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/*
 * Copyright 2015-2018 The OpenZipkin Authors
 *
 * 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 zipkin.internal;

import java.util.List;
import zipkin.Span;
import zipkin.SpanDecoder;

/**
 * Detecting decoder used in transports which don't include means to identify the type of the data.
 *
 * For example, we can identify the encoding and also the format in http via the request path and
 * content-type. However, in Kafka it could be that folks send mixed Zipkin data without identifying
 * its format. For example, Kafka historically has no content-type and users don't always segregate
 * different queues by instrumentation format.
 */
// In TBinaryProtocol encoding, the first byte is the TType, in a range 0-16
// .. If the first byte isn't in that range, it isn't a thrift.
//
// When byte(0) == '[' (91), assume it is a list of json-encoded spans
//
// When byte(0) == 10, assume it is a proto3-encoded span or trace ID field
//
// When byte(0) <= 16, assume it is a TBinaryProtocol-encoded thrift
// .. When serializing a Span (Struct), the first byte will be the type of a field
// .. When serializing a List[ThriftSpan], the first byte is the member type, TType.STRUCT(12)
// .. As ThriftSpan has no STRUCT fields: so, if the first byte is TType.STRUCT(12), it is a list.
public final class DetectingSpanDecoder implements SpanDecoder {
  /**
   * Zipkin v2 json will have "localEndpoint" or "remoteEndpoint" fields, and others won't.
   *
   * 
Note: Technically, it is also possible that one can thwart this by creating an binary
   * annotation of type string with a name or value literally ending in Endpoint. This would be
   * strange, especially as the convention to identify a local endpoint is the key "lc". To prevent
   * a secondary check, this scenario is also ignored.
   */
  static final byte[] ENDPOINT_FIELD_SUFFIX = "Endpoint\"".getBytes(Util.UTF_8);

  /**
   * Technically, it is possible to have a v2 span with no endpoints. This should catch the case
   * where someone reported a tag without reporting the "localEndpoint".
   *
   * Note: we don't check for annotations as that exists in both v1 and v2 formats.
   */
  static final byte[] TAGS_FIELD = {'"', 't', 'a', 'g', 's', '"'};

  static final SpanDecoder JSON2_DECODER = new V2JsonSpanDecoder();
  static final SpanDecoder PROTO3_DECODER = new V2Proto3SpanDecoder();

  @Override
  public Span readSpan(byte[] span) {
    SpanDecoder decoder = detectFormat(span);
    if (span[0] == 12 /* List[ThriftSpan] */ || span[0] == '[') {
      throw new IllegalArgumentException("Expected json or thrift object, not list encoding");
    }
    return decoder.readSpan(span);
  }

  @Override
  public List readSpans(byte[] span) {
    SpanDecoder decoder = detectFormat(span);
    if (span[0] != 12 /* List[ThriftSpan] */ && !protobuf3(span) && span[0] != '[') {
      throw new IllegalArgumentException("Expected json, proto3 or thrift list encoding");
    }
    return decoder.readSpans(span);
  }

  /** @throws IllegalArgumentException if the input isn't a json or thrift list or object. */
  public static SpanDecoder detectFormat(byte[] bytes) {
    if (bytes[0] <= 16) { // binary format
      if (protobuf3(bytes)) return PROTO3_DECODER;
      return THRIFT_DECODER; /* the first byte is the TType, in a range 0-16 */
    } else if (bytes[0] != '[' && bytes[0] != '{') {
      throw new IllegalArgumentException("Could not detect the span format");
    }
    if (contains(bytes, ENDPOINT_FIELD_SUFFIX)) return JSON2_DECODER;
    if (contains(bytes, TAGS_FIELD)) return JSON2_DECODER;
    return SpanDecoder.JSON_DECODER;
  }

  static boolean contains(byte[] bytes, byte[] subsequence) {
    bytes:
    for (int i = 0; i < bytes.length - subsequence.length + 1; i++) {
      for (int j = 0; j < subsequence.length; j++) {
        if (bytes[i + j] != subsequence[j]) {
          continue bytes;
        }
      }
      return true;
    }
    return false;
  }

  /* span key or trace ID key */
  static boolean protobuf3(byte[] bytes) {
    return bytes[0] == 10 && bytes[1] != 0; // varint follows and won't be zero
  }
}