bug fix: In bruteforce.h, searchKnn return less element than expected when using filter.

[RaspStudio]

Bug Description

This bug will occur when less than k elements are added into topResults, and the remaining elements all failed to satisfy dist <= topResults.top().first(Notice that topResults.size() < k now).

If there sure is k elements satisfy dist <= topResults.top().first, the function should return them regardless of their internal order.

Example

When calling searchKnn with filter, the following code may cannot fill the topResults to size k.

for (int i = 0; i < k; i++) {
    dist_t dist = fstdistfunc_(query_data, data_ + size_per_element_ * i, dist_func_param_);
    labeltype label = *((labeltype*) (data_ + size_per_element_ * i + data_size_));
    if ((!isIdAllowed) || (*isIdAllowed)(label)) { // <- not every element satisfies
        topResults.emplace(dist, label);
    }
}

If the above loop only fill one nearest element into topResults, the following loop will never satisfy dist <= lastdist, as lastdist is the nearest element. Under this circumstance, the function will only return 1 element regardless of k.

// now topResults.size() == 1, and the only element in the container is the nearest element of all elements.
dist_t lastdist = topResults.empty() ? std::numeric_limits<dist_t>::max() : topResults.top().first;
for (int i = k; i < cur_element_count; i++) {
    dist_t dist = fstdistfunc_(query_data, data_ + size_per_element_ * i, dist_func_param_);
    if (dist <= lastdist) { // <- will never satisfy
        labeltype label = *((labeltype *) (data_ + size_per_element_ * i + data_size_));
        if ((!isIdAllowed) || (*isIdAllowed)(label)) {
            topResults.emplace(dist, label);
        }
        if (topResults.size() > k)
            topResults.pop();
        if (!topResults.empty()) {
            lastdist = topResults.top().first;
        }
    }
}
return topResults;