expected

the
the quick
the quick brown
the quick brown fox
quick
quick brown
quick brown fox
quick brown fox jumped
brown
brown fox
brown fox jumped
brown fox jumped over
fox
fox jumped
fox jumped over
fox jumped over the
jumped
jumped over
jumped over the
jumped over the lazy
over
over the
over the lazy
over the lazy dog
the lazy dog
the lazy
the
lazy dog
lazy
dog

indexer.cc

#include <iostream>                                                             
#include <string>                                                               
#include <vector>                                                               

std::vector<std::string_view> create_index(std::string_view source, int max) {  
  std::vector<std::string_view> idx;                                            

  const char *this_word_start = source.begin();                                 
  const char *next_word_start;                                                  
  int cur_phrase_len = 1;                                                       

  for (auto it = source.begin(); it != source.end(); it++) {                    
    if (*it != ' ') {                                                           
      continue;
    }

    if (cur_phrase_len == 1) {
      next_word_start = it + 1;
    }

    idx.push_back(
        std::string_view(this_word_start, std::distance(this_word_start, it)));

    if (cur_phrase_len++ == max) {
      it = next_word_start;
      this_word_start = next_word_start;
      cur_phrase_len = 1;
    }
  }

  return idx;
}

int main() {
  auto idx = create_index("the quick brown fox jumped over the lazy dog", 4);
  for (auto &str : idx) {
    std::cout << str << "\n";
  }
}

stdout

the
the quick
the quick brown
the quick brown fox
quick
quick brown
quick brown fox
quick brown fox jumped
brown
brown fox
brown fox jumped
brown fox jumped over
fox
fox jumped
fox jumped over
fox jumped over the
jumped
jumped over
jumped over the
jumped over the lazy
over
over the
over the lazy