};

int do_precomposition(int base, int comb) {
  int min = 0;
  int max = sizeof(precompositions) / sizeof(precompositions[0]) - 1;
  int mid;
  unsigned int sought = ((unsigned)base << 16) | (unsigned)comb, that;

  /* binary search */
  while (max >= min) {
    mid = (min + max) / 2;
    that = ((unsigned)precompositions[mid].base << 16) | ((unsigned)precompositions[mid].comb);
    if (that < sought) {
      min = mid + 1;
    } else if (that > sought) {
      max = mid - 1;
    } else {
      return precompositions[mid].replacement;
    }
  }
  /* no match */
  return -1;
}
