content: /*
   * Copyright © 2018-2025 Commonwealth Scientific and Industrial Research
   * Organisation (CSIRO) ABN 41 687 119 230.
   *
   * Licensed 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 au.csiro.pathling.fhirpath.operator;
  
  import au.csiro.pathling.fhirpath.collection.Collection;
  import au.csiro.pathling.fhirpath.collection.IntegerCollection;
  import jakarta.annotation.Nonnull;
  import org.apache.spark.sql.functions;
  
  /**
   * Operations for subsetting collections by index.
   */
  public class SubsettingOperations {
  
    private SubsettingOperations() {
    }
  
    /**
     * Returns a singleton collection containing the element at the specified index.
     *
     * @param subject The collection to index
     * @param index The index to use
     * @return A singleton collection containing the element at the specified index
     * @see <a href="https://hl7.org/fhirpath/#index-integer-collection">Indexer operation</a>
     */
    @FhirPathOperator(name = "[]")
    @Nonnull
    public static Collection index(@Nonnull final Collection subject,
        @Nonnull final IntegerCollection index) {
      return subject.map(
          rep -> rep.vectorize(
              // If the subject is non-singular, use the `element_at` function to extract the element 
              // at the specified index. 
              c -> functions.element_at(c, index.getColumnValue().plus(1)),
              // If the subject is singular, return the subject itself. 
              c -> c
          ));
    }
  }
  