rotate

#include <stdio.h>
#include <assert.h>
#include <math.h>
#include <sys/types.h>

size_t fix_offset(ssize_t offset, size_t len) {
    /* Behavior of modulus of negative is, I believe, not standardized by c89,
       so I don't use it...
    */
    int neg = 0;
    if (offset < 0)
        neg = 1;
    offset = abs(offset) % len;
    if(neg) return len - offset;
    return offset;
}

void rotate(int *start, size_t len, ssize_t offset) {
    if (len < 2) return;
    offset = fix_offset(offset, len);
    size_t res_len = len - offset;
    int *scratch = start + res_len;
    int ii = 0;
    for (; ii < res_len; ++ii) {
        int tmp = scratch[ii % offset];
        scratch[ii % offset] = start[ii];
        start[ii] = tmp;
    }
    size_t scratch_offset = offset - (ii % offset);
    /*gcc -O3 does tail call optimization on this if you look at the asm...*/
    rotate(scratch, offset, scratch_offset);
}