/* 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;
    //寻找key的位置
    p = zipmapLookupRaw(zm,key,klen,&zmlen);
    if (p == NULL) {
        /* Key not found: enlarge */
        //key的位置没有找到，调整zipmap的大小，准备添加操作
        zm = zipmapResize(zm, zmlen+reqlen);
        p = zm+zmlen-1;
        zmlen = zmlen+reqlen;

        /* Increase zipmap length (this is an insert) */
        //如果头字节还没有达到最大值，则递增
        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的位置以及找到，判断是否有空间插入新的值
        freelen = zipmapRawEntryLength(p);
        if (freelen < reqlen) {
            /* 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. */
             //如果没有空间插入新的值，则调整大小
            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. */
            //移动空间以便增加新的值
            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  bytes to the front, then resize
         * the zipmap to be  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: */
    //定位到插入的位置，首先写入key值
    p += zipmapEncodeLength(p,klen);
    memcpy(p,key,klen);
    p += klen;
    /* Value: */
    //key值后面是value值，再次写入
    p += zipmapEncodeLength(p,vlen);
    *p++ = vempty;
    memcpy(p,val,vlen);
    return zm;
}

/* Return the number of entries inside a zipmap */
/* 返回map的长度 */
unsigned int zipmapLen(unsigned char *zm) {
    unsigned int len = 0;
    //如果第一个长度小于最大值，则直接返回
    if (zm[0] < ZIPMAP_BIGLEN) {
        len = zm[0];
    } else {
    	//否则变量计算长度
        unsigned char *p = zipmapRewind(zm);
        while((p = zipmapNext(p,NULL,NULL,NULL,NULL)) != NULL) len++;

        /* Re-store length if small enough */
        if (len < ZIPMAP_BIGLEN) zm[0] = len;
    }
    return len;
}

    zm = zipmapSet(zm,(unsigned char*) "name",4, (unsigned char*) "foo",3,NULL);
    zm = zipmapSet(zm,(unsigned char*) "surname",7, (unsigned char*) "foo",3,NULL);
    zm = zipmapSet(zm,(unsigned char*) "age",3, (unsigned char*) "foo",3,NULL);