redis源码分析之zipmap.c

/**
 * 创建空zipmap
 * @return
 */
/* Create a new empty zipmap. */
unsigned char *zipmapNew(void) {
    unsigned char *zm = zmalloc(2);

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

/**
 * 读取zipmap元素的长度（1字节或者5字节，五字节时，第一个字节为标志位）
 * @param p
 * @return
 */
/* 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));
    memrev32ifbe(&len);
    return len;
}

/**
 * 计算编码zipmap value长度所需要的字节数，并且储存在p中，若p为空则只返回占用字节数
 * @param p
 * @param len
 * @return
 */
/* Encode the length 'l' writing it in 'p'. If p is NULL it just returns
 * the amount of bytes required to encode such a length. */
static unsigned int zipmapEncodeLength(unsigned char *p, unsigned int len) {
    if (p == NULL) {
        return ZIPMAP_LEN_BYTES(len);
    } else {
        if (len < ZIPMAP_BIGLEN) {
            p[0] = len;
            return 1;
        } else {
            p[0] = ZIPMAP_BIGLEN;
            memcpy(p+1,&len,sizeof(len));
            memrev32ifbe(p+1);
            return 1+sizeof(len);
        }
    }
}

/**
 * zipmap查找key，如果totlen指针不为空，则返回zip整个长度
 * @param zm
 * @param key
 * @param klen
 * @param totlen
 * @return
 */
/* Search for a matching key, returning a pointer to the entry inside the
 * zipmap. Returns NULL if the key is not found.
 *
 * If NULL is returned, and totlen is not NULL, it is set to the entire
 * size of the zimap, so that the calling function will be able to
 * reallocate the original zipmap to make room for more entries. */
static unsigned char *zipmapLookupRaw(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned int *totlen) {
    unsigned char *p = zm+1, *k = NULL;
    unsigned int l,llen;

    while(*p != ZIPMAP_END) {
        unsigned char free;

        /* Match or skip the key */
        l = zipmapDecodeLength(p);
        llen = zipmapEncodeLength(NULL,l);
        if (key != NULL && k == NULL && l == klen && !memcmp(p+llen,key,l)) {
            /* Only return when the user doesn't care
             * for the total length of the zipmap. */
            if (totlen != NULL) {
                k = p;
            } else {
                return p;
            }
        }
        //zmlen–len–“foo”–len–free–“bar”–len–“hello”–len–free–“world”-end
        //跳过key
        p += llen+l;
        //跳过value
        /* Skip the value as well */
        l = zipmapDecodeLength(p);
        p += zipmapEncodeLength(NULL,l);
        free = p[0];
        p += l+1+free; /* +1 to skip the free byte */
    }
    if (totlen != NULL) *totlen = (unsigned int)(p-zm)+1;//算上end占用的一个字节
    return k;
}

/**
 * 计算zipmap entry需要的长度
 * @param klen
 * @param vlen
 * @return
 */
static unsigned long zipmapRequiredLength(unsigned int klen, unsigned int vlen) {
    unsigned int l;

    l = klen+vlen+3;
    if (klen >= ZIPMAP_BIGLEN) l += 4;
    if (vlen >= ZIPMAP_BIGLEN) l += 4;
    return l;
}

/**
 * zipmap entry key占用的字节数
 * @param p
 * @return
 */
/* Return the total amount used by a key (encoded length + payload) */
static unsigned int zipmapRawKeyLength(unsigned char *p) {
    unsigned int l = zipmapDecodeLength(p);
    return zipmapEncodeLength(NULL,l) + l;
}

/**
 * zipmap entry value占用的字节数，包括空闲长度
 * @param p
 * @return
 */
/* Return the total amount used by a value
 * (encoded length + single byte free count + payload) */
static unsigned int zipmapRawValueLength(unsigned char *p) {
    unsigned int l = zipmapDecodeLength(p);
    unsigned int used;

    used = zipmapEncodeLength(NULL,l);
    //p[used]为空闲长度
    used += p[used] + 1 + l;
    return used;
}

/**
 * 计算zipmap entry的字节数
 * @param p
 * @return
 */
/* If 'p' points to a key, this function returns the total amount of
 * bytes used to store this entry (entry = key + associated value + trailing
 * free space if any). */
static unsigned int zipmapRawEntryLength(unsigned char *p) {
    unsigned int l = zipmapRawKeyLength(p);
    return l + zipmapRawValueLength(p+l);
}

/**
 * zipmap 空间重分配
 * @param zm
 * @param len
 * @return
 */
static inline unsigned char *zipmapResize(unsigned char *zm, unsigned int len) {
    zm = zrealloc(zm, len);
    zm[len-1] = ZIPMAP_END;
    return zm;
}

/**
 * zipmap设置或者更新entry
 * @param zm
 * @param key
 * @param klen
 * @param val
 * @param vlen
 * @param update
 * @return
 */
/* 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;

        /* Increase zipmap length (this is an insert) */
        if (zm[0] < ZIPMAP_BIGLEN) zm[0]++;//大于等于254就不起作用了
    } else {
        /* Key found. Is there enough space for the new value? */
        /* Compute the total length: */
        if (update) *update = 1;
        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. */
            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. */
            //移动末尾字节位置到p+reqlen，在扩容的时候已经重置end，所以移动的时候少一个字节
            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. */
    //如果reqlen小的话，并且达到阈值（4字节）的话，进行缩容操作
    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 {
        //如果不缩容的话，需要记录free空间占用
        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;
}

/**
 * zipmap 删除指定key的entry
 * @param zm
 * @param key
 * @param klen
 * @param deleted
 * @return
 */
/* 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]--;//如果元素个数为254个，则不做修改

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

/**
 * zipmap 跳过zipmap len
 * @param zm
 * @return
 */
/* Call before iterating through elements via zipmapNext() */
unsigned char *zipmapRewind(unsigned char *zm) {
    return zm+1;
}

/**
 * zipmap next元素
 * @param zm
 * @param key
 * @param klen
 * @param value
 * @param vlen
 * @return
 */
/* 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;
        *vlen = zipmapDecodeLength(zm);
        *value += ZIPMAP_LEN_BYTES(*vlen);
    }
    zm += zipmapRawValueLength(zm);
    return zm;
}

/**
 * zipmap 获取指定key的value
 * @param zm
 * @param key
 * @param klen
 * @param value
 * @param vlen
 * @return
 */
/* 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;
}

/**
 * zipmap 判断元素是否存在
 * @param zm
 * @param key
 * @param klen
 * @return
 */
/* Return 1 if the key exists, otherwise 0 is returned. */
int zipmapExists(unsigned char *zm, unsigned char *key, unsigned int klen) {
    return zipmapLookupRaw(zm,key,klen,NULL) != NULL;
}

/**
 * zipmap获取zipmap长度，如果超过了长度则重置长度（zipmap整个api中其实不强依赖于这个字节，可能只是为了判别要转换为dict的阈值）
 * @param zm
 * @return
 */
/* Return the number of entries inside a zipmap */
unsigned int zipmapLen(unsigned char *zm) {
    unsigned int len = 0;
    if (zm[0] < ZIPMAP_BIGLEN) {
        len = zm[0];
    } else {
        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;
}

/**
 * zipmap总占用字节数
 * @param zm
 * @return
 */
/* Return the raw size in bytes of a zipmap, so that we can serialize
 * the zipmap on disk (or everywhere is needed) just writing the returned
 * amount of bytes of the C array starting at the zipmap pointer. */
size_t zipmapBlobLen(unsigned char *zm) {
    unsigned int totlen;
    zipmapLookupRaw(zm,NULL,0,&totlen);
    return totlen;
}

#ifdef REDIS_TEST
static void zipmapRepr(unsigned char *p) {
    unsigned int l;

    printf("{status %u}",*p++);
    while(1) {
        if (p[0] == ZIPMAP_END) {
            printf("{end}");
            break;
        } else {
            unsigned char e;

            l = zipmapDecodeLength(p);
            printf("{key %u}",l);
            p += zipmapEncodeLength(NULL,l);
            if (l != 0 && fwrite(p,l,1,stdout) == 0) perror("fwrite");
            p += l;

            l = zipmapDecodeLength(p);
            printf("{value %u}",l);
            p += zipmapEncodeLength(NULL,l);
            e = *p++;
            if (l != 0 && fwrite(p,l,1,stdout) == 0) perror("fwrite");
            p += l+e;
            if (e) {
                printf("[");
                while(e--) printf(".");
                printf("]");
            }
        }
    }
    printf("\n");
}

#define UNUSED(x) (void)(x)
int zipmapTest(int argc, char *argv[]) {
    unsigned char *zm;

    UNUSED(argc);
    UNUSED(argv);

    zm = zipmapNew();

    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);
    zipmapRepr(zm);

    zm = zipmapSet(zm,(unsigned char*) "hello",5, (unsigned char*) "world!",6,NULL);
    zm = zipmapSet(zm,(unsigned char*) "foo",3, (unsigned char*) "bar",3,NULL);
    zm = zipmapSet(zm,(unsigned char*) "foo",3, (unsigned char*) "!",1,NULL);
    zipmapRepr(zm);
    zm = zipmapSet(zm,(unsigned char*) "foo",3, (unsigned char*) "12345",5,NULL);
    zipmapRepr(zm);
    zm = zipmapSet(zm,(unsigned char*) "new",3, (unsigned char*) "xx",2,NULL);
    zm = zipmapSet(zm,(unsigned char*) "noval",5, (unsigned char*) "",0,NULL);
    zipmapRepr(zm);
    zm = zipmapDel(zm,(unsigned char*) "new",3,NULL);
    zipmapRepr(zm);

    printf("\nLook up large key:\n");
    {
        unsigned char buf[512];
        unsigned char *value;
        unsigned int vlen, i;
        for (i = 0; i < 512; i++) buf[i] = 'a';

        zm = zipmapSet(zm,buf,512,(unsigned char*) "long",4,NULL);
        if (zipmapGet(zm,buf,512,&value,&vlen)) {
            printf("  <long key> is associated to the %d bytes value: %.*s\n",
                vlen, vlen, value);
        }
    }

    printf("\nPerform a direct lookup:\n");
    {
        unsigned char *value;
        unsigned int vlen;

        if (zipmapGet(zm,(unsigned char*) "foo",3,&value,&vlen)) {
            printf("  foo is associated to the %d bytes value: %.*s\n",
                vlen, vlen, value);
        }
    }
    printf("\nIterate through elements:\n");
    {
        unsigned char *i = zipmapRewind(zm);
        unsigned char *key, *value;
        unsigned int klen, vlen;

        while((i = zipmapNext(i,&key,&klen,&value,&vlen)) != NULL) {
            printf("  %d:%.*s => %d:%.*s\n", klen, klen, key, vlen, vlen, value);
        }
    }
    return 0;
}
#endif

数据结构详见
https://blog.youkuaiyun.com/u012658346/article/details/51329360