Redis系列（九）：数据结构Hash源码解析和HSET、HGET命令

2.源码解析

1.相关命令如下：

    {"hset",hsetCommand,,"wmF",,NULL,,,,,},
    {"hsetnx",hsetnxCommand,,"wmF",,NULL,,,,,},
    {"hget",hgetCommand,,"rF",,NULL,,,,,},
    {"hmset",hmsetCommand,-,"wm",,NULL,,,,,},
    {"hmget",hmgetCommand,-,"r",,NULL,,,,,},
    {"hincrby",hincrbyCommand,,"wmF",,NULL,,,,,},
    {"hincrbyfloat",hincrbyfloatCommand,,"wmF",,NULL,,,,,},
    {"hdel",hdelCommand,-,"wF",,NULL,,,,,},
    {"hlen",hlenCommand,,"rF",,NULL,,,,,},
    {"hstrlen",hstrlenCommand,,"rF",,NULL,,,,,},
    {"hkeys",hkeysCommand,,"rS",,NULL,,,,,},
    {"hvals",hvalsCommand,,"rS",,NULL,,,,,},
    {"hgetall",hgetallCommand,,"r",,NULL,,,,,},
    {"hexists",hexistsCommand,,"rF",,NULL,,,,,},
    {"hscan",hscanCommand,-,"rR",,NULL,,,,,},

2.ziplist数据结构

/* We use this function to receive information about a ziplist entry.
 * Note that this is not how the data is actually encoded, is just what we
 * get filled by a function in order to operate more easily. */
typedef struct zlentry {
    unsigned int prevrawlensize; /* Bytes used to encode the previous entry len*/
    unsigned int prevrawlen;     /* Previous entry len. */
    unsigned int lensize;        /* Bytes used to encode this entry type/len.
                                    For example strings have a 1, 2 or 5 bytes
                                    header. Integers always use a single byte.*/
    unsigned int len;            /* Bytes used to represent the actual entry.
                                    For strings this is just the string length
                                    while for integers it is 1, 2, 3, 4, 8 or
                                    0 (for 4 bit immediate) depending on the
                                    number range. */
    unsigned int headersize;     /* prevrawlensize + lensize. */
    unsigned char encoding;      /* Set to ZIP_STR_* or ZIP_INT_* depending on
                                    the entry encoding. However for 4 bits
                                    immediate integers this can assume a range
                                    of values and must be range-checked. */
    unsigned char *p;            /* Pointer to the very start of the entry, that
                                    is, this points to prev-entry-len field. */
} zlentry;

3.hashtable数据结构

typedef struct dictEntry {
    void *key;
    union {
        void *val;
        uint64_t u64;
        int64_t s64;
        double d;
    } v;
    struct dictEntry *next;
} dictEntry;

typedef struct dictType {
    uint64_t (*hashFunction)(const void *key);
    void *(*keyDup)(void *privdata, const void *key);
    void *(*valDup)(void *privdata, const void *obj);
    int (*keyCompare)(void *privdata, const void *key1, const void *key2);
    void (*keyDestructor)(void *privdata, void *key);
    void (*valDestructor)(void *privdata, void *obj);
} dictType;

/* This is our hash table structure. Every dictionary has two of this as we
 * implement incremental rehashing, for the old to the new table. */
typedef struct dictht {
    dictEntry **table;
    unsigned long size;
    unsigned long sizemask;
    unsigned long used;
} dictht;

typedef struct dict {
    dictType *type;
    void *privdata;
    dictht ht[];
    long rehashidx; /* rehashing not in progress if rehashidx == -1 */
    unsigned long iterators; /* number of iterators currently running */
} dict;

hset

// t_hash.c, set key field value
void hsetCommand(client *c) {
    int update;
    robj *o;
    // 1. 查找hash的key是否存在，不存在则新建一个，然后在其上进行数据操作
    if ((o = hashTypeLookupWriteOrCreate(c,c->argv[])) == NULL) return;
    // 2. 检查2-3个参数是否需要将简单版（ziplist）hash表转换为复杂的hash表，转换后的表通过 o->ptr 体现
    hashTypeTryConversion(o,c->argv,,);
    // 3. 添加kv到 o 的hash表中
    update = hashTypeSet(o,c->argv[]->ptr,c->argv[]->ptr,HASH_SET_COPY);
    addReply(c, update ? shared.czero : shared.cone);
    // 变更命令传播
    signalModifiedKey(c->db,c->argv[]);
    notifyKeyspaceEvent(NOTIFY_HASH,"hset",c->argv[],c->db->id);
    server.dirty++;
}

// 1. 获取db外部的key, 即整体hash数据实例
// t_hash.c
robj *hashTypeLookupWriteOrCreate(client *c, robj *key) {
    robj *o = lookupKeyWrite(c->db,key);
    if (o == NULL) {
        // 此处创建的hashObject是以 ziplist 形式的
        o = createHashObject();
        dbAdd(c->db,key,o);
    } else {
        // 不是hash类型的键已存在，不可覆盖，返回错误
        if (o->type != OBJ_HASH) {
            addReply(c,shared.wrongtypeerr);
            return NULL;
        }
    }
    return o;
}
// object.c, 创建hashObject, 以 ziplist 形式创建
robj *createHashObject(void) {
    unsigned char *zl = ziplistNew();
    robj *o = createObject(OBJ_HASH, zl);
    o->encoding = OBJ_ENCODING_ZIPLIST;
    return o;
}
// ziplist.c
static unsigned char *createList() {
    unsigned char *zl = ziplistNew();
    zl = ziplistPush(zl, (unsigned char*)"foo", , ZIPLIST_TAIL);
    zl = ziplistPush(zl, (unsigned char*)"quux", , ZIPLIST_TAIL);
    zl = ziplistPush(zl, (unsigned char*)"hello", , ZIPLIST_HEAD);
    zl = ziplistPush(zl, (unsigned char*)"", , ZIPLIST_TAIL);
    return zl;
}

// 2. 检查参数，是否需要将 ziplist 形式的hash表转换为真正的hash表
/* Check the length of a number of objects to see if we need to convert a
 * ziplist to a real hash. Note that we only check string encoded objects
 * as their string length can be queried in constant time. */
void hashTypeTryConversion(robj *o, robj **argv, int start, int end) {
    int i;

    if (o->encoding != OBJ_ENCODING_ZIPLIST) return;

    for (i = start; i <= end; i++) {
        // 参数大于设置的 hash_max_ziplist_value (默认: 64)时，会直接将 ziplist 转换为 ht
        // OBJ_ENCODING_RAW, OBJ_ENCODING_EMBSTR
        // 循环检查参数，只要发生了一次转换就结束检查（没必要继续了）
        if (sdsEncodedObject(argv[i]) &&
            sdslen(argv[i]->ptr) > server.hash_max_ziplist_value)
        {
            // 这个转换过程很有意思，我们深入看看
            hashTypeConvert(o, OBJ_ENCODING_HT);
            break;
        }
    }
}
// t_hash.c, 转换编码方式 (如上, ziplist -> ht)
void hashTypeConvert(robj *o, int enc) {
    if (o->encoding == OBJ_ENCODING_ZIPLIST) {
        // 此处我们只处理这种情况
        hashTypeConvertZiplist(o, enc);
    } else if (o->encoding == OBJ_ENCODING_HT) {
        serverPanic("Not implemented");
    } else {
        serverPanic("Unknown hash encoding");
    }
}
// t_hash.c, 转换编码 ziplist 为目标 enc (实际只能是 OBJ_ENCODING_HT)
void hashTypeConvertZiplist(robj *o, int enc) {
    serverAssert(o->encoding == OBJ_ENCODING_ZIPLIST);

    if (enc == OBJ_ENCODING_ZIPLIST) {
        /* Nothing to do... */

    } else if (enc == OBJ_ENCODING_HT) {
        hashTypeIterator *hi;
        dict *dict;
        int ret;
        // 迭代器创建
        hi = hashTypeInitIterator(o);
        // 一个hash的数据结构就是一个 dict, 从这个级别来说, hash 与 db 是一个级别的
        dict = dictCreate(&hashDictType, NULL);
        // 依次迭代 o, 赋值到 hi->fptr, hi->vptr
        // 依次添加到 dict 中
        while (hashTypeNext(hi) != C_ERR) {
            sds key, value;
            // 从 hi->fptr 中获取key
            // 从 hi->vptr 中获取value
            key = hashTypeCurrentObjectNewSds(hi,OBJ_HASH_KEY);
            value = hashTypeCurrentObjectNewSds(hi,OBJ_HASH_VALUE);
            // 添加到 dict 中
            ret = dictAdd(dict, key, value);
            if (ret != DICT_OK) {
                serverLogHexDump(LL_WARNING,"ziplist with dup elements dump",
                    o->ptr,ziplistBlobLen(o->ptr));
                serverPanic("Ziplist corruption detected");
            }
        }
        // 释放迭代器
        hashTypeReleaseIterator(hi);
        zfree(o->ptr);
        // 将变更反映到o对象上返回
        o->encoding = OBJ_ENCODING_HT;
        o->ptr = dict;
    } else {
        serverPanic("Unknown hash encoding");
    }
}
// 2.1. 迭代ziplist元素
// t_hash.c, 迭代器
/* Move to the next entry in the hash. Return C_OK when the next entry
 * could be found and C_ERR when the iterator reaches the end. */
int hashTypeNext(hashTypeIterator *hi) {
    if (hi->encoding == OBJ_ENCODING_ZIPLIST) {
        unsigned char *zl;
        unsigned char *fptr, *vptr;
        // 每次都是基于原始字符器进行计算偏移
        // 迭代的是 fptr,vptr
        zl = hi->subject->ptr;
        fptr = hi->fptr;
        vptr = hi->vptr;
        // 第一次查找时使用index查找，后续则使用 fptr,vptr 进行迭代
        if (fptr == NULL) {
            /* Initialize cursor */
            serverAssert(vptr == NULL);
            fptr = ziplistIndex(zl, );
        } else {
            /* Advance cursor */
            serverAssert(vptr != NULL);
            fptr = ziplistNext(zl, vptr);
        }
        if (fptr == NULL) return C_ERR;

        /* Grab pointer to the value (fptr points to the field) */
        vptr = ziplistNext(zl, fptr);
        serverAssert(vptr != NULL);

        /* fptr, vptr now point to the first or next pair */
        hi->fptr = fptr;
        hi->vptr = vptr;
    } else if (hi->encoding == OBJ_ENCODING_HT) {
        if ((hi->de = dictNext(hi->di)) == NULL) return C_ERR;
    } else {
        serverPanic("Unknown hash encoding");
    }
    return C_OK;
}
// ziplist.c, 查找 index 的元素
/* Returns an offset to use for iterating with ziplistNext. When the given
 * index is negative, the list is traversed back to front. When the list
 * doesn't contain an element at the provided index, NULL is returned. */
unsigned char *ziplistIndex(unsigned char *zl, int index) {
    unsigned char *p;
    unsigned int prevlensize, prevlen = ;
    if (index < ) {
        // 小于0时，反向查找
        index = (-index)-;
        p = ZIPLIST_ENTRY_TAIL(zl);
        if (p[] != ZIP_END) {
            ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);
            while (prevlen >  && index--) {
                p -= prevlen;
                ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);
            }
        }
    } else {
        p = ZIPLIST_ENTRY_HEAD(zl);
        while (p[] != ZIP_END && index--) {
            p += zipRawEntryLength(p);
        }
    }
    // 迭代完成还没找到元素 p[0]=ZIP_END
    // index 超出整体ziplist大小则遍历完成后 index>0
    return (p[] == ZIP_END || index > ) ? NULL : p;
}
// ziplist.c, 由 fptr,vptr 进行迭代元素
/* Return pointer to next entry in ziplist.
 *
 * zl is the pointer to the ziplist
 * p is the pointer to the current element
 *
 * The element after 'p' is returned, otherwise NULL if we are at the end. */
unsigned char *ziplistNext(unsigned char *zl, unsigned char *p) {
    ((void) zl);

    /* "p" could be equal to ZIP_END, caused by ziplistDelete,
     * and we should return NULL. Otherwise, we should return NULL
     * when the *next* element is ZIP_END (there is no next entry). */
    if (p[] == ZIP_END) {
        return NULL;
    }
    // 当前指针偏移当前元素长度（根据ziplist协议），即到下一元素指针位置
    p += zipRawEntryLength(p);
    if (p[] == ZIP_END) {
        return NULL;
    }

    return p;
}
/* Return the total number of bytes used by the entry pointed to by 'p'. */
static unsigned int zipRawEntryLength(unsigned char *p) {
    unsigned int prevlensize, encoding, lensize, len;
    ZIP_DECODE_PREVLENSIZE(p, prevlensize);
    ZIP_DECODE_LENGTH(p + prevlensize, encoding, lensize, len);
    return prevlensize + lensize + len;
}

// 2.2. t_hash.c, 获取 hashTypeIterator 的具体值，写入 vstr, vlen 中
/* Return the key or value at the current iterator position as a new
 * SDS string. */
sds hashTypeCurrentObjectNewSds(hashTypeIterator *hi, int what) {
    unsigned char *vstr;
    unsigned int vlen;
    long long vll;

    hashTypeCurrentObject(hi,what,&vstr,&vlen,&vll);
    if (vstr) return sdsnewlen(vstr,vlen);
    return sdsfromlonglong(vll);
}
/* Higher level function of hashTypeCurrent*() that returns the hash value
 * at current iterator position.
 *
 * The returned element is returned by reference in either *vstr and *vlen if
 * it's returned in string form, or stored in *vll if it's returned as
 * a number.
 *
 * If *vll is populated *vstr is set to NULL, so the caller
 * can always check the function return by checking the return value
 * type checking if vstr == NULL. */
void hashTypeCurrentObject(hashTypeIterator *hi, int what, unsigned char **vstr, unsigned int *vlen, long long *vll) {
    if (hi->encoding == OBJ_ENCODING_ZIPLIST) {
        *vstr = NULL;
        hashTypeCurrentFromZiplist(hi, what, vstr, vlen, vll);
    } else if (hi->encoding == OBJ_ENCODING_HT) {
        sds ele = hashTypeCurrentFromHashTable(hi, what);
        *vstr = (unsigned char*) ele;
        *vlen = sdslen(ele);
    } else {
        serverPanic("Unknown hash encoding");
    }
}

// t_hash.c, 从ziplist中获取某个 hashTypeIterator 的具体值，结果定稿 vstr, vlen
/* Get the field or value at iterator cursor, for an iterator on a hash value
 * encoded as a ziplist. Prototype is similar to `hashTypeGetFromZiplist`. */
void hashTypeCurrentFromZiplist(hashTypeIterator *hi, int what,
                                unsigned char **vstr,
                                unsigned int *vlen,
                                long long *vll)
{
    int ret;

    serverAssert(hi->encoding == OBJ_ENCODING_ZIPLIST);
    // OBJ_HASH_KEY 从 fptr 中获取, 否则从 vptr 中获取
    if (what & OBJ_HASH_KEY) {
        ret = ziplistGet(hi->fptr, vstr, vlen, vll);
        serverAssert(ret);
    } else {
        ret = ziplistGet(hi->vptr, vstr, vlen, vll);
        serverAssert(ret);
    }
}
// ziplist.c,
/* Get entry pointed to by 'p' and store in either '*sstr' or 'sval' depending
 * on the encoding of the entry. '*sstr' is always set to NULL to be able
 * to find out whether the string pointer or the integer value was set.
 * Return 0 if 'p' points to the end of the ziplist, 1 otherwise. */
unsigned int ziplistGet(unsigned char *p, unsigned char **sstr, unsigned int *slen, long long *sval) {
    zlentry entry;
    if (p == NULL || p[] == ZIP_END) return ;
    if (sstr) *sstr = NULL;
    // 按照ziplist的编码协议, 获取头部信息
    zipEntry(p, &entry);
    if (ZIP_IS_STR(entry.encoding)) {
        if (sstr) {
            *slen = entry.len;
            *sstr = p+entry.headersize;
        }
    } else {
        if (sval) {
            *sval = zipLoadInteger(p+entry.headersize,entry.encoding);
        }
    }
    return ;
}
// ziplist.c, 解析原始字符串为 zlentry
/* Return a struct with all information about an entry. */
static void zipEntry(unsigned char *p, zlentry *e) {
    // 按照ziplist的编码协议,依次读取 prevrawlensize, prevrawlen
    ZIP_DECODE_PREVLEN(p, e->prevrawlensize, e->prevrawlen);
    // 指向下一位置偏移，按照ziplist的编码协议,依次读取 encoding, lensize, len
    ZIP_DECODE_LENGTH(p + e->prevrawlensize, e->encoding, e->lensize, e->len);
    // 除去header得到 body偏移
    e->headersize = e->prevrawlensize + e->lensize;
    e->p = p;
}

header

// ziplist.c
/* Decode the length of the previous element, from the perspective of the entry
 * pointed to by 'ptr'. */
#define ZIP_DECODE_PREVLEN(ptr, prevlensize, prevlen) do {                     \
    // 解析第1个字符为 prevlensize
    ZIP_DECODE_PREVLENSIZE(ptr, prevlensize);                                  \
    if ((prevlensize) == ) {                                                  \
        (prevlen) = (ptr)[];                                                  \
    } else if ((prevlensize) == ) {                                           \
        assert(sizeof((prevlensize)) == );                                    \
        // 当ptr[0]>254时，代表内容有点大，需要使用 5个字符保存上一字符长度
        memcpy(&(prevlen), ((char*)(ptr)) + , );                             \
        memrev32ifbe(&prevlen);                                                \
    }                                                                          \
} while();
/* Decode the number of bytes required to store the length of the previous
 * element, from the perspective of the entry pointed to by 'ptr'. */
#define ZIP_DECODE_PREVLENSIZE(ptr, prevlensize) do {                          \
    if ((ptr)[] < ZIP_BIGLEN) {                                               \
        (prevlensize) = ;                                                     \
    } else {                                                                   \
        (prevlensize) = ;                                                     \
    }                                                                          \
} while();
/* Decode the length encoded in 'ptr'. The 'encoding' variable will hold the
 * entries encoding, the 'lensize' variable will hold the number of bytes
 * required to encode the entries length, and the 'len' variable will hold the
 * entries length. */
#define ZIP_DECODE_LENGTH(ptr, encoding, lensize, len) do {                    \
    // 解析第1个字符为 编码格式 &ZIP_STR_MASK=0xc0
    ZIP_ENTRY_ENCODING((ptr), (encoding));                                     \
    if ((encoding) < ZIP_STR_MASK) {                                           \
        // 0 << 6 =0
        // 具体解析如下代码,
        if ((encoding) == ZIP_STR_06B) {                                       \
            (lensize) = ;                                                     \
            (len) = (ptr)[] & 0x3f;                                           \
        }
        // 1 << 6 =64
        else if ((encoding) == ZIP_STR_14B) {                                  \
            (lensize) = ;                                                     \
            (len) = (((ptr)[] & 0x3f) << ) | (ptr)[];                       \
        }
        // 2 << 6 =128
        else if (encoding == ZIP_STR_32B) {                                    \
            (lensize) = ;                                                     \
            (len) = ((ptr)[] << ) |                                         \
                    ((ptr)[] << ) |                                         \
                    ((ptr)[] <<  ) |                                         \
                    ((ptr)[]);                                                \
        } else {                                                               \
            assert(NULL);                                                      \
        }                                                                      \
    } else {                                                                   \
        // 超过 0xc0 的长度了，直接使用 1,2,3,4 表示len
        (lensize) = ;                                                         \
        (len) = zipIntSize(encoding);                                          \
    }                                                                          \
} while();
/* Extract the encoding from the byte pointed by 'ptr' and set it into
 * 'encoding'. */
#define ZIP_ENTRY_ENCODING(ptr, encoding) do {  \
    (encoding) = (ptr[]); \
    if ((encoding) < ZIP_STR_MASK) (encoding) &= ZIP_STR_MASK; \
} while()

/* Different encoding/length possibilities */
#define ZIP_STR_MASK 0xc0
#define ZIP_INT_MASK 0x30
#define ZIP_STR_06B (0 << 6)        // 0x00
#define ZIP_STR_14B (1 << 6)        // 0x40
#define ZIP_STR_32B (2 << 6)        // 0x80
#define ZIP_INT_16B (0xc0 | 0<<4)    // 0xc0
#define ZIP_INT_32B (0xc0 | 1<<4)    // 0xd0
#define ZIP_INT_64B (0xc0 | 2<<4)    // 0xe0
#define ZIP_INT_24B (0xc0 | 3<<4)    // 0xf0
#define ZIP_INT_8B 0xfe                // 0xfe

添加kv到对应的key实例中：

// 3. 添加kv到 hash表中, 稍微复杂
// t_hash.c, 做变更到hash表中
int hashTypeSet(robj *o, sds field, sds value, int flags) {
    int update = ;
    // 针对ziplist 的添加, 与 ht 编码的添加, 自然是分别处理
    if (o->encoding == OBJ_ENCODING_ZIPLIST) {
        unsigned char *zl, *fptr, *vptr;

        zl = o->ptr;
        // 找到ziplist 的头节点指针
        fptr = ziplistIndex(zl, ZIPLIST_HEAD);
        if (fptr != NULL) {
            // 尝试查找该 field 对应的元素(从1开始)，如果找到则先删除原值，然后统一添加
            fptr = ziplistFind(fptr, (unsigned char*)field, sdslen(field), );
            if (fptr != NULL) {
                /* Grab pointer to the value (fptr points to the field) */
                // value 不可以为null, 否则 ziplist 将无法工作
                vptr = ziplistNext(zl, fptr);
                serverAssert(vptr != NULL);
                update = ;

                /* Delete value */
                // 先删除旧的 value, 再以插入的形式更新, 后续讲删除时再详解
                zl = ziplistDelete(zl, &vptr);

                /* Insert new value */
                // 重点，将value添加到 ziplist 中
                zl = ziplistInsert(zl, vptr, (unsigned char*)value,
                        sdslen(value));
            }
        }
        // 没有找到对应元素，则直接将元素添加到尾部即可
        if (!update) {
            /* Push new field/value pair onto the tail of the ziplist */
            zl = ziplistPush(zl, (unsigned char*)field, sdslen(field),
                    ZIPLIST_TAIL);
            zl = ziplistPush(zl, (unsigned char*)value, sdslen(value),
                    ZIPLIST_TAIL);
        }
        o->ptr = zl;

        /* Check if the ziplist needs to be converted to a hash table */
        // 大于设置的阀值后，转换ziplist为ht(默认: 512)
        if (hashTypeLength(o) > server.hash_max_ziplist_entries)
            hashTypeConvert(o, OBJ_ENCODING_HT);
    } else if (o->encoding == OBJ_ENCODING_HT) {
        dictEntry *de = dictFind(o->ptr,field);
        if (de) {
            sdsfree(dictGetVal(de));
            if (flags & HASH_SET_TAKE_VALUE) {
                dictGetVal(de) = value;
                value = NULL;
            } else {
                dictGetVal(de) = sdsdup(value);
            }
            update = ;
        } else {
            sds f,v;
            if (flags & HASH_SET_TAKE_FIELD) {
                f = field;
                field = NULL;
            } else {
                f = sdsdup(field);
            }
            if (flags & HASH_SET_TAKE_VALUE) {
                v = value;
                value = NULL;
            } else {
                v = sdsdup(value);
            }
            dictAdd(o->ptr,f,v);
        }
    } else {
        serverPanic("Unknown hash encoding");
    }

    /* Free SDS strings we did not referenced elsewhere if the flags
     * want this function to be responsible. */
    if (flags & HASH_SET_TAKE_FIELD && field) sdsfree(field);
    if (flags & HASH_SET_TAKE_VALUE && value) sdsfree(value);
    return update;
}
// 3.1. 使用ziplist进行保存 field -> value
// ziplist.c, 查找某个 field 是否存在于ziplist中
/* Find pointer to the entry equal to the specified entry. Skip 'skip' entries
 * between every comparison. Returns NULL when the field could not be found. */
unsigned char *ziplistFind(unsigned char *p, unsigned char *vstr, unsigned int vlen, unsigned int skip) {
    int skipcnt = ;
    unsigned char vencoding = ;
    long long vll = ;

    while (p[] != ZIP_END) {
        unsigned int prevlensize, encoding, lensize, len;
        unsigned char *q;
        // 解析整个字符串p的 prevlensize,encoding,lensize,len
        ZIP_DECODE_PREVLENSIZE(p, prevlensize);
        ZIP_DECODE_LENGTH(p + prevlensize, encoding, lensize, len);
        q = p + prevlensize + lensize;
        // 传入1, 代表要跳过一个元素, 比如: 查找key时,跳过1个v,然后继续迭代
        // 跳过了n个元素后，再从此开始key的比对过程
        if (skipcnt == ) {
            /* Compare current entry with specified entry */
            // 针对不同的编码使用不同的比较方式
            if (ZIP_IS_STR(encoding)) {
                // 找到相应的元素，直接返回 p 指针
                if (len == vlen && memcmp(q, vstr, vlen) == ) {
                    return p;
                }
            } else {
                /* Find out if the searched field can be encoded. Note that
                 * we do it only the first time, once done vencoding is set
                 * to non-zero and vll is set to the integer value. */
                if (vencoding == ) {
                    if (!zipTryEncoding(vstr, vlen, &vll, &vencoding)) {
                        /* If the entry can't be encoded we set it to
                         * UCHAR_MAX so that we don't retry again the next
                         * time. */
                        vencoding = UCHAR_MAX;
                    }
                    /* Must be non-zero by now */
                    assert(vencoding);
                }

                /* Compare current entry with specified entry, do it only
                 * if vencoding != UCHAR_MAX because if there is no encoding
                 * possible for the field it can't be a valid integer. */
                if (vencoding != UCHAR_MAX) {
                    long long ll = zipLoadInteger(q, encoding);
                    if (ll == vll) {
                        return p;
                    }
                }
            }

            /* Reset skip count */
            // 查找一次，跳过skip次
            skipcnt = skip;
        } else {
            /* Skip entry */
            skipcnt--;
        }

        /* Move to next entry */
        p = q + len;
    }

    return NULL;
}
// ziplist.c, 添加value到ziplist中
// zl:ziplist实例, p:要插入的key字串, s:要插入的value字串, len:要插入的value的长度
/* Insert an entry at "p". */
unsigned char *ziplistInsert(unsigned char *zl, unsigned char *p, unsigned char *s, unsigned int slen) {
    return __ziplistInsert(zl,p,s,slen);
}
/* Insert item at "p". */
static unsigned char *__ziplistInsert(unsigned char *zl, unsigned char *p, unsigned char *s, unsigned int slen) {
    size_t curlen = intrev32ifbe(ZIPLIST_BYTES(zl)), reqlen;
    unsigned int prevlensize, prevlen = ;
    size_t offset;
    int nextdiff = ;
    unsigned char encoding = ;
    long long value = ; /* initialized to avoid warning. Using a value
                                    that is easy to see if for some reason
                                    we use it uninitialized. */
    zlentry tail;

    /* Find out prevlen for the entry that is inserted. */
    if (p[] != ZIP_END) {
        ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);
    } else {
        unsigned char *ptail = ZIPLIST_ENTRY_TAIL(zl);
        if (ptail[] != ZIP_END) {
            prevlen = zipRawEntryLength(ptail);
        }
    }

    /* See if the entry can be encoded */
    if (zipTryEncoding(s,slen,&value,&encoding)) {
        /* 'encoding' is set to the appropriate integer encoding */
        reqlen = zipIntSize(encoding);
    } else {
        /* 'encoding' is untouched, however zipEncodeLength will use the
         * string length to figure out how to encode it. */
        reqlen = slen;
    }
    /* We need space for both the length of the previous entry and
     * the length of the payload. */
    // 加上prevlen,encoding,slen 的长度，以计算value的存放位置
    reqlen += zipPrevEncodeLength(NULL,prevlen);
    reqlen += zipEncodeLength(NULL,encoding,slen);

    /* When the insert position is not equal to the tail, we need to
     * make sure that the next entry can hold this entry's length in
     * its prevlen field. */
    nextdiff = (p[] != ZIP_END) ? zipPrevLenByteDiff(p,reqlen) : ;

    /* Store offset because a realloc may change the address of zl. */
    // 存储当前偏移位置，以便在扩容之后，还能找到相应位置
    // p = p -zl + zl
    offset = p-zl;
    zl = ziplistResize(zl,curlen+reqlen+nextdiff);
    p = zl+offset;

    /* Apply memory move when necessary and update tail offset. */
    if (p[] != ZIP_END) {
        /* Subtract one because of the ZIP_END bytes */
        // 字符拷贝
        memmove(p+reqlen,p-nextdiff,curlen-offset-+nextdiff);

        /* Encode this entry's raw length in the next entry. */
        zipPrevEncodeLength(p+reqlen,reqlen);

        /* Update offset for tail */
        ZIPLIST_TAIL_OFFSET(zl) =
            intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))+reqlen);

        /* When the tail contains more than one entry, we need to take
         * "nextdiff" in account as well. Otherwise, a change in the
         * size of prevlen doesn't have an effect on the *tail* offset. */
        zipEntry(p+reqlen, &tail);
        if (p[reqlen+tail.headersize+tail.len] != ZIP_END) {
            ZIPLIST_TAIL_OFFSET(zl) =
                intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))+nextdiff);
        }
    } else {
        /* This element will be the new tail. */
        ZIPLIST_TAIL_OFFSET(zl) = intrev32ifbe(p-zl);
    }

    /* When nextdiff != 0, the raw length of the next entry has changed, so
     * we need to cascade the update throughout the ziplist */
    if (nextdiff != ) {
        // 如果本次更新后数据位置变化，则需要更新后续的元素位置
        offset = p-zl;
        zl = __ziplistCascadeUpdate(zl,p+reqlen);
        p = zl+offset;
    }

    /* Write the entry */
    // 将 value 写入 p 中, 即写入了 ziplist 中
    p += zipPrevEncodeLength(p,prevlen);
    p += zipEncodeLength(p,encoding,slen);
    if (ZIP_IS_STR(encoding)) {
        memcpy(p,s,slen);
    } else {
        zipSaveInteger(p,value,encoding);
    }
    ZIPLIST_INCR_LENGTH(zl,);
    return zl;
}
// 另外，如果没有旧的元素值时，直接在hash表的末尾添加对应的field->value 即可
// ziplist.c, 在尾部进行添加元素，没有许多的情况要考虑，但是代码完全复用 __ziplistInsert()
unsigned char *ziplistPush(unsigned char *zl, unsigned char *s, unsigned int slen, int where) {
    unsigned char *p;
    p = (where == ZIPLIST_HEAD) ? ZIPLIST_ENTRY_HEAD(zl) : ZIPLIST_ENTRY_END(zl);
    return __ziplistInsert(zl,p,s,slen);
}

深入理解ziplist

　看起来没ziplist好像没那么简单呢，为啥还要搞这么复杂呢？其实以上代码，仅是在人看来复杂，对机器来说就是更多的移位计算操作，多消耗点cpu就换来了空间上的节省，是可以的。软件本身的复杂性带来了效益，是软件的价值体现，所以，并非所有的东西都是简单即美。

　　接下来，我们来看一下使用 HT 的编码又如何存储field->value呢？

// 3.2. OBJ_ENCODING_HT 的 field -> value 的添加
    if (o->encoding == OBJ_ENCODING_HT) {
        // hash 表中查找对应的 field
        dictEntry *de = dictFind(o->ptr,field);
        if (de) {
            sdsfree(dictGetVal(de));
            // hset 时使用 HASH_SET_COPY, 所以直接使用 sdsdup() 即可
            if (flags & HASH_SET_TAKE_VALUE) {
                dictGetVal(de) = value;
                value = NULL;
            } else {
                dictGetVal(de) = sdsdup(value);
            }
            update = ;
        } else {
            // 新增 field -> value
            sds f,v;
            if (flags & HASH_SET_TAKE_FIELD) {
                f = field;
                field = NULL;
            } else {
                f = sdsdup(field);
            }
            if (flags & HASH_SET_TAKE_VALUE) {
                v = value;
                value = NULL;
            } else {
                v = sdsdup(value);
            }
            // 添加到 hash 表中，前些篇章讲解过，大概就是计算hash，放入v的过程
            dictAdd(o->ptr,f,v);
        }
    }

如此看来，OBJ_ENCODING_HT 的实现反而简单了哦。

总结下 hash的插入过程，hash 初始创建时都是使用ziplist 进行容纳元素的，在特定情况下会触发 ziplist 为 ht 的编码方式, 比如:

　　　　1. hset时自身的参数大于设置值(默认: 64)时直接转换 ziplist -> ht;

　　　　2. hash表的元素数量大于设置值(默认: 512)时转换 ziplist -> ht;

　　这么设计的原因是，元素较少且占用空间较小时，使用ziplist会节省空间，且时间消耗与hash表相关并不大，所以 ziplist 是优先的选择了。但是大量数据还是必须要使用hash表存储的。