A snippet of a merge function to a 3-way merge sort in C. The idea was to optimize the code size and the amount of whiles and ifs.

merge3.c

typedef struct {
  int *data;
  int size;
  int current;
} mergeArray;

#define COPY_MERGE_ARRAY(dest, source) dest.data[dest.current++] = source.data[source.current++]

int copyItemToFinalArray(mergeArray array, mergeArray final) {
  COPY_MERGE_ARRAY(final, array);
  return array.current == array.size ? 1 : 0;
}

int finishArrayToFinalArray(mergeArray array, mergeArray final) {
  for (int i = array.current; i < array.size; i++) {
    COPY_MERGE_ARRAY(final, array);
  }
}

int* merge(int *allData, int start, int leftSize, int midSize, int endSize) {
  /* cria mergeArrays, current começa em 0, data tem já o endereço do primeiro elemento de cada parte da Arraya */
  mergeArray startArray, midArray, endArray;

  /* cria mergeArray pra ter o array final, size no tamanho certo, current começa em 0, ponteiro na primeira posição */
  mergeArray finalArray;

  char startFinished, midFinished, endFinished;
  startFinished = midFinished = endFinished = 0;

  while (startFinished + midFinished + endFinished < 2) {
    int startItem = startArray.data[startArray.current];
    int midItem = midArray.data[midArray.current];
    int endItem = endArray.data[endArray.current];

    if (!startFinished && startItem > midItem && startItem > endItem ){
      startFinished = copyItemToFinalArray(startArray, finalArray);
    } else if (!midFinished && midItem > startItem && midItem > endItem ) {
      midFinished = copyItemToFinalArray(midArray, finalArray);
    } else {
      endFinished = copyItemToFinalArray(endArray, finalArray);
    }
  }

  if (!startFinished) finishArrayToFinalArray(startArray, finalArray);
  if (!midFinished) finishArrayToFinalArray(midArray, finalArray);
  if (!endFinished) finishArrayToFinalArray(endArray, finalArray);

  return finalArray.data;
}