    }
    return true;
  }

  @Override
  public boolean needsScores() {
    return needsScores;
  }

  @Override
  public boolean isCacheable(LeafReaderContext ctx) {
    for (DoubleValuesSource v : variables) {
      if (v.isCacheable(ctx) == false) return false;
    }
    return true;
  }

  @Override
  public Explanation explain(LeafReaderContext ctx, int docId, Explanation scoreExplanation)
      throws IOException {
    Explanation[] explanations = new Explanation[variables.length];
    DoubleValues dv =
        getValues(
            ctx,
            DoubleValuesSource.constant(scoreExplanation.getValue().doubleValue())
                .getValues(ctx, null));
    if (dv.advanceExact(docId) == false) {
      return Explanation.noMatch(expression.sourceText);
    }
    int i = 0;
    for (DoubleValuesSource var : variables) {
      explanations[i++] = var.explain(ctx, docId, scoreExplanation);
    }
    return Explanation.match(
        dv.doubleValue(), expression.sourceText + ", computed from:", explanations);
  }

  @Override
  public DoubleValuesSource rewrite(IndexSearcher searcher) throws IOException {
    boolean changed = false;
    DoubleValuesSource[] rewritten = new DoubleValuesSource[variables.length];
    for (int i = 0; i < variables.length; i++) {
      rewritten[i] = variables[i].rewrite(searcher);
      changed |= (rewritten[i] != variables[i]);
    }
    if (changed) {
      return new ExpressionValueSource(rewritten, expression, needsScores);
    }
    return this;
  }
}