| #include "bm25.h" |
| #include "tokenize.h" |
| #include <algorithm> |
| #include <cmath> |
|
|
| int32_t BM25::add(const std::string& text) { |
| const int32_t id = n_docs_++; |
| auto toks = tokenize(text); |
| doc_len_.push_back(static_cast<int32_t>(toks.size())); |
| total_len_ += toks.size(); |
|
|
| |
| std::unordered_map<std::string,int32_t> tf; |
| tf.reserve(toks.size()); |
| for (auto& t : toks) tf[t]++; |
| for (auto& [term, f] : tf) postings_[term].emplace_back(id, f); |
| finalized_ = false; |
| return id; |
| } |
|
|
| void BM25::finalize() { |
| avgdl_ = n_docs_ ? static_cast<float>(total_len_ / n_docs_) : 0.0f; |
| idf_.clear(); |
| idf_.reserve(postings_.size()); |
| for (auto& [term, plist] : postings_) { |
| const double nq = static_cast<double>(plist.size()); |
| |
| idf_[term] = static_cast<float>( |
| std::log((n_docs_ - nq + 0.5) / (nq + 0.5) + 1.0)); |
| } |
| finalized_ = true; |
| } |
|
|
| std::vector<ScoredDoc> BM25::search(const std::string& query, int k) const { |
| std::vector<ScoredDoc> out; |
| if (!finalized_ || n_docs_ == 0) return out; |
|
|
| std::vector<float> acc(n_docs_, 0.0f); |
| for (auto& term : tokenize(query)) { |
| auto it = postings_.find(term); |
| if (it == postings_.end()) continue; |
| const float idf = idf_.at(term); |
| for (auto& [doc, f] : it->second) { |
| const float denom = f + k1_ * (1.0f - b_ + b_ * doc_len_[doc] / avgdl_); |
| acc[doc] += idf * (f * (k1_ + 1.0f)) / denom; |
| } |
| } |
| out.reserve(n_docs_); |
| for (int32_t i = 0; i < n_docs_; i++) |
| if (acc[i] > 0.0f) out.push_back({i, acc[i]}); |
|
|
| const int kk = std::min<int>(k, static_cast<int>(out.size())); |
| std::partial_sort(out.begin(), out.begin() + kk, out.end(), |
| [](const ScoredDoc& a, const ScoredDoc& b){ return a.score > b.score; }); |
| out.resize(kk); |
| return out; |
| } |
|
|
