gbit.c

<Win2k系统编程>源码.次数为国人自编,内容丰富,还是不错的.

/****************************** Module Header *******************************
* Module Name: GBIT.C
*
* Bitmap allocation routines to manage a bit-mapped free list, and find
* free sections.
*
* Functions:
*
* gbit_set()
* gbit_init()
* gbit_alloc()
* gbit_free()
* gbit_findfree()
*
* Comments:
*
* Each map is an array of unsigned longs where bit 0 of the first 
* long represents block 1.
*
****************************************************************************/

#include <windows.h>
#include "gutils.h"


BOOL gbit_set(DWORD FAR * map, long blknr, long nblks, BOOL op_set);

/***************************************************************************
 * Function: gbit_init
 *
 * Purpose:
 *
 * Initialise a pre-allocated map of ulongs to represent a free
 * area of nblks
 */
void APIENTRY
gbit_init(DWORD FAR * map, long nblks)
{
        long i;
        long leftover = nblks % 32;
        long blks = nblks / 32;
        DWORD last = 0;

        for (i=0; i < blks; i++) {
                map[i] = 0xffffffff;
        }
        for (i = 0; i < leftover; i++) {
                last = (last << 1) | 1;
        }
        if(leftover)
                map[blks] = last;
}

/***************************************************************************
 * Function: gbit_alloc
 *
 * Purpose:
 *
 * Mark a region starting at blknr for nblks, as busy (ie 0) 
 */
BOOL APIENTRY
gbit_alloc(DWORD FAR * map, long blknr, long nblks)
{
        return(gbit_set(map, blknr, nblks, FALSE));
}


/***************************************************************************
 * Function: gbit_set
 *
 * Purpose:
 *
 * Mark region - if op_set, to 1s, otherwise to 0s 
 */
BOOL
gbit_set(DWORD FAR * map, long blknr, long nblks, BOOL op_set)
{
        long first;
        long last;
        long fullwords;
        long startbit, startword;
        long i;
        DWORD dword = 0;

        blknr--;
        first = min(32 - (blknr % 32), nblks);
        nblks -= first;
        last = nblks % 32;
        fullwords = (nblks - last) / 32;
        
        startword = blknr / 32;
        startbit = blknr % 32;
        for (i = 0; i < first; i++) {
                dword = (dword << 1) | 1;
        }
        dword <<= startbit;
        if (op_set) {
                map[startword] |= dword;
                dword = 0xffffffff;
        } else {
                map[startword] &= ~dword;
                dword = 0;
        }
        startword++;
        for (i = 0; i < fullwords; i++) {
                map[startword+i] = dword;
        }
        startword += fullwords;
        for(i = 0, dword = 0; i < last; i++) {
                dword = (dword << 1) | 1;
        }
        if (last) {
                if (op_set) {
                        map[startword] |= dword;
                } else {
                        map[startword] &= ~dword;
                }
        }

        return(TRUE);
}

/***************************************************************************
 * Function: gbit_free
 *
 * Purpose:
 *
 * Mark region of nblks starting at blknr to 0s - ie not busy 
 */
BOOL APIENTRY
gbit_free(DWORD FAR * map, long blknr, long nblks)
{
        return(gbit_set(map, blknr, nblks, TRUE));
}


/***************************************************************************
 * Function: gbit_findfree
 *
 * Purpose:
 *
 * Find a free segment (ie contiguous sequence of 1s) of nblks in length.
 * If not found, find longest sequence. Store address of segment in *blknr.
 *
 * Return value is nr of blks in sequence found. Region is *not* marked busy.
 */
long APIENTRY
gbit_findfree(DWORD FAR* map, long nblks, long mapsize, long FAR * blknr)
{
        long curblk, startblk, len, i;
        long startbit, nfull, nlast, nbitsleft;
        DWORD mask;
        long mapblks = (mapsize + 31) / 32;
        long aubegin = 0, aulen = 0;
        long curbit = 0;

        /* main loop looking at segments */
        for (curblk = 0; curblk < mapblks; ) {
loop:
                /* loop finding first 1 */
                for (; curblk < mapblks; curblk++, curbit = 0) {
                        if (map[curblk] > 0) {
                                break;
                        }
                }
                if (curblk >= mapblks) 
                        break;
                
                /* find first 1 in this long */
                startblk = curblk;
                for (mask = 1, i = 0; i < curbit; i++) {
                        mask <<= 1;
                }
                for(; curbit < 32; curbit++, mask <<= 1) {
                        if (map[curblk] & mask) {
                                break;
                        }
                } 
                if (curbit >= 32) {
                        /* abandon this word - start again with next word */
                        curblk++;
                        curbit = 0;
                        goto loop;
                }

                /* we've now found a 1 - calc remaining
                 * bits in this word, complete words etc required.
                 */
                startbit = curbit;
                nbitsleft = min( (32 - curbit), nblks);
                nfull = (nblks - nbitsleft) / 32;
                nlast = (nblks - nbitsleft) % 32;

                /* check for required sequence within this word */

                for (i = 0; i < nbitsleft; i++, curbit++, mask <<= 1) {
                        if ((map[curblk] & mask) == 0) {
                                /* abandon and start again - start
                                 * next pass at curbit in same word
                                 */
                                /* store free region if longest yet */
                                if (i > aulen) {
                                        aulen = i;
                                        aubegin = curblk * 32 + startbit +1;
                                }
                                goto loop;
                        }
                }
                
                /* check for nfull full words */
                for (curblk++; curblk <= startblk + nfull; curblk++) {
                        if (curblk >= mapblks) {
                                /* end of map - abandon here and exit at top
                                 * of loop
                                 */
                                len = nbitsleft +
                                        ((curblk - (startblk + 1)) * 32);
                                if (len > aulen) {
                                        aubegin = startblk * 32 + startbit + 1;
                                        aulen = len;
                                }
                                goto loop;
                        }
                        if (map[curblk] != 0xffffffff) {
                                /* not a full word - start again at this bit */
                                len = 0;
                                curbit = 0;
                                for (mask = 1; mask & map[curblk]; mask <<= 1) {
                                        len++;
                                        curbit++;
                                }
                                len += nbitsleft +
                                        (curblk - (startblk+ 1)) * 32;
                                if (len > aulen) {
                                        aulen = len;
                                        aubegin = startblk * 32 + startbit + 1;
                                }
                                /* continue with current blk, bit */
                                goto loop;
                        }
                }

                /* left-over bits required in last word */
                mask = 1;
                for (curbit = 0; curbit < nlast;  curbit++, mask <<= 1) {
                        if ((map[curblk] & mask) == 0) {
                                len = nbitsleft + (nfull * 32);
                                len += curbit;
                                if (len > aulen) {
                                        aulen = len;
                                        aubegin = startblk * 32 + startbit + 1;
                                }
                                goto loop;
                        }
                }
                /* ok - found a block big enough! */
                aubegin = startblk * 32 + startbit + 1;
                *blknr = aubegin;
                return(nblks);
        }

        /* end of map - return longest sequence */
        *blknr = aubegin;
        return(aulen);
}