Function CalculateReserveForVector

be/src/gutil/strings/split.cc:347–366

If we know how much to allocate for a vector of strings, we can allocate the vector only once and directly to the right size. This saves in between 33-66 % of memory space needed for the result, and runs faster in the microbenchmarks. The reserve is only implemented for the single character delim. The implementation for counting is cut-and-pasted from SplitStringToIteratorUsing. I could

// and use the existing template function, but probably this is more clear
// and more sure to get optimized to reasonable code.
static int CalculateReserveForVector(const string& full, const char* delim) {
  int count = 0;
  if (delim[0] != '\0' && delim[1] == '\0') {
    // Optimize the common case where delim is a single character.
    char c = delim[0];
    const char* p = full.data();
    const char* end = p + full.size();
    while (p != end) {
      if (*p == c) {  // This could be optimized with hasless(v,1) trick.
        ++p;
      } else {
        while (++p != end && *p != c) {
          // Skip to the next occurence of the delimiter.
        }
        ++count;
      }
    }
  }
  return count;
}

// ----------------------------------------------------------------------
// SplitStringUsing()