- SV * temp;
- struct partition_stack_entry partition_stack[QSORT_MAX_STACK];
- int next_stack_entry = 0;
- int part_left;
- int part_right;
-#ifdef QSORT_ORDER_GUESS
- int qsort_break_even;
- int swapped;
-#endif
-
- PERL_ARGS_ASSERT_QSORTSVU;
-
- /* Make sure we actually have work to do.
- */
- if (num_elts <= 1) {
- return;
- }
-
- /* Inoculate large partitions against quadratic behavior */
- if (num_elts > QSORT_PLAY_SAFE) {
- size_t n;
- SV ** const q = array;
- for (n = num_elts; n > 1; ) {
- const size_t j = (size_t)(n-- * Drand01());
- temp = q[j];
- q[j] = q[n];
- q[n] = temp;
- }
- }
-
- /* Setup the initial partition definition and fall into the sorting loop
- */
- part_left = 0;
- part_right = (int)(num_elts - 1);
-#ifdef QSORT_ORDER_GUESS
- qsort_break_even = QSORT_BREAK_EVEN;
-#else
-#define qsort_break_even QSORT_BREAK_EVEN
-#endif
- for ( ; ; ) {
- if ((part_right - part_left) >= qsort_break_even) {
- /* OK, this is gonna get hairy, so lets try to document all the
- concepts and abbreviations and variables and what they keep
- track of:
-
- pc: pivot chunk - the set of array elements we accumulate in the
- middle of the partition, all equal in value to the original
- pivot element selected. The pc is defined by:
-
- pc_left - the leftmost array index of the pc
- pc_right - the rightmost array index of the pc
-
- we start with pc_left == pc_right and only one element
- in the pivot chunk (but it can grow during the scan).
-
- u: uncompared elements - the set of elements in the partition
- we have not yet compared to the pivot value. There are two
- uncompared sets during the scan - one to the left of the pc
- and one to the right.
-
- u_right - the rightmost index of the left side's uncompared set
- u_left - the leftmost index of the right side's uncompared set
-
- The leftmost index of the left sides's uncompared set
- doesn't need its own variable because it is always defined
- by the leftmost edge of the whole partition (part_left). The
- same goes for the rightmost edge of the right partition
- (part_right).
-
- We know there are no uncompared elements on the left once we
- get u_right < part_left and no uncompared elements on the
- right once u_left > part_right. When both these conditions
- are met, we have completed the scan of the partition.
-
- Any elements which are between the pivot chunk and the
- uncompared elements should be less than the pivot value on
- the left side and greater than the pivot value on the right
- side (in fact, the goal of the whole algorithm is to arrange
- for that to be true and make the groups of less-than and
- greater-then elements into new partitions to sort again).
-
- As you marvel at the complexity of the code and wonder why it
- has to be so confusing. Consider some of the things this level
- of confusion brings:
-
- Once I do a compare, I squeeze every ounce of juice out of it. I
- never do compare calls I don't have to do, and I certainly never
- do redundant calls.
-
- I also never swap any elements unless I can prove there is a
- good reason. Many sort algorithms will swap a known value with
- an uncompared value just to get things in the right place (or
- avoid complexity :-), but that uncompared value, once it gets
- compared, may then have to be swapped again. A lot of the
- complexity of this code is due to the fact that it never swaps
- anything except compared values, and it only swaps them when the
- compare shows they are out of position.
- */
- int pc_left, pc_right;
- int u_right, u_left;
-
- int s;
-
- pc_left = ((part_left + part_right) / 2);
- pc_right = pc_left;
- u_right = pc_left - 1;
- u_left = pc_right + 1;
-
- /* Qsort works best when the pivot value is also the median value
- in the partition (unfortunately you can't find the median value
- without first sorting :-), so to give the algorithm a helping
- hand, we pick 3 elements and sort them and use the median value
- of that tiny set as the pivot value.
-
- Some versions of qsort like to use the left middle and right as
- the 3 elements to sort so they can insure the ends of the
- partition will contain values which will stop the scan in the
- compare loop, but when you have to call an arbitrarily complex
- routine to do a compare, its really better to just keep track of
- array index values to know when you hit the edge of the
- partition and avoid the extra compare. An even better reason to
- avoid using a compare call is the fact that you can drop off the
- edge of the array if someone foolishly provides you with an
- unstable compare function that doesn't always provide consistent
- results.
-
- So, since it is simpler for us to compare the three adjacent
- elements in the middle of the partition, those are the ones we
- pick here (conveniently pointed at by u_right, pc_left, and
- u_left). The values of the left, center, and right elements
- are refered to as l c and r in the following comments.
- */
-
-#ifdef QSORT_ORDER_GUESS
- swapped = 0;
-#endif
- s = qsort_cmp(u_right, pc_left);
- if (s < 0) {
- /* l < c */
- s = qsort_cmp(pc_left, u_left);
- /* if l < c, c < r - already in order - nothing to do */
- if (s == 0) {
- /* l < c, c == r - already in order, pc grows */
- ++pc_right;
- qsort_all_asserts(pc_left, pc_right, u_left + 1, u_right - 1);
- } else if (s > 0) {
- /* l < c, c > r - need to know more */
- s = qsort_cmp(u_right, u_left);
- if (s < 0) {
- /* l < c, c > r, l < r - swap c & r to get ordered */
- qsort_swap(pc_left, u_left);
- qsort_all_asserts(pc_left, pc_right, u_left + 1, u_right - 1);
- } else if (s == 0) {
- /* l < c, c > r, l == r - swap c&r, grow pc */
- qsort_swap(pc_left, u_left);
- --pc_left;
- qsort_all_asserts(pc_left, pc_right, u_left + 1, u_right - 1);
- } else {
- /* l < c, c > r, l > r - make lcr into rlc to get ordered */
- qsort_rotate(pc_left, u_right, u_left);
- qsort_all_asserts(pc_left, pc_right, u_left + 1, u_right - 1);
- }
- }
- } else if (s == 0) {
- /* l == c */
- s = qsort_cmp(pc_left, u_left);
- if (s < 0) {
- /* l == c, c < r - already in order, grow pc */
- --pc_left;
- qsort_all_asserts(pc_left, pc_right, u_left + 1, u_right - 1);
- } else if (s == 0) {
- /* l == c, c == r - already in order, grow pc both ways */
- --pc_left;
- ++pc_right;
- qsort_all_asserts(pc_left, pc_right, u_left + 1, u_right - 1);
- } else {
- /* l == c, c > r - swap l & r, grow pc */
- qsort_swap(u_right, u_left);
- ++pc_right;
- qsort_all_asserts(pc_left, pc_right, u_left + 1, u_right - 1);
- }
- } else {
- /* l > c */
- s = qsort_cmp(pc_left, u_left);
- if (s < 0) {
- /* l > c, c < r - need to know more */
- s = qsort_cmp(u_right, u_left);
- if (s < 0) {
- /* l > c, c < r, l < r - swap l & c to get ordered */
- qsort_swap(u_right, pc_left);
- qsort_all_asserts(pc_left, pc_right, u_left + 1, u_right - 1);
- } else if (s == 0) {
- /* l > c, c < r, l == r - swap l & c, grow pc */
- qsort_swap(u_right, pc_left);
- ++pc_right;
- qsort_all_asserts(pc_left, pc_right, u_left + 1, u_right - 1);
- } else {
- /* l > c, c < r, l > r - rotate lcr into crl to order */
- qsort_rotate(u_right, pc_left, u_left);
- qsort_all_asserts(pc_left, pc_right, u_left + 1, u_right - 1);
- }
- } else if (s == 0) {
- /* l > c, c == r - swap ends, grow pc */
- qsort_swap(u_right, u_left);
- --pc_left;
- qsort_all_asserts(pc_left, pc_right, u_left + 1, u_right - 1);
- } else {
- /* l > c, c > r - swap ends to get in order */
- qsort_swap(u_right, u_left);
- qsort_all_asserts(pc_left, pc_right, u_left + 1, u_right - 1);
- }
- }
- /* We now know the 3 middle elements have been compared and
- arranged in the desired order, so we can shrink the uncompared
- sets on both sides
- */
- --u_right;
- ++u_left;
- qsort_all_asserts(pc_left, pc_right, u_left, u_right);
-
- /* The above massive nested if was the simple part :-). We now have
- the middle 3 elements ordered and we need to scan through the
- uncompared sets on either side, swapping elements that are on
- the wrong side or simply shuffling equal elements around to get
- all equal elements into the pivot chunk.
- */
-
- for ( ; ; ) {
- int still_work_on_left;
- int still_work_on_right;
https://perl5.git.perl.org / perl5.git / blobdiff