docs/HLL/HllCppExample.md

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# Hyper Log Log Sketch C++ Example

    #include <iostream>
    #include <fstream>
    
    #include <hll.hpp>
    
    //simplified file operations and no error handling for clarity
    int main(int argc, char **argv) {
      const int lg_k = 11;
      const auto type = datasketches::HLL_4; // this is the default, but explicit here for illustration
    
      // this section generates two sketches with some overlap and serializes them into files
      {
        // 100000 distinct keys
        datasketches::hll_sketch sketch1(lg_k, type); // type is optional, defaults to HLL_4
        for (int key = 0; key < 100000; key++) sketch1.update(key);
        std::ofstream os1("hll_sketch1.bin");
        sketch1.serialize_compact(os1);
    
        // 100000 distinct keys
        datasketches::hll_sketch sketch2(lg_k, type); // type is optional, defaults to HLL_4
        for (int key = 50000; key < 150000; key++) sketch2.update(key);
        std::ofstream os2("hll_sketch2.bin");
        sketch2.serialize_compact(os2);
      }
    
      // this section deserializes the sketches, produces union and prints the result
      {
        std::ifstream is1("hll_sketch1.bin");
        auto sketch1 = datasketches::hll_sketch::deserialize(is1);
    
        std::ifstream is2("hll_sketch2.bin");
        auto sketch2 = datasketches::hll_sketch::deserialize(is2);
    
        datasketches::hll_union u(lg_k);
        u.update(sketch1);
        u.update(sketch2);
        auto sketch = u.get_result(type); // type is optional, defaults to HLL_4
    
        // debug summary of the union result sketch
        std::cout << sketch.to_string();
    
        std::cout << "Distinct count estimate: " << sketch.get_estimate() << std::endl;
        std::cout << "Distinct count lower bound 95% confidence: " << sketch.get_lower_bound(2) << std::endl;
        std::cout << "Distinct count upper bound 95% confidence: " << sketch.get_upper_bound(2) << std::endl;
      }
    
      return 0;
    }

    ### HLL SKETCH SUMMARY: 
      Log Config K   : 11
      Hll Target     : HLL_4
      Current Mode   : HLL
      LB             : 148634
      Estimate       : 152041
      UB             : 155614
      OutOfOrder flag: true
      CurMin         : 4
      NumAtCurMin    : 21
      HipAccum       : 147291
      KxQ0           : 19.889
      KxQ1           : 0
    Distinct count estimate: 152041
    Distinct count lower bound 95% confidence: 145234
    Distinct count upper bound 95% confidence: 159184