redis基础数据结构-zipmap简介

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cnt |key1.len|key1.context|val1.len|free1.len|val1.context|null1.context|key2.len|key2.context|val2.len|free2.len|val2.context|null2.context|0xFF| 
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/* Create a new empty zipmap. */
unsigned char *zipmapNew(void) {
    unsigned char *zm = zmalloc(2);

    zm[0] = 0; /* Length */
    zm[1] = ZIPMAP_END;
    return zm;
}

/* Decode the encoded length pointed by 'p' */
static unsigned int zipmapDecodeLength(unsigned char *p) {
    unsigned int len = *p;

    if (len < ZIPMAP_BIGLEN) return len;
    memcpy(&len,p+1,sizeof(unsigned int));
    return len;
}

/* Set key to value, creating the key if it does not already exist.
 * If 'update' is not NULL, *update is set to 1 if the key was
 * already preset, otherwise to 0. */
unsigned char *zipmapSet(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned char *val, unsigned int vlen, int *update) {
    unsigned int zmlen, offset;
    unsigned int freelen, reqlen = zipmapRequiredLength(klen,vlen);
    unsigned int empty, vempty;
    unsigned char *p;
   
    freelen = reqlen;
    if (update) *update = 0;
    p = zipmapLookupRaw(zm,key,klen,&zmlen);
    if (p == NULL) {
        /* Key not found: enlarge */
        zm = zipmapResize(zm, zmlen+reqlen);
        p = zm+zmlen-1;
        zmlen = zmlen+reqlen;	/* 新的总长 */

        /* Increase zipmap length (this is an insert) */
        if (zm[0] < ZIPMAP_BIGLEN) zm[0]++;
    } else {
        /* Key found. Is there enough space for the new value? */
        /* Compute the total length: */
        if (update) *update = 1;	/* key已存在，则更新key对应的val */
        freelen = zipmapRawEntryLength(p);
        if (freelen < reqlen) {	/* 现存的空间不够用 */
            /* Store the offset of this key within the current zipmap, so
             * it can be resized. Then, move the tail backwards so this
             * pair fits at the current position. */
            offset = p-zm;
            zm = zipmapResize(zm, zmlen-freelen+reqlen);
            p = zm+offset;

            /* The +1 in the number of bytes to be moved is caused by the
             * end-of-zipmap byte. Note: the *original* zmlen is used. 
             * void *memmove(void *dest, const void *src, size_t n) */
            memmove(p+reqlen, p+freelen, zmlen-(offset+freelen+1));
            zmlen = zmlen-freelen+reqlen;
            freelen = reqlen;
        }
    }

    /* We now have a suitable block where the key/value entry can
     * be written. If there is too much free space, move the tail
     * of the zipmap a few bytes to the front and shrink the zipmap,
     * as we want zipmaps to be very space efficient. */
    empty = freelen-reqlen;
    if (empty >= ZIPMAP_VALUE_MAX_FREE) {
        /* First, move the tail <empty> bytes to the front, then resize
         * the zipmap to be <empty> bytes smaller. */
        offset = p-zm;
        memmove(p+reqlen, p+freelen, zmlen-(offset+freelen+1));
        zmlen -= empty;
        zm = zipmapResize(zm, zmlen);
        p = zm+offset;
        vempty = 0;
    } else {
        vempty = empty;
    }

    /* Just write the key + value and we are done. */
    /* Key: */
    p += zipmapEncodeLength(p,klen);
    memcpy(p,key,klen);
    p += klen;
    /* Value: */
    p += zipmapEncodeLength(p,vlen);
    *p++ = vempty;
    memcpy(p,val,vlen);
    return zm;
}

/* Remove the specified key. If 'deleted' is not NULL the pointed integer is
 * set to 0 if the key was not found, to 1 if it was found and deleted. */
unsigned char *zipmapDel(unsigned char *zm, unsigned char *key, unsigned int klen, int *deleted) {
    unsigned int zmlen, freelen;
    unsigned char *p = zipmapLookupRaw(zm,key,klen,&zmlen);
    if (p) {
        freelen = zipmapRawEntryLength(p);
        memmove(p, p+freelen, zmlen-((p-zm)+freelen+1));
        zm = zipmapResize(zm, zmlen-freelen);

        /* Decrease zipmap length */
        if (zm[0] < ZIPMAP_BIGLEN) zm[0]--;

        if (deleted) *deleted = 1;
    } else {
        if (deleted) *deleted = 0;
    }
    return zm;
}

/* Search a key and retrieve the pointer and len of the associated value.
 * If the key is found the function returns 1, otherwise 0. */
int zipmapGet(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned char **value, unsigned int *vlen) {
    unsigned char *p;

    if ((p = zipmapLookupRaw(zm,key,klen,NULL)) == NULL) return 0;
    p += zipmapRawKeyLength(p);
    *vlen = zipmapDecodeLength(p);
    *value = p + ZIPMAP_LEN_BYTES(*vlen) + 1;
    return 1;
}

/* Call it before to iterate trought elements via zipmapNext() */
unsigned char *zipmapRewind(unsigned char *zm) {
    return zm+1;
}

/* This function is used to iterate through all the zipmap elements.
 * In the first call the first argument is the pointer to the zipmap + 1.
 * In the next calls what zipmapNext returns is used as first argument.
 * Example:
 *
 * unsigned char *i = zipmapRewind(my_zipmap);
 * while((i = zipmapNext(i,&key,&klen,&value,&vlen)) != NULL) {
 *     printf("%d bytes key at $p\n", klen, key);
 *     printf("%d bytes value at $p\n", vlen, value);
 * }
 */
unsigned char *zipmapNext(unsigned char *zm, unsigned char **key, unsigned int *klen, unsigned char **value, unsigned int *vlen) {
    if (zm[0] == ZIPMAP_END) return NULL;
    if (key) {
        *key = zm;
        *klen = zipmapDecodeLength(zm);
        *key += ZIPMAP_LEN_BYTES(*klen);
    }
    zm += zipmapRawKeyLength(zm);
    if (value) {
        *value = zm+1;	/* +1是free.len.head格式占用的一个字节空间 */
        *vlen = zipmapDecodeLength(zm);
        *value += ZIPMAP_LEN_BYTES(*vlen);
    }
    zm += zipmapRawValueLength(zm);
    return zm;
}