Return to Answer

###Bug###

You never reset i back to TABSPACES - 1, so only the first tab will be placed correctly. The rest of the time, you will convert four spaces into three spaces plus a tab.

###Inefficient moving of tail###

Every time you replace four spaces with a tab, you have a loop that moves the tail of the string forward one step at a time, three times total. First of all, you could speed that up 3x by moving the tail three steps in one pass, perhaps with one call to memmove(). Second of all, you could avoid the move altogether if you just moved each character into place like this:

while (mgetline(line, MAXLINESIZE) > 0) {
 char *tail = linep;
 while (*linep) {
 *tail++ = *linep;
 if (*linep++ == ' ') {
 if (++tab == TABSPACES) {
 // Erase spaces already copied to tail.
 tail -= TABSPACES;
 // Replace with tab instead.
 *tail++ = '\t';
 tab = 0;
 }
 } else {
 tab = 0;
 }
 }
 printf("%s", line);
 linep = line;
}

With this solution you can get rid of your kick(), countx(), and swap() functions.

Bug

You never reset i back to TABSPACES - 1, so only the first tab will be placed correctly. The rest of the time, you will convert four spaces into three spaces plus a tab.

Inefficient moving of tail

Every time you replace four spaces with a tab, you have a loop that moves the tail of the string forward one step at a time, three times total. First of all, you could speed that up 3x by moving the tail three steps in one pass, perhaps with one call to memmove(). Second of all, you could avoid the move altogether if you just moved each character into place like this:

while (mgetline(line, MAXLINESIZE) > 0) {
 char *tail = linep;
 while (*linep) {
 *tail++ = *linep;
 if (*linep++ == ' ') {
 if (++tab == TABSPACES) {
 // Erase spaces already copied to tail.
 tail -= TABSPACES;
 // Replace with tab instead.
 *tail++ = '\t';
 tab = 0;
 }
 } else {
 tab = 0;
 }
 }
 printf("%s", line);
 linep = line;
}

With this solution you can get rid of your kick(), countx(), and swap() functions.