mergesort for array of pointers to structs - c

Question

I'm trying to write a generic mergesort for arrays holding pointers to different structs. I'm at a point that the compiler doesn't give any errors, but the sort function doesn't give the right output yet.
The two structures and the two compare functions are defined as follows:
typedef struct Query {int day, rank;} Query;
typedef struct Event {int day, frame; char action;} Event;
int compEvents (const void *a, const void *b) {
return (*(Event**)a)->day - (*(Event**)b)->day;
}
int compQueries (const void *a, const void *b) {
return (*(Query**)a)->day - (*(Query**)b)->day;
}
If I use qsort, everything works fine and the output is correct:
qsort (queries, size, sizeof(Query*), compQueries);
If I use a different mergesort for each type, and don't use the compare functions, everything works fine too:
void queryMerge(Query **arr, int left, int mid, int right) {
Query **temp = newQueryArray(right-left+1);
int i = left, j = mid + 1, t = 0;
while (i <= mid && j <= right) {
if (arr[i]->day < arr[j]->day) temp[t++] = arr[i++];
else temp[t++] = arr[j++];
}
while (i <= mid) temp[t++] = arr[i++];
while (j <= right) temp[t++] = arr[j++];
for (int i = left; i <= right; i++) arr[i] = temp[i-left];
free(temp);
}
void querySort(Query **arr, int left, int right) {
if (left < right) {
int mid = left + (right - left)/2;
querySort(arr, left, mid);
querySort(arr, mid+1, right);
queryMerge(arr, left, mid, right);
}
}
void eventMerge(Event **arr, int left, int mid, int right) {
Event **temp = newEventArray(right-left+1);
int i = left, j = mid + 1, t = 0;
while (i <= mid && j <= right) {
if (arr[i]->day < arr[j]->day) temp[t++] = arr[i++];
else temp[t++] = arr[j++];
}
while (i <= mid) temp[t++] = arr[i++];
while (j <= right) temp[t++] = arr[j++];
for (int i = left; i <= right; i++) arr[i] = temp[i-left];
free(temp);
}
void eventSort(Event **arr, int left, int right) {
if (left < right) {
int mid = left + (right - left)/2;
eventSort(arr, left, mid);
eventSort(arr, mid+1, right);
eventMerge(arr, left, mid, right);
}
}
However, if I try to use a generic mergesort that would work for both types just like qsort, it isn't working correctly, even though the compiler and valgrind have no complaints:
void merge(void *arr, int left, int mid, int right, int width, int (*comp)(const void*, const void*)) {
void *temp = malloc((right - left + 1) * width);
int i = left, j = mid + 1, t = 0;
while (i <= mid && j <= right) {
if (comp((char*)arr + i*width, (char*)arr + j*width)) {
memcpy((char*)temp + t*width, (char*)arr + i*width, width);
i++; t++;
} else {
memcpy((char*)temp + t*width, (char*)arr + j*width, width);
j++; t++;
}
}
while (i <= mid) {
memcpy((char*)temp + t*width, (char*)arr + i*width, width);
i++; t++;
}
while (j <= right) {
memcpy((char*)temp + t*width, (char*)arr + j*width, width);
j++; t++;
}
for (int i = left; i <= right; i++) {
memcpy((char*)arr + i*width, (char*)temp + (i - left)*width, width);
}
free(temp);
}
void mergeSort(void *arr, int left, int right, int width, int (*comp)(const void*, const void*)) {
if (left < right) {
int mid = left + (right - left)/2;
mergeSort(arr, left, mid, width, comp);
mergeSort(arr, mid+1, right, width, comp);
merge(arr, left, mid, right, width, comp);
}
}
This is the way it is called:
mergeSort(queries, 0, size-1, sizeof(Query*), compQueries);
I tried different things to get the void pointers right, but I'm still not sure.
Any ideas?

Answer 1

Your usage of comparison function is broken:
if (comp((char*)arr + i*width, (char*)arr + j*width)))
is the same as
if (comp((char*)arr + i*width, (char*)arr + j*width)) != 0)
while you need
if (comp((char*)arr + i*width, (char*)arr + j*width)) < 0)
With your code you will ignore which was larger but only if they are same. As a result you do not sort at all.
Adding the <0 fixes the sorting.
Note:
Your specific function uses correct comparison result:
if (arr[i]->day < arr[j]->day)