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added a comment - - edited I just looked at this. Didn't find the problem, but did make some comments in the code and clarified it a bit. My guess is that the main partition loop leaves b and c in the wrong order. If i get another chance to look at this, I will put in some assertions that match what I think the loop invariants should be. That will probably make things obvious.
Here is my commented version fo the key sort routine:
private static void quickSort0(int start, int end, IntComparator comp, Swapper swap) {
int length = end - start;
if (length < 7) {
insertionSort(start, end, comp, swap);
return;
}
int middle = (start + end) / 2;
if (length > 7) {
int bottom = start;
int top = end - 1;
if (length > 40) {
// for lots of data, bottom, middle and top are medians near the beginning, middle or end of the data
int skosh = length / 8;
bottom = med3(bottom, bottom + skosh, bottom + (2 * skosh), comp);
middle = med3(middle - skosh, middle, middle + skosh, comp);
top = med3(top - (2 * skosh), top - skosh, top, comp);
}
middle = med3(bottom, middle, top, comp);
}
int partitionIndex = middle; // an index, not a value.
// regions from a to b and from c to d are what we will recursively sort
int a, b, c, d;
a = b = start;
c = d = end - 1;
while (b <= c) {
// copy all values equal to the partition value to before a..b. In the process, advance b
// as long as values less than the partition or equal are found, also stop when a..b collides with c..d
int comparison = comp.compare(b, partitionIndex);
while (b <= c && comparison <= 0) {
if (comparison == 0) {
swap.swap(a, b);
a++;
}
b++;
comparison = comp.compare(b, partitionIndex);
}
// at this point [start..a) has partition values, [a..b) has values < partition
// also, either b>c or v[b] > partition value
comparison = comp.compare(c, partitionIndex);
while (c >= b && comparison >= 0) {
if (comparison == 0) {
swap.swap(c, d);
d--;
}
c--;
comparison = comp.compare(c, partitionIndex);
}
// now we also know that [d..end] contains partition values,
// [c..d) contains values > partition value
// also, either b>c or (v[b] > partition OR v[c] < partition)
if (b <= c) {
// v[b] > partition OR v[c] < partition
// swapping will let us continue to grow the two regions
swap.swap(b, c);
b++;
c--;
}
}
// now we know
// b = c+1
// [start..a) and [d..end) contain partition value
// all of [a..b) are less than partition
// all of [c..d) are greater than partition
// shift [a..b) to beginning
length = Math.min(a - start, b - a);
int l = start;
int h = b - length;
while (length-- > 0) {
swap.swap(l, h);
l++;
h++;
}
// shift [c..d) to end
length = Math.min(d - c, end - 1 - d);
l = b;
h = end - length;
while (length-- > 0) {
swap.swap(l, h);
l++;
h++;
}
// recurse left and right
length = b - a;
if (length > 0) {
quickSort0(start, start + length, comp, swap);
}
length = d - c;
if (length > 0) {
quickSort0(end - length, end, comp, swap);
}
}
/**
* In-place insertion sort that is fast for pre-sorted data.
*
* @param start Where to start sorting (inclusive)
* @param end Where to stop (exclusive)
* @param comp Sort order.
* @param swap How to swap items.
*/
private static void insertionSort(int start, int end, IntComparator comp, Swapper swap) {
for (int i = start + 1; i < end; i++) {
for (int j = i; j > start && comp.compare(j - 1, j) > 0; j--) {
swap.swap(j - 1, j);
}
}
}

This doesn't work, and I'm a bit stumped.