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kc3-lang/SDL/src/stdlib/SDL_qsort.c
Gabriel Jacobo
- src/stdlib/SDL_qsort.c
-
/* qsort.c * (c) 1998 Gareth McCaughan * * This is a drop-in replacement for the C library's |qsort()| routine. * * Features: * - Median-of-three pivoting (and more) * - Truncation and final polishing by a single insertion sort * - Early truncation when no swaps needed in pivoting step * - Explicit recursion, guaranteed not to overflow * - A few little wrinkles stolen from the GNU |qsort()|. * - separate code for non-aligned / aligned / word-size objects * * This code may be reproduced freely provided * - this file is retained unaltered apart from minor * changes for portability and efficiency * - no changes are made to this comment * - any changes that *are* made are clearly flagged * - the _ID string below is altered by inserting, after * the date, the string " altered" followed at your option * by other material. (Exceptions: you may change the name * of the exported routine without changing the ID string. * You may change the values of the macros TRUNC_* and * PIVOT_THRESHOLD without changing the ID string, provided * they remain constants with TRUNC_nonaligned, TRUNC_aligned * and TRUNC_words/WORD_BYTES between 8 and 24, and * PIVOT_THRESHOLD between 32 and 200.) * * You may use it in anything you like; you may make money * out of it; you may distribute it in object form or as * part of an executable without including source code; * you don't have to credit me. (But it would be nice if * you did.) * * If you find problems with this code, or find ways of * making it significantly faster, please let me know! * My e-mail address, valid as of early 1998 and certainly * OK for at least the next 18 months, is * gjm11@dpmms.cam.ac.uk * Thanks! * * Gareth McCaughan Peterhouse Cambridge 1998 */ #include "../SDL_internal.h" /* #include <assert.h> #include <stdlib.h> #include <string.h> */ #include "SDL_stdinc.h" #include "SDL_assert.h" #if defined(HAVE_QSORT) void SDL_qsort(void *base, size_t nmemb, size_t size, int (*compare) (const void *, const void *)) { qsort(base, nmemb, size, compare); } #else #ifdef assert #undef assert #endif #define assert(X) SDL_assert(X) #ifdef malloc #undef malloc #endif #define malloc SDL_malloc #ifdef free #undef free #endif #define free SDL_free #ifdef memcpy #undef memcpy #endif #define memcpy SDL_memcpy #ifdef memmove #undef memmove #endif #define memmove SDL_memmove #ifdef qsort #undef qsort #endif #define qsort SDL_qsort static const char _ID[] = "<qsort.c gjm 1.12 1998-03-19>"; /* How many bytes are there per word? (Must be a power of 2, * and must in fact equal sizeof(int).) */ #define WORD_BYTES sizeof(int) /* How big does our stack need to be? Answer: one entry per * bit in a |size_t|. */ #define STACK_SIZE (8*sizeof(size_t)) /* Different situations have slightly different requirements, * and we make life epsilon easier by using different truncation * points for the three different cases. * So far, I have tuned TRUNC_words and guessed that the same * value might work well for the other two cases. Of course * what works well on my machine might work badly on yours. */ #define TRUNC_nonaligned 12 #define TRUNC_aligned 12 #define TRUNC_words 12*WORD_BYTES /* nb different meaning */ /* We use a simple pivoting algorithm for shortish sub-arrays * and a more complicated one for larger ones. The threshold * is PIVOT_THRESHOLD. */ #define PIVOT_THRESHOLD 40 typedef struct { char *first; char *last; } stack_entry; #define pushLeft {stack[stacktop].first=ffirst;stack[stacktop++].last=last;} #define pushRight {stack[stacktop].first=first;stack[stacktop++].last=llast;} #define doLeft {first=ffirst;llast=last;continue;} #define doRight {ffirst=first;last=llast;continue;} #define pop {if (--stacktop<0) break;\ first=ffirst=stack[stacktop].first;\ last=llast=stack[stacktop].last;\ continue;} /* Some comments on the implementation. * 1. When we finish partitioning the array into "low" * and "high", we forget entirely about short subarrays, * because they'll be done later by insertion sort. * Doing lots of little insertion sorts might be a win * on large datasets for locality-of-reference reasons, * but it makes the code much nastier and increases * bookkeeping overhead. * 2. We always save the shorter and get to work on the * longer. This guarantees that every time we push * an item onto the stack its size is <= 1/2 of that * of its parent; so the stack can't need more than * log_2(max-array-size) entries. * 3. We choose a pivot by looking at the first, last * and middle elements. We arrange them into order * because it's easy to do that in conjunction with * choosing the pivot, and it makes things a little * easier in the partitioning step. Anyway, the pivot * is the middle of these three. It's still possible * to construct datasets where the algorithm takes * time of order n^2, but it simply never happens in * practice. * 3' Newsflash: On further investigation I find that * it's easy to construct datasets where median-of-3 * simply isn't good enough. So on large-ish subarrays * we do a more sophisticated pivoting: we take three * sets of 3 elements, find their medians, and then * take the median of those. * 4. We copy the pivot element to a separate place * because that way we can always do our comparisons * directly against a pointer to that separate place, * and don't have to wonder "did we move the pivot * element?". This makes the inner loop better. * 5. It's possible to make the pivoting even more * reliable by looking at more candidates when n * is larger. (Taking this to its logical conclusion * results in a variant of quicksort that doesn't * have that n^2 worst case.) However, the overhead * from the extra bookkeeping means that it's just * not worth while. * 6. This is pretty clean and portable code. Here are * all the potential portability pitfalls and problems * I know of: * - In one place (the insertion sort) I construct * a pointer that points just past the end of the * supplied array, and assume that (a) it won't * compare equal to any pointer within the array, * and (b) it will compare equal to a pointer * obtained by stepping off the end of the array. * These might fail on some segmented architectures. * - I assume that there are 8 bits in a |char| when * computing the size of stack needed. This would * fail on machines with 9-bit or 16-bit bytes. * - I assume that if |((int)base&(sizeof(int)-1))==0| * and |(size&(sizeof(int)-1))==0| then it's safe to * get at array elements via |int*|s, and that if * actually |size==sizeof(int)| as well then it's * safe to treat the elements as |int|s. This might * fail on systems that convert pointers to integers * in non-standard ways. * - I assume that |8*sizeof(size_t)<=INT_MAX|. This * would be false on a machine with 8-bit |char|s, * 16-bit |int|s and 4096-bit |size_t|s. :-) */ /* The recursion logic is the same in each case: */ #define Recurse(Trunc) \ { size_t l=last-ffirst,r=llast-first; \ if (l<Trunc) { \ if (r>=Trunc) doRight \ else pop \ } \ else if (l<=r) { pushLeft; doRight } \ else if (r>=Trunc) { pushRight; doLeft }\ else doLeft \ } /* and so is the pivoting logic: */ #define Pivot(swapper,sz) \ if ((size_t)(last-first)>PIVOT_THRESHOLD*sz) mid=pivot_big(first,mid,last,sz,compare);\ else { \ if (compare(first,mid)<0) { \ if (compare(mid,last)>0) { \ swapper(mid,last); \ if (compare(first,mid)>0) swapper(first,mid);\ } \ } \ else { \ if (compare(mid,last)>0) swapper(first,last)\ else { \ swapper(first,mid); \ if (compare(mid,last)>0) swapper(mid,last);\ } \ } \ first+=sz; last-=sz; \ } #ifdef DEBUG_QSORT #include <stdio.h> #endif /* and so is the partitioning logic: */ #define Partition(swapper,sz) { \ int swapped=0; \ do { \ while (compare(first,pivot)<0) first+=sz; \ while (compare(pivot,last)<0) last-=sz; \ if (first<last) { \ swapper(first,last); swapped=1; \ first+=sz; last-=sz; } \ else if (first==last) { first+=sz; last-=sz; break; }\ } while (first<=last); \ if (!swapped) pop \ } /* and so is the pre-insertion-sort operation of putting * the smallest element into place as a sentinel. * Doing this makes the inner loop nicer. I got this * idea from the GNU implementation of qsort(). */ #define PreInsertion(swapper,limit,sz) \ first=base; \ last=first + (nmemb>limit ? limit : nmemb-1)*sz;\ while (last!=base) { \ if (compare(first,last)>0) first=last; \ last-=sz; } \ if (first!=base) swapper(first,(char*)base); /* and so is the insertion sort, in the first two cases: */ #define Insertion(swapper) \ last=((char*)base)+nmemb*size; \ for (first=((char*)base)+size;first!=last;first+=size) { \ char *test; \ /* Find the right place for |first|. \ * My apologies for var reuse. */ \ for (test=first-size;compare(test,first)>0;test-=size) ; \ test+=size; \ if (test!=first) { \ /* Shift everything in [test,first) \ * up by one, and place |first| \ * where |test| is. */ \ memcpy(pivot,first,size); \ memmove(test+size,test,first-test); \ memcpy(test,pivot,size); \ } \ } #define SWAP_nonaligned(a,b) { \ register char *aa=(a),*bb=(b); \ register size_t sz=size; \ do { register char t=*aa; *aa++=*bb; *bb++=t; } while (--sz); } #define SWAP_aligned(a,b) { \ register int *aa=(int*)(a),*bb=(int*)(b); \ register size_t sz=size; \ do { register int t=*aa;*aa++=*bb; *bb++=t; } while (sz-=WORD_BYTES); } #define SWAP_words(a,b) { \ register int t=*((int*)a); *((int*)a)=*((int*)b); *((int*)b)=t; } /* ---------------------------------------------------------------------- */ static char * pivot_big(char *first, char *mid, char *last, size_t size, int compare(const void *, const void *)) { size_t d = (((last - first) / size) >> 3) * size; char *m1, *m2, *m3; { char *a = first, *b = first + d, *c = first + 2 * d; #ifdef DEBUG_QSORT fprintf(stderr, "< %d %d %d\n", *(int *) a, *(int *) b, *(int *) c); #endif m1 = compare(a, b) < 0 ? (compare(b, c) < 0 ? b : (compare(a, c) < 0 ? c : a)) : (compare(a, c) < 0 ? a : (compare(b, c) < 0 ? c : b)); } { char *a = mid - d, *b = mid, *c = mid + d; #ifdef DEBUG_QSORT fprintf(stderr, ". %d %d %d\n", *(int *) a, *(int *) b, *(int *) c); #endif m2 = compare(a, b) < 0 ? (compare(b, c) < 0 ? b : (compare(a, c) < 0 ? c : a)) : (compare(a, c) < 0 ? a : (compare(b, c) < 0 ? c : b)); } { char *a = last - 2 * d, *b = last - d, *c = last; #ifdef DEBUG_QSORT fprintf(stderr, "> %d %d %d\n", *(int *) a, *(int *) b, *(int *) c); #endif m3 = compare(a, b) < 0 ? (compare(b, c) < 0 ? b : (compare(a, c) < 0 ? c : a)) : (compare(a, c) < 0 ? a : (compare(b, c) < 0 ? c : b)); } #ifdef DEBUG_QSORT fprintf(stderr, "-> %d %d %d\n", *(int *) m1, *(int *) m2, *(int *) m3); #endif return compare(m1, m2) < 0 ? (compare(m2, m3) < 0 ? m2 : (compare(m1, m3) < 0 ? m3 : m1)) : (compare(m1, m3) < 0 ? m1 : (compare(m2, m3) < 0 ? m3 : m2)); } /* ---------------------------------------------------------------------- */ static void qsort_nonaligned(void *base, size_t nmemb, size_t size, int (*compare) (const void *, const void *)) { stack_entry stack[STACK_SIZE]; int stacktop = 0; char *first, *last; char *pivot = malloc(size); size_t trunc = TRUNC_nonaligned * size; assert(pivot != 0); first = (char *) base; last = first + (nmemb - 1) * size; if ((size_t) (last - first) > trunc) { char *ffirst = first, *llast = last; while (1) { /* Select pivot */ { char *mid = first + size * ((last - first) / size >> 1); Pivot(SWAP_nonaligned, size); memcpy(pivot, mid, size); } /* Partition. */ Partition(SWAP_nonaligned, size); /* Prepare to recurse/iterate. */ Recurse(trunc)} } PreInsertion(SWAP_nonaligned, TRUNC_nonaligned, size); Insertion(SWAP_nonaligned); free(pivot); } static void qsort_aligned(void *base, size_t nmemb, size_t size, int (*compare) (const void *, const void *)) { stack_entry stack[STACK_SIZE]; int stacktop = 0; char *first, *last; char *pivot = malloc(size); size_t trunc = TRUNC_aligned * size; assert(pivot != 0); first = (char *) base; last = first + (nmemb - 1) * size; if ((size_t) (last - first) > trunc) { char *ffirst = first, *llast = last; while (1) { /* Select pivot */ { char *mid = first + size * ((last - first) / size >> 1); Pivot(SWAP_aligned, size); memcpy(pivot, mid, size); } /* Partition. */ Partition(SWAP_aligned, size); /* Prepare to recurse/iterate. */ Recurse(trunc)} } PreInsertion(SWAP_aligned, TRUNC_aligned, size); Insertion(SWAP_aligned); free(pivot); } static void qsort_words(void *base, size_t nmemb, int (*compare) (const void *, const void *)) { stack_entry stack[STACK_SIZE]; int stacktop = 0; char *first, *last; char *pivot = malloc(WORD_BYTES); assert(pivot != 0); first = (char *) base; last = first + (nmemb - 1) * WORD_BYTES; if (last - first > TRUNC_words) { char *ffirst = first, *llast = last; while (1) { #ifdef DEBUG_QSORT fprintf(stderr, "Doing %d:%d: ", (first - (char *) base) / WORD_BYTES, (last - (char *) base) / WORD_BYTES); #endif /* Select pivot */ { char *mid = first + WORD_BYTES * ((last - first) / (2 * WORD_BYTES)); Pivot(SWAP_words, WORD_BYTES); *(int *) pivot = *(int *) mid; } #ifdef DEBUG_QSORT fprintf(stderr, "pivot=%d\n", *(int *) pivot); #endif /* Partition. */ Partition(SWAP_words, WORD_BYTES); /* Prepare to recurse/iterate. */ Recurse(TRUNC_words)} } PreInsertion(SWAP_words, (TRUNC_words / WORD_BYTES), WORD_BYTES); /* Now do insertion sort. */ last = ((char *) base) + nmemb * WORD_BYTES; for (first = ((char *) base) + WORD_BYTES; first != last; first += WORD_BYTES) { /* Find the right place for |first|. My apologies for var reuse */ int *pl = (int *) (first - WORD_BYTES), *pr = (int *) first; *(int *) pivot = *(int *) first; for (; compare(pl, pivot) > 0; pr = pl, --pl) { *pr = *pl; } if (pr != (int *) first) *pr = *(int *) pivot; } free(pivot); } /* ---------------------------------------------------------------------- */ void qsort(void *base, size_t nmemb, size_t size, int (*compare) (const void *, const void *)) { if (nmemb <= 1) return; if (((uintptr_t) base | size) & (WORD_BYTES - 1)) qsort_nonaligned(base, nmemb, size, compare); else if (size != WORD_BYTES) qsort_aligned(base, nmemb, size, compare); else qsort_words(base, nmemb, compare); } #endif /* !SDL_qsort */ /* vi: set ts=4 sw=4 expandtab: */