I need to update the midpoint variable to "chase" the midpoint value if the midpoint gets swapped:

uint partition(Tester &tester, uint start, uint end) {
    uint midpoint = (start + end) >> 1;
    for (;;) {
        while (tester.compare(start, midpoint) < 0)
            ++start;
        while (tester.compare(midpoint, end) < 0)
            --end;
        if (start >= end)
            return end;
        if (midpoint == start)
            midpoint = end;
        else if (midpoint == end)
            midpoint = start;
        tester.swap(start++, end--);
    }
}