/*
 * 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.beam.sdk.schemas.transforms;

import java.util.Map;
import org.apache.beam.sdk.annotations.Experimental;
import org.apache.beam.sdk.annotations.Experimental.Kind;
import org.apache.beam.sdk.schemas.FieldAccessDescriptor;
import org.apache.beam.sdk.schemas.Schema;
import org.apache.beam.sdk.schemas.Schema.Field;
import org.apache.beam.sdk.schemas.Schema.FieldType;
import org.apache.beam.sdk.transforms.DoFn;
import org.apache.beam.sdk.transforms.PTransform;
import org.apache.beam.sdk.transforms.ParDo;
import org.apache.beam.sdk.values.PCollection;
import org.apache.beam.sdk.values.Row;

/**
 * A {@link PTransform} for selecting a subset of fields from a schema type.
 *
 * <p>This transforms allows projecting out a subset of fields from a schema type. The output of
 * this transform is of type {@link Row}, though that can be converted into any other type with
 * matching schema using the {@link Convert} transform.
 *
 * <p>For example, consider the following POJO type:
 *
 * <pre>{@code
 * {@literal @}DefaultSchema(JavaFieldSchema.class)
 * public class UserEvent {
 *   public String userId;
 *   public String eventId;
 *   public int eventType;
 *   public Location location;
 * }
 *
 * {@literal @}DefaultSchema(JavaFieldSchema.class)
 * public class Location {
 *   public double latitude;
 *   public double longtitude;
 * }
 * }</pre>
 *
 * Say you want to select just the set of userId, eventId pairs from each element, you would write
 * the following:
 *
 * <pre>{@code
 * PCollection<UserEvent> events = readUserEvents();
 * PCollection<Row> rows = event.apply(Select.fieldNames("userId", "eventId"));
 * }</pre>
 *
 * It's possible to select a nested field as well. For example, if you want just the location
 * information from each element:
 *
 * <pre>{@code
 * PCollection<UserEvent> events = readUserEvents();
 * PCollection<Row> rows = event.apply(Select.fieldAccess(FieldAccessDescriptor.create()
 *      .withNestedField("location",
 *          FieldAccessDescriptor.withAllFields())));
 * }</pre>
 */
@Experimental(Kind.SCHEMAS)
public class Select<T> extends PTransform<PCollection<T>, PCollection<Row>> {
  private final FieldAccessDescriptor fieldAccessDescriptor;

  private Select(FieldAccessDescriptor fieldAccessDescriptor) {
    this.fieldAccessDescriptor = fieldAccessDescriptor;
  }

  /** Select a set of top-level field ids from the row. */
  public static <T> Select<T> fieldIds(Integer... ids) {
    return new Select(FieldAccessDescriptor.withFieldIds(ids));
  }

  /** Select a set of top-level field names from the row. */
  public static <T> Select<T> fieldNames(String... names) {
    return new Select(FieldAccessDescriptor.withFieldNames(names));
  }

  /**
   * Select a set of fields described in a {@link FieldAccessDescriptor}.
   *
   * <p>This allows for nested fields to be selected as well.
   */
  public static <T> Select<T> fieldAccess(FieldAccessDescriptor fieldAccessDescriptor) {
    return new Select(fieldAccessDescriptor);
  }

  // TODO: Support Xpath or JsonPath as a way of describing fields.

  @Override
  public PCollection<Row> expand(PCollection<T> input) {
    Schema inputSchema = input.getSchema();
    FieldAccessDescriptor resolved = fieldAccessDescriptor.resolve(inputSchema);
    Schema outputSchema = getOutputSchema(inputSchema, resolved);

    PCollection<Row> selected =
        input
            .apply(
                ParDo.of(
                    new DoFn<T, Row>() {
                      // TODO: This should be the same as resolved so that Beam knows which fields
                      // are being accessed. Currently Beam only supports wildcard descriptors.
                      // Once BEAM-4457 is fixed, fix this.
                      @FieldAccess("filterFields")
                      final FieldAccessDescriptor fieldAccessDescriptor =
                          FieldAccessDescriptor.withAllFields();

                      @ProcessElement
                      public void process(
                          @FieldAccess("filterFields") Row row, OutputReceiver<Row> r) {
                        r.output(selectRow(row, resolved, inputSchema, outputSchema));
                      }
                    }))
            .setRowSchema(outputSchema);

    return selected;
  }

  // Currently we don't flatten selected nested fields. We should consider whether to flatten them
  // or leave them as is.
  static Schema getOutputSchema(Schema inputSchema, FieldAccessDescriptor fieldAccessDescriptor) {
    if (fieldAccessDescriptor.allFields()) {
      return inputSchema;
    }
    Schema.Builder builder = new Schema.Builder();
    for (int fieldId : fieldAccessDescriptor.fieldIdsAccessed()) {
      builder.addField(inputSchema.getField(fieldId));
    }

    for (Map.Entry<Integer, FieldAccessDescriptor> nested :
        fieldAccessDescriptor.nestedFields().entrySet()) {
      Field field = inputSchema.getField(nested.getKey());
      FieldAccessDescriptor nestedDescriptor = nested.getValue();
      FieldType nestedType =
          FieldType.row(getOutputSchema(field.getType().getRowSchema(), nestedDescriptor));

      nestedType = nestedType.withNullable(field.getType().getNullable());
      builder.addField(field.getName(), nestedType);
    }
    return builder.build();
  }

  static Row selectRow(
      Row input,
      FieldAccessDescriptor fieldAccessDescriptor,
      Schema inputSchema,
      Schema outputSchema) {
    if (fieldAccessDescriptor.allFields()) {
      return input;
    } else {
      Row.Builder output = Row.withSchema(outputSchema);
      for (int fieldId : fieldAccessDescriptor.fieldIdsAccessed()) {
        output.addValue(input.getValue(fieldId));
      }
      for (Map.Entry<Integer, FieldAccessDescriptor> nested :
          fieldAccessDescriptor.nestedFields().entrySet()) {
        String fieldName = inputSchema.nameOf(nested.getKey());
        Schema nestedInputSchema = inputSchema.getField(nested.getKey()).getType().getRowSchema();
        Schema nestedOutputSchema = outputSchema.getField(fieldName).getType().getRowSchema();
        output.addValue(
            selectRow(
                input.getValue(fieldName),
                nested.getValue(),
                nestedInputSchema,
                nestedOutputSchema));
      }
      return output.build();
    }
  }
}
