Use protobuf to serialize URL

[krisukox]

Google Flights uses protobuf to serialize the URL. Firstly, I did serialization manually, but I think adding protobuf, even for a really simple task, is the right approach. With this, it can be easily ported to any language with a protocol buffer compiler.

The downside of this method is that the final binary will be ~4 MB larger. (e.g. example3 binary 8 MB -> 12 MB).

Travelers:
When I tried to reverse engineer the URL, I found that it is serialized with protobuf:
Los Angeles -> Miami, 3 Adults
https://www.google.com/travel/flights/search?tfs=CBwQAholEgoyMDIzLTExLTA2ag4IAxIKL20vMDMwcWIzdHIHCAESA01JQRolEgoyMDIzLTExLTEzagcIARIDTUlBcg4IAxIKL20vMDMwcWIzdEABQAFAAUgBcAGCAQsI____________AZgBAQ

The tfs property is encoded with protobuf and with base64 encoding defined in RFC 4648.

Result of: protoc --decode_raw < proto.bin

1: 28
2: 2
3 {
  2: "2023-11-06"
  13 {
    1: 3
    2: "/m/030qb3t"
  }
  14 {
    1: 1
    2: "MIA"
  }
}
3 {
  2: "2023-11-13"
  13 {
    1: 1
    2: "MIA"
  }
  14 {
    1: 3
    2: "/m/030qb3t"
  }
}
8: 1
8: 1
8: 1
9: 1
14: 1
16 {
  1: 18446744073709551615
}
19: 1

Google Flights serializes travelers a bit differently than the implemented solution (3 adults):

• Google Flights

8: 1
8: 1
8: 1

• The client library:

8: "\001\001\001"