漫话Redis源码之六

redis中有很多内存操作的经典使用，越看越有意思：

/* Return a ziplist element as an SDS string. */
sds ziplistGetObject(unsigned char *sptr) {
    unsigned char *vstr;
    unsigned int vlen;
    long long vlong;
 
    serverAssert(sptr != NULL);
    serverAssert(ziplistGet(sptr,&vstr,&vlen,&vlong));
 
    if (vstr) {
        return sdsnewlen((char*)vstr,vlen);
    } else {
        return sdsfromlonglong(vlong);
    }
}
 
/* Compare element in sorted set with given element. */
int zzlCompareElements(unsigned char *eptr, unsigned char *cstr, unsigned int clen) {
    unsigned char *vstr;
    unsigned int vlen;
    long long vlong;
    unsigned char vbuf[32];
    int minlen, cmp;
 
    serverAssert(ziplistGet(eptr,&vstr,&vlen,&vlong));
    if (vstr == NULL) {
        /* Store string representation of long long in buf. */
        vlen = ll2string((char*)vbuf,sizeof(vbuf),vlong);
        vstr = vbuf;
    }
 
    minlen = (vlen < clen) ? vlen : clen;
    cmp = memcmp(vstr,cstr,minlen);
    if (cmp == 0) return vlen-clen;
    return cmp;
}
 
unsigned int zzlLength(unsigned char *zl) {
    return ziplistLen(zl)/2;
}
 
/* Move to next entry based on the values in eptr and sptr. Both are set to
 * NULL when there is no next entry. */
void zzlNext(unsigned char *zl, unsigned char **eptr, unsigned char **sptr) {
    unsigned char *_eptr, *_sptr;
    serverAssert(*eptr != NULL && *sptr != NULL);
 
    _eptr = ziplistNext(zl,*sptr);
    if (_eptr != NULL) {
        _sptr = ziplistNext(zl,_eptr);
        serverAssert(_sptr != NULL);
    } else {
        /* No next entry. */
        _sptr = NULL;
    }
 
    *eptr = _eptr;
    *sptr = _sptr;
}
 
/* Move to the previous entry based on the values in eptr and sptr. Both are
 * set to NULL when there is no next entry. */
void zzlPrev(unsigned char *zl, unsigned char **eptr, unsigned char **sptr) {
    unsigned char *_eptr, *_sptr;
    serverAssert(*eptr != NULL && *sptr != NULL);
 
    _sptr = ziplistPrev(zl,*eptr);
    if (_sptr != NULL) {
        _eptr = ziplistPrev(zl,_sptr);
        serverAssert(_eptr != NULL);
    } else {
        /* No previous entry. */
        _eptr = NULL;
    }
 
    *eptr = _eptr;
    *sptr = _sptr;
}
 
/* Returns if there is a part of the zset is in range. Should only be used
 * internally by zzlFirstInRange and zzlLastInRange. */
int zzlIsInRange(unsigned char *zl, zrangespec *range) {
    unsigned char *p;
    double score;
 
    /* Test for ranges that will always be empty. */
    if (range->min > range->max ||
            (range->min == range->max && (range->minex || range->maxex)))
        return 0;
 
    p = ziplistIndex(zl,-1); /* Last score. */
    if (p == NULL) return 0; /* Empty sorted set */
    score = zzlGetScore(p);
    if (!zslValueGteMin(score,range))
        return 0;
 
    p = ziplistIndex(zl,1); /* First score. */
    serverAssert(p != NULL);
    score = zzlGetScore(p);
    if (!zslValueLteMax(score,range))
        return 0;
 
    return 1;
}
 
/* Find pointer to the first element contained in the specified range.
 * Returns NULL when no element is contained in the range. */
unsigned char *zzlFirstInRange(unsigned char *zl, zrangespec *range) {
    unsigned char *eptr = ziplistIndex(zl,0), *sptr;
    double score;
 
    /* If everything is out of range, return early. */
    if (!zzlIsInRange(zl,range)) return NULL;
 
    while (eptr != NULL) {
        sptr = ziplistNext(zl,eptr);
        serverAssert(sptr != NULL);
 
        score = zzlGetScore(sptr);
        if (zslValueGteMin(score,range)) {
            /* Check if score <= max. */
            if (zslValueLteMax(score,range))
                return eptr;
            return NULL;
        }
 
        /* Move to next element. */
        eptr = ziplistNext(zl,sptr);
    }
 
    return NULL;
}
 
/* Find pointer to the last element contained in the specified range.
 * Returns NULL when no element is contained in the range. */
unsigned char *zzlLastInRange(unsigned char *zl, zrangespec *range) {
    unsigned char *eptr = ziplistIndex(zl,-2), *sptr;
    double score;
 
    /* If everything is out of range, return early. */
    if (!zzlIsInRange(zl,range)) return NULL;
 
    while (eptr != NULL) {
        sptr = ziplistNext(zl,eptr);
        serverAssert(sptr != NULL);
 
        score = zzlGetScore(sptr);
        if (zslValueLteMax(score,range)) {
            /* Check if score >= min. */
            if (zslValueGteMin(score,range))
                return eptr;
            return NULL;
        }
 
        /* Move to previous element by moving to the score of previous element.
         * When this returns NULL, we know there also is no element. */
        sptr = ziplistPrev(zl,eptr);
        if (sptr != NULL)
            serverAssert((eptr = ziplistPrev(zl,sptr)) != NULL);
        else
            eptr = NULL;
    }
 
    return NULL;
}
 
int zzlLexValueGteMin(unsigned char *p, zlexrangespec *spec) {
    sds value = ziplistGetObject(p);
    int res = zslLexValueGteMin(value,spec);
    sdsfree(value);
    return res;
}
 
int zzlLexValueLteMax(unsigned char *p, zlexrangespec *spec) {
    sds value = ziplistGetObject(p);
    int res = zslLexValueLteMax(value,spec);
    sdsfree(value);
    return res;
}