syntax = "proto3";

// An index stored in CIFF is a single file comprised of exactly the following:
//  - A Header protobuf message,
//  - Exactly the number of PostingsList messages specified in the num_postings_lists field of the Header
//  - Exactly the number of DocRecord messages specified in the num_doc_records field of the Header
// The protobuf messages are defined below.

// This is the CIFF header. It always comes first.
message Header {
  int32 version = 1;              // Version.

  int32 num_postings_lists = 2;   // Exactly the number of PostingsList messages that follow the Header.
  int32 num_docs = 3;             // Exactly the number of DocRecord messages that follow the PostingsList messages.

  // The total number of postings lists in the collection; the vocabulary size. This might differ from
  // num_postings_lists, for example, because we only export the postings lists of query terms.
  int32 total_postings_lists = 4;

  // The total number of documents in the collection; might differ from num_doc_records for a similar reason as above.
  int32 total_docs = 5;

  // The total number of terms in the entire collection. This is the sum of all document lengths of all documents in
  // the collection.
  int64 total_terms_in_collection = 6;

  // The average document length. We store this value explicitly in case the exporting application wants a particular
  // level of precision.
  double average_doclength = 7;

  // Description of this index, meant for human consumption. Describing, for example, the exporting application,
  // document processing and tokenization pipeline, etc.
  string description = 8;
}

// An individual posting.
message Posting {
  int32 docid = 1;
  int32 tf = 2;
}

// A postings list, comprised of one ore more postings.
message PostingsList {
  string term = 1;   // The term.
  int64 df = 2;      // The document frequency.
  int64 cf = 3;      // The collection frequency.
  repeated Posting postings = 4;
}

// A record containing metadata about an individual document.
message DocRecord {
  int32 docid = 1;               // Refers to the docid in the postings lists.
  string collection_docid = 2;   // Refers to a docid in the external collection.
  int32 doclength = 3;           // Length of this document.
}