結合redis設計與實現的redis源碼學習-9-zipmap（壓縮圖)

在學習了所有redis設計與實現的數據結構後，我發現在redis中還有一些特殊的數據結構，今天我們來學習zipmap。
zipmap和ziplist比較相似，使用特殊的內存存儲方法來提升個數較少的鍵值對。具有以下特點：
1、使用1個字節的len來存儲元素個數，當長度大於最大值那麼只能遍歷獲得size了；
2、len後面是key的長度，一般使用1個字節或者5個字節表示；
3、free用來表示value長度的空閒值，當value的數據被改變後可能會發生變化，一般情況下free的值較小，如果空閒太多，zipmap會進行調整使整體變得儘可能小；
4、數據佈局格式：<zmlen><len>"foo"<len><free>"bar"<len>"hello"<len><free>"world" <總長度>

#ifndef _ZIPMAP_H
#define _ZIPMAP_H

unsigned char *zipmapNew(void);//創建一個新壓縮圖
unsigned char *zipmapSet(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned char *val, unsigned int vlen, int *update);//設置一個鍵值對
unsigned char *zipmapDel(unsigned char *zm, unsigned char *key, unsigned int klen, int *deleted);//刪除一個鍵值對
unsigned char *zipmapRewind(unsigned char *zm);//
unsigned char *zipmapNext(unsigned char *zm, unsigned char **key, unsigned int *klen, unsigned char **value, unsigned int *vlen);//取得下一個鍵值對
int zipmapGet(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned char **value, unsigned int *vlen);//獲取某個鍵值對
int zipmapExists(unsigned char *zm, unsigned char *key, unsigned int klen);//判斷某個鍵值對是否存在
unsigned int zipmapLen(unsigned char *zm);//獲取鍵值對個數
size_t zipmapBlobLen(unsigned char *zm);//獲取序列化到文件中所需的大小
void zipmapRepr(unsigned char *p);//輸出壓縮圖的具體信息

#ifdef REDIS_TEST
int zipmapTest(int argc, char *argv[]);
#endif

#endif

zipmap.c

#include <stdio.h>
#include <string.h>
#include "zmalloc.h"
#include "endianconv.h"

#define ZIPMAP_BIGLEN 254
#define ZIPMAP_END 255

/* The following defines the max value for the <free> field described in the
 * comments above, that is, the max number of trailing bytes in a value. */
#define ZIPMAP_VALUE_MAX_FREE 4

/* The following macro returns the number of bytes needed to encode the length
 * for the integer value _l, that is, 1 byte for lengths < ZIPMAP_BIGLEN and 返回應該開闢的大小
 * 5 bytes for all the other lengths. */
#define ZIPMAP_LEN_BYTES(_l) (((_l) < ZIPMAP_BIGLEN) ? 1 : sizeof(unsigned int)+1)

/* Create a new empty zipmap. */
unsigned char *zipmapNew(void) {
    unsigned char *zm = zmalloc(2);
    //一個空的壓縮圖只有長度標誌位和結束符
    zm[0] = 0; /* Length */
    zm[1] = ZIPMAP_END;
    return zm;
}
/* Decode the encoded length pointed by 'p' 返回zipmap的長度*/
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;
}

/* 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. 返回zipmap的編碼長度*/
static unsigned int zipmapEncodeLength(unsigned char *p, unsigned int len) {
    if (p == NULL) {
        return ZIPMAP_LEN_BYTES(len);
    } else {
        if (len < ZIPMAP_BIGLEN) {//小於最大長度,返回1，p[0]可以表示該長度範圍
            p[0] = len;
            return 1;
        } else {
            p[0] = ZIPMAP_BIGLEN;//等於最大長度,返回長度+1
            memcpy(p+1,&len,sizeof(len));
            memrev32ifbe(p+1);
            return 1+sizeof(len);
        }
    }
}
/* 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;//p是存儲數據的開始位置
    unsigned int l,llen;

    while(*p != ZIPMAP_END) {//沒有到結束位
        unsigned char free;

        /* Match or skip the key 對比或者跳過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. 只有在用戶不在意zipmap的總長度的時候返回*/
            if (totlen != NULL) {
                k = p;
            } else {
                return p;
            }
        }
        p += llen+l;
        /* Skip the value as well 總是跳過該值*/
        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;
    return k;
}
//返回zipmap要求的長度，根據zipmap的最大長度判斷，key或value大於就+4
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;
}

/* Return the total amount used by a key (encoded length + payload) 返回key的長度*/
static unsigned int zipmapRawKeyLength(unsigned char *p) {
    unsigned int l = zipmapDecodeLength(p);
    return zipmapEncodeLength(NULL,l) + l;
}
/* Return the total amount used by a value
 * (encoded length + single byte free count + payload) 獲取值佔用的長度*/
static unsigned int zipmapRawValueLength(unsigned char *p) {
    unsigned int l = zipmapDecodeLength(p);
    unsigned int used;

    used = zipmapEncodeLength(NULL,l);
    used += p[used] + 1 + l;
    return used;
}

/* 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). 如果p是key的話，會返回條目總共使用的長度*/
static unsigned int zipmapRawEntryLength(unsigned char *p) {
    unsigned int l = zipmapRawKeyLength(p);
    return l + zipmapRawValueLength(p+l);
}
//重置zipmap的大小
static inline unsigned char *zipmapResize(unsigned char *zm, unsigned int len) {
    zm = zrealloc(zm, len);
    zm[len-1] = ZIPMAP_END;
    return zm;
}
/* 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. 設置一個還不存在的key和他的value，如果update不是空的話且key已經存在的話,會把他的值設爲一*/
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 找不到key，重設大小，插入key*/
        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]++;
    } else {
        /* Key found. Is there enough space for the new value? */
        /* Compute the total length: 找到key了，是否有足夠的空間給新值，計算總長度*/
        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. */
            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. 移動結尾的空字節到開始位置，然後重置zipmap的大小*/
        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. 現在將key和value存入*/
    /* 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. 刪除指定的key，返回0是沒有找到，1的話就是正常刪除*/
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);//找啊找
    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;
}
/* Call before iterating through elements via zipmapNext() 返回第一個元素的地址，配合zipmapNext來遍歷zipmap*/
unsigned char *zipmapRewind(unsigned char *zm) {
    return zm+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);
 * }
 可以用來遍歷zipmap*/
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;
}
/* Search a key and retrieve the pointer and len of the associated value.
 * If the key is found the function returns 1, otherwise 0. 查找一個key*/
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;
}
/* Return 1 if the key exists, otherwise 0 is returned. 判斷key是否存在*/
int zipmapExists(unsigned char *zm, unsigned char *key, unsigned int klen) {
    return zipmapLookupRaw(zm,key,klen,NULL) != NULL;
}
/* Return the number of entries inside a zipmap 返回zipmap的元素個數*/
unsigned int zipmapLen(unsigned char *zm) {
    unsigned int len = 0;
    if (zm[0] < ZIPMAP_BIGLEN) {
        len = zm[0];
    } else {
        unsigned char *p = zipmapRewind(zm);//如果個數太多就要遍歷計數
        while((p = zipmapNext(p,NULL,NULL,NULL,NULL)) != NULL) len++;

        /* Re-store length if small enough 防止判斷失誤*/
        if (len < ZIPMAP_BIGLEN) zm[0] = len;
    }
    return len;
}
/* 返回zipmap的總長度 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;
}