void compute_deltas(pmt_t* ppi, int x, int ix, int iy, int y, int* deltas)
{
    int i;
    int in_same_cycle;
    point_t* px   = &ppi->points[x];
    point_t* pix  = &ppi->points[ix];
    point_t* piiy = &ppi->points[iy];
    point_t* py   = &ppi->points[y];

    for (i = 1; i <= ppi->N; i++)
        deltas[i] = 0;

    // In the same cycle, but neighbors.
    if (pix == py) { // No change.
        printf("-- delta case 1.\n");
    }

    // In different cycles, but one of x and iy is in a one-cycle.
    else if (px == pix) {
        int ycyclen = get_cycle_length(ppi, y);
        printf("-- delta case 2.\n");
        deltas[1]--; deltas[ycyclen]--; deltas[ycyclen+1]++;
    }

    else if (piiy == py) {
        int xcyclen = get_cycle_length(ppi, x);
        printf("-- delta case 3.\n");
        deltas[1]--; deltas[xcyclen]--; deltas[xcyclen+1]++;
    }

    // In the same cycle, but neighbors.
    else if (pix == piiy) {
        int xcyclen = get_cycle_length(ppi, x);
        printf("-- delta case 4.\n");
        deltas[xcyclen]--; deltas[xcyclen-1]++; deltas[1]++;
    }
    else if (px == py) {
        int ycyclen = get_cycle_length(ppi, y);
        printf("-- delta case 5.\n");
        deltas[ycyclen]--; deltas[ycyclen-1]++; deltas[1]++;
    }

    else {
        int xcyclen, ycyclen, dxy, dyx;
        in_same_cycle = are_in_same_cycle(ppi, x, y,
            &xcyclen, &ycyclen, &dxy, &dyx);
        if (in_same_cycle) {
            // It splits into separate cycles.
            printf("-- delta case 6.\n");
            printf("-- same:  d(%d,%d)=%d d(%d,%d)=%d\n", x,y, dxy, y,x,dyx);
            deltas[dxy+dyx]--; deltas[dxy-1]++; deltas[dyx+1]++;
        }
        else {
            // They merge into a common cycle.
            printf("-- delta case 7.\n");
            printf("-- diff:  len(%d)=%d len(%d)=%d\n", x,xcyclen, y,ycyclen);
            deltas[xcyclen]--; deltas[ycyclen]--; deltas[xcyclen+ycyclen]++;
        }
    }
}