/* qsort.c: * **************************************************************** * Copyright (C) 1991, 1992, 1996, 1997, 1999 Free Software Foundation, Inc. * Modifications (C) 2002, Tom Lord * * See the file "COPYING" for further information about * the copyright and warranty status of this work. * * This file is snarfed from the GNU C Library and modified for libhackerlab. * Originally Written by Douglas C. Schmidt (schmidt@ics.uci.edu). * */ #include #include #include #include "hackerlab/sort/qsort.h" /* This implementation incorporates four optimizations discussed in Sedgewick: * * 1. Non-recursive, using an explicit stack of pointer that store the * next array partition to sort. To save time, this maximum amount * of space required to store an array of SIZE_MAX is allocated on the * stack. Assuming a 32-bit (64 bit) integer for size_t, this needs * only 32 * sizeof(stack_node) == 256 bytes (for 64 bit: 1024 bytes). * Pretty cheap, actually. * * 2. Chose the pivot element using a median-of-three decision tree. * This reduces the probability of selecting a bad pivot value and * eliminates certain extraneous comparisons. * * 3. Only quicksorts TOTAL_ELEMS / MAX_THRESH partitions, leaving * insertion sort to order the MAX_THRESH items within each partition. * This is a big win, since insertion sort is faster for small, mostly * sorted array segments. * * 4. The larger of the two sub-partitions is always pushed onto the * stack first, with the algorithm then concentrating on the * smaller partition. This *guarantees* no more than log (total_elems) * stack size is needed (actually O(1) in this case)! */ /* Discontinue quicksort algorithm when partition gets below this size. * This particular magic number was chosen to work best on a Sun 4/260. */ #define MAX_THRESH 4 /* Stack node declarations used to store unfulfilled partition obligations. */ struct stack_node { char *lo; char *hi; }; /* The next 4 #defines implement a very fast in-line stack abstraction. * * The stack needs log (total_elements) entries (we could even subtract * log(MAX_THRESH)). Since total_elements has type size_t, we get as * upper bound for log (total_elements): * bits per byte (CHAR_BIT) * sizeof(size_t) */ #define STACK_SIZE (8 * sizeof(size_t)) #define PUSH(low, high) ((void) ((top->lo = (low)), (top->hi = (high)), ++top)) #define POP(low, high) ((void) (--top, (low = top->lo), (high = top->hi))) #define STACK_NOT_EMPTY (stack < top) /* Byte-wise swap two items of size SIZE. */ #define SWAP(a, b, size) \ do \ { \ size_t __size = (size); \ char * __a; \ char * __b; \ \ __size = (size); \ __a = (a); \ __b = (b); \ \ do \ { \ char __tmp; \ \ __tmp = *__a; \ *__a++ = *__b; \ *__b++ = __tmp; \ } \ while (--__size > 0); \ } \ while (0) void quicksort (void * base, size_t n_elts, size_t sizeof_elt, quicksort_cmp cmp, void * closure) { char * base_ptr; size_t max_thresh; if (n_elts == 0) return; base_ptr = (char *)base; max_thresh = MAX_THRESH * sizeof_elt; if (n_elts > MAX_THRESH) { struct stack_node stack[STACK_SIZE]; struct stack_node *top; char *lo; char *hi; top = stack + 1; lo = base_ptr; hi = &lo[sizeof_elt * (n_elts - 1)]; while (STACK_NOT_EMPTY) { char *left_ptr; char *right_ptr; char *mid; /* Select median value from among LO, MID, and HI. Rearrange * LO and HI so the three values are sorted. This lowers the * probability of picking a pathological pivot value and * skips a comparison for both the LEFT_PTR and RIGHT_PTR in * the while loops. */ mid = lo + sizeof_elt * ((hi - lo) / sizeof_elt >> 1); if ((*cmp) ((void *) mid, (void *) lo, closure) < 0) { SWAP (mid, lo, sizeof_elt); } if ((*cmp) ((void *) hi, (void *) mid, closure) < 0) { SWAP (mid, hi, sizeof_elt); } else goto jump_over; if ((*cmp) ((void *) mid, (void *) lo, closure) < 0) SWAP (mid, lo, sizeof_elt); jump_over: left_ptr = lo + sizeof_elt; right_ptr = hi - sizeof_elt; /* Here's the famous ``collapse the walls'' section of quicksort. * Gotta like those tight inner loops! They are the main reason * that this algorithm runs much faster than others. */ do { while ((*cmp) ((void *) left_ptr, (void *) mid, closure) < 0) left_ptr += sizeof_elt; while ((*cmp) ((void *) mid, (void *) right_ptr, closure) < 0) right_ptr -= sizeof_elt; if (left_ptr < right_ptr) { SWAP (left_ptr, right_ptr, sizeof_elt); if (mid == left_ptr) mid = right_ptr; else if (mid == right_ptr) mid = left_ptr; left_ptr += sizeof_elt; right_ptr -= sizeof_elt; } else if (left_ptr == right_ptr) { left_ptr += sizeof_elt; right_ptr -= sizeof_elt; break; } } while (left_ptr <= right_ptr); /* Set up pointers for next iteration. First determine whether * left and right partitions are below the threshold size. If so, * ignore one or both. Otherwise, push the larger partition's * bounds on the stack and continue sorting the smaller one. */ if ((size_t) (right_ptr - lo) <= max_thresh) { if ((size_t) (hi - left_ptr) <= max_thresh) { /* Ignore both small partitions. */ POP (lo, hi); } else { /* Ignore small left partition. */ lo = left_ptr; } } else if ((size_t) (hi - left_ptr) <= max_thresh) { /* Ignore small right partition. */ hi = right_ptr; } else if ((right_ptr - lo) > (hi - left_ptr)) { /* Push larger left partition indices. */ PUSH (lo, right_ptr); lo = left_ptr; } else { /* Push larger right partition indices. */ PUSH (left_ptr, hi); hi = right_ptr; } } } /* Once the BASE_PTR array is partially sorted by quicksort the rest * is completely sorted using insertion sort, since this is efficient * for partitions below MAX_THRESH size. BASE_PTR points to the beginning * of the array to sort, and END_PTR points at the very last element in * the array (*not* one beyond it!). */ #define min(x, y) ((x) < (y) ? (x) : (y)) { char * end_ptr; char * tmp_ptr; char * thresh; char * run_ptr; end_ptr = &base_ptr[sizeof_elt * (n_elts - 1)]; tmp_ptr = base_ptr; thresh = min(end_ptr, base_ptr + max_thresh); /* Find smallest element in first threshold and place it at the * array's beginning. This is the smallest array element, * and the operation speeds up insertion sort's inner loop. */ for (run_ptr = tmp_ptr + sizeof_elt; run_ptr <= thresh; run_ptr += sizeof_elt) if ((*cmp) ((void *) run_ptr, (void *) tmp_ptr, closure) < 0) tmp_ptr = run_ptr; if (tmp_ptr != base_ptr) { SWAP (tmp_ptr, base_ptr, sizeof_elt); } /* Insertion sort, running from left-hand-side up to right-hand-side. */ run_ptr = base_ptr + sizeof_elt; while ((run_ptr += sizeof_elt) <= end_ptr) { tmp_ptr = run_ptr - sizeof_elt; while ((*cmp) ((void *) run_ptr, (void *) tmp_ptr, closure) < 0) tmp_ptr -= sizeof_elt; tmp_ptr += sizeof_elt; if (tmp_ptr != run_ptr) { char *trav; trav = run_ptr + sizeof_elt; while (--trav >= run_ptr) { char c; char * hi; char * lo; c = *trav; for (hi = lo = trav; (lo -= sizeof_elt) >= tmp_ptr; hi = lo) *hi = *lo; *hi = c; } } } } } /* tag: Tom Lord Fri Feb 22 06:34:02 2002 (qsort.c) */