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The official MongoDB Apache Kafka Connect Connector.
There is a newer version: 1.13.1
/*
 * Copyright 2008-present MongoDB, Inc.
 *
 * 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 com.mongodb.kafka.connect.source.schema;

import java.util.List;
import java.util.Map;
import java.util.Objects;

import org.apache.kafka.connect.data.Decimal;
import org.apache.kafka.connect.data.Schema;
import org.apache.kafka.connect.data.SchemaBuilder;
import org.apache.kafka.connect.data.Timestamp;

import org.bson.BsonDocument;
import org.bson.BsonValue;

public final class BsonDocumentToSchema {

  private static final String ID_FIELD = "_id";
  private static final Schema DEFAULT_INFER_SCHEMA_TYPE = Schema.OPTIONAL_STRING_SCHEMA;
  public static final String DEFAULT_FIELD_NAME = "default";

  public static Schema inferDocumentSchema(final BsonDocument document) {
    return createSchemaBuilder(DEFAULT_FIELD_NAME, document).required().build();
  }

  private static Schema inferDocumentSchema(final String fieldPath, final BsonDocument document) {
    return createSchemaBuilder(fieldPath, document).optional().build();
  }

  private static SchemaBuilder createSchemaBuilder(
      final String fieldPath, final BsonDocument document) {
    SchemaBuilder builder = SchemaBuilder.struct();
    builder.name(fieldPath);
    if (document.containsKey(ID_FIELD)) {
      builder.field(ID_FIELD, inferSchema(ID_FIELD, document.get(ID_FIELD)));
    }
    document.entrySet().stream()
        .filter(kv -> !kv.getKey().equals(ID_FIELD))
        .sorted(Map.Entry.comparingByKey())
        .forEach(
            kv ->
                builder.field(
                    kv.getKey(),
                    inferSchema(createFieldPath(fieldPath, kv.getKey()), kv.getValue())));
    return builder;
  }

  private static Schema inferSchema(final String fieldPath, final BsonValue bsonValue) {
    switch (bsonValue.getBsonType()) {
      case BOOLEAN:
        return Schema.OPTIONAL_BOOLEAN_SCHEMA;
      case INT32:
        return Schema.OPTIONAL_INT32_SCHEMA;
      case INT64:
        return Schema.OPTIONAL_INT64_SCHEMA;
      case DOUBLE:
        return Schema.OPTIONAL_FLOAT64_SCHEMA;
      case DECIMAL128:
        return Decimal.builder(bsonValue.asDecimal128().getValue().bigDecimalValue().scale())
            .optional()
            .build();
      case DATE_TIME:
      case TIMESTAMP:
        return Timestamp.builder().optional().build();
      case DOCUMENT:
        return inferDocumentSchema(fieldPath, bsonValue.asDocument());
      case ARRAY:
        List values = bsonValue.asArray().getValues();
        Schema firstItemSchema =
            values.isEmpty() ? DEFAULT_INFER_SCHEMA_TYPE : inferSchema(fieldPath, values.get(0));
        if (values.isEmpty()
            || values.stream()
                .anyMatch(bv -> !Objects.equals(inferSchema(fieldPath, bv), firstItemSchema))) {
          return SchemaBuilder.array(DEFAULT_INFER_SCHEMA_TYPE).name(fieldPath).optional().build();
        }
        return SchemaBuilder.array(inferSchema(fieldPath, bsonValue.asArray().getValues().get(0)))
            .name(fieldPath)
            .optional()
            .build();
      case BINARY:
        return Schema.OPTIONAL_BYTES_SCHEMA;
      case SYMBOL:
      case STRING:
      case NULL:
        return Schema.OPTIONAL_STRING_SCHEMA;
      case OBJECT_ID:
      case REGULAR_EXPRESSION:
      case DB_POINTER:
      case JAVASCRIPT:
      case JAVASCRIPT_WITH_SCOPE:
      case MIN_KEY:
      case MAX_KEY:
      case UNDEFINED:
      default:
        return DEFAULT_INFER_SCHEMA_TYPE;
    }
  }

  private static String createFieldPath(final String fieldPath, final String fieldName) {
    if (fieldPath.equals(DEFAULT_FIELD_NAME)) {
      return fieldName;
    } else {
      return fieldPath + "_" + fieldName;
    }
  }

  private BsonDocumentToSchema() {}
}