Redis的t_hash.c是对hash字典数据结构的实现。
主要是基于ziplist、dict、sds实现。

当元素较少的时候，使用ziplist来实现字典数据结构。
当元素较多的时候，使用dict来实现字典数据结构。

每次添加元素的时候，都会去检查是否需要转换类型。

在redis.conf中存在如下配置：

# Hashes are encoded using a memory efficient data structure when they have a
# small number of entries, and the biggest entry does not exceed a given
# threshold. These thresholds can be configured using the following directives.
hash-max-ziplist-entries 512
hash-max-ziplist-value 64

在t_hash.c存在如下转换ziplist和dict的代码：

/* 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. */
//当元素较少的时候，用ziplist存储，数量较多的时候转为Hash存储。
void hashTypeTryConversion(robj *o, robj **argv, int start, int end) {
    int i;

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

    //达到hash_max_ziplist_value的时候，会转为dict
    for (i = start; i <= end; i++) {
        if (sdsEncodedObject(argv[i]) &&
            sdslen(argv[i]->ptr) > server.hash_max_ziplist_value)
        {
            hashTypeConvert(o, OBJ_ENCODING_HT);
            break;
        }
    }
}

1. 新增元素

//hset命令
void hsetCommand(client *c) {
    int i, created = 0;
    robj *o;

    if ((c->argc % 2) == 1) {
        addReplyError(c,"wrong number of arguments for HMSET");
        return;
    }

    //根据key查找
    if ((o = hashTypeLookupWriteOrCreate(c,c->argv[1])) == NULL) return;
    //尝试转换类型ziplist -> dict
    hashTypeTryConversion(o,c->argv,2,c->argc-1);

    for (i = 2; i < c->argc; i += 2)
        created += !hashTypeSet(o,c->argv[i]->ptr,c->argv[i+1]->ptr,HASH_SET_COPY);

    /* HMSET (deprecated) and HSET return value is different. */
    char *cmdname = c->argv[0]->ptr;
    if (cmdname[1] == 's' || cmdname[1] == 'S') {
        /* HSET */
        addReplyLongLong(c, created);
    } else {
        /* HMSET */
        addReply(c, shared.ok);
    }
    //事务 watch key
    signalModifiedKey(c->db,c->argv[1]);
    notifyKeyspaceEvent(NOTIFY_HASH,"hset",c->argv[1],c->db->id);
    server.dirty++;
}

//hsetnx命令
void hsetnxCommand(client *c) {
    robj *o;
    //根据key查找
    if ((o = hashTypeLookupWriteOrCreate(c,c->argv[1])) == NULL) return;
    //类型转换
    hashTypeTryConversion(o,c->argv,2,3);

    //判断field是否存在
    if (hashTypeExists(o, c->argv[2]->ptr)) {
        addReply(c, shared.czero);
    } else {
        //添加
        hashTypeSet(o,c->argv[2]->ptr,c->argv[3]->ptr,HASH_SET_COPY);
        addReply(c, shared.cone);
        signalModifiedKey(c->db,c->argv[1]);
        notifyKeyspaceEvent(NOTIFY_HASH,"hset",c->argv[1],c->db->id);
        server.dirty++;
    }
}

static void addHashFieldToReply(client *c, robj *o, sds field) {
    int ret;

    //如果为空，返回"$-1\r\n"
    if (o == NULL) {
        addReply(c, shared.nullbulk);
        return;
    }

    //ziplist
    if (o->encoding == OBJ_ENCODING_ZIPLIST) {
        unsigned char *vstr = NULL;
        unsigned int vlen = UINT_MAX;
        long long vll = LLONG_MAX;

        //根据field获取
        ret = hashTypeGetFromZiplist(o, field, &vstr, &vlen, &vll);
        if (ret < 0) {
            //如果为空，返回"$-1\r\n"
            addReply(c, shared.nullbulk);
        } else {
            if (vstr) {
                addReplyBulkCBuffer(c, vstr, vlen);
            } else {
                addReplyBulkLongLong(c, vll);
            }
        }

    }
    //dict
    else if (o->encoding == OBJ_ENCODING_HT) {
        sds value = hashTypeGetFromHashTable(o, field);
        if (value == NULL)
            addReply(c, shared.nullbulk);
        else
            addReplyBulkCBuffer(c, value, sdslen(value));
    } else {
        serverPanic("Unknown hash encoding");
    }
}

2. 删除元素

//hdel命令
void hdelCommand(client *c) {
    robj *o;
    int j, deleted = 0, keyremoved = 0;

    //根据key查找并校验类型
    if ((o = lookupKeyWriteOrReply(c,c->argv[1],shared.czero)) == NULL ||
        checkType(c,o,OBJ_HASH)) return;

    //删除
    for (j = 2; j < c->argc; j++) {
        if (hashTypeDelete(o,c->argv[j]->ptr)) {
            deleted++;
            if (hashTypeLength(o) == 0) {
                dbDelete(c->db,c->argv[1]);
                keyremoved = 1;
                break;
            }
        }
    }

    if (deleted) {
        signalModifiedKey(c->db,c->argv[1]);
        notifyKeyspaceEvent(NOTIFY_HASH,"hdel",c->argv[1],c->db->id);
        if (keyremoved)
            notifyKeyspaceEvent(NOTIFY_GENERIC,"del",c->argv[1],
                                c->db->id);
        server.dirty += deleted;
    }

    //响应client
    addReplyLongLong(c,deleted);
}

3. 查找元素

//hget命令
void hgetCommand(client *c) {
    robj *o;

    //根据key查找，并检验类型
    if ((o = lookupKeyReadOrReply(c,c->argv[1],shared.nullbulk)) == NULL ||
        checkType(c,o,OBJ_HASH)) return;

    addHashFieldToReply(c, o, c->argv[2]->ptr);
}

//hmget命令
void hmgetCommand(client *c) {
    robj *o;
    int i;

    /* Don't abort when the key cannot be found. Non-existing keys are empty
     * hashes, where HMGET should respond with a series of null bulks. */
    //根据key查找
    o = lookupKeyRead(c->db, c->argv[1]);
    if (o != NULL && o->type != OBJ_HASH) {
        addReply(c, shared.wrongtypeerr);
        return;
    }

    addReplyMultiBulkLen(c, c->argc-2);
    //返回
    for (i = 2; i < c->argc; i++) {
        addHashFieldToReply(c, o, c->argv[i]->ptr);
    }
}

4. 增加value值

//hincrby命令
void hincrbyCommand(client *c) {
    long long value, incr, oldvalue;
    robj *o;
    sds new;
    unsigned char *vstr;
    unsigned int vlen;

    //增加的值，类型转换，值处理
    if (getLongLongFromObjectOrReply(c,c->argv[3],&incr,NULL) != C_OK) return;
    //根据key查找
    if ((o = hashTypeLookupWriteOrCreate(c,c->argv[1])) == NULL) return;
    //获取当前value
    if (hashTypeGetValue(o,c->argv[2]->ptr,&vstr,&vlen,&value) == C_OK) {
        if (vstr) {
            if (string2ll((char*)vstr,vlen,&value) == 0) {
                addReplyError(c,"hash value is not an integer");
                return;
            }
        } /* Else hashTypeGetValue() already stored it into &value */
    } else {
        value = 0;
    }

    oldvalue = value;
    //判断是否越域
    if ((incr < 0 && oldvalue < 0 && incr < (LLONG_MIN-oldvalue)) ||
        (incr > 0 && oldvalue > 0 && incr > (LLONG_MAX-oldvalue))) {
        addReplyError(c,"increment or decrement would overflow");
        return;
    }
    //赋值
    value += incr;
    new = sdsfromlonglong(value);
    //修改value，HASH_SET_TAKE_VALUE
    hashTypeSet(o,c->argv[2]->ptr,new,HASH_SET_TAKE_VALUE);
    //响应client
    addReplyLongLong(c,value);
    signalModifiedKey(c->db,c->argv[1]);
    notifyKeyspaceEvent(NOTIFY_HASH,"hincrby",c->argv[1],c->db->id);
    server.dirty++;
}

//hincrbyfloat命令
void hincrbyfloatCommand(client *c) {
    long double value, incr;
    long long ll;
    robj *o;
    sds new;
    unsigned char *vstr;
    unsigned int vlen;

    //增加的值，类型转换，值处理
    if (getLongDoubleFromObjectOrReply(c,c->argv[3],&incr,NULL) != C_OK) return;
    //根据key查找
    if ((o = hashTypeLookupWriteOrCreate(c,c->argv[1])) == NULL) return;
    //获取原来的值
    if (hashTypeGetValue(o,c->argv[2]->ptr,&vstr,&vlen,&ll) == C_OK) {
        if (vstr) {
            if (string2ld((char*)vstr,vlen,&value) == 0) {
                addReplyError(c,"hash value is not a float");
                return;
            }
        } else {
            value = (long double)ll;
        }
    } else {
        value = 0;
    }

    //增加
    value += incr;
    if (isnan(value) || isinf(value)) {
        addReplyError(c,"increment would produce NaN or Infinity");
        return;
    }

    char buf[MAX_LONG_DOUBLE_CHARS];
    int len = ld2string(buf,sizeof(buf),value,1);
    new = sdsnewlen(buf,len);
    //修改value
    hashTypeSet(o,c->argv[2]->ptr,new,HASH_SET_TAKE_VALUE);
    addReplyBulkCBuffer(c,buf,len);
    signalModifiedKey(c->db,c->argv[1]);
    notifyKeyspaceEvent(NOTIFY_HASH,"hincrbyfloat",c->argv[1],c->db->id);
    server.dirty++;

    /* Always replicate HINCRBYFLOAT as an HSET command with the final value
     * in order to make sure that differences in float pricision or formatting
     * will not create differences in replicas or after an AOF restart. */

    //新值引用次数增加，老值引用次数减少
    robj *aux, *newobj;
    aux = createStringObject("HSET",4);
    newobj = createRawStringObject(buf,len);
    rewriteClientCommandArgument(c,0,aux);
    decrRefCount(aux);
    rewriteClientCommandArgument(c,3,newobj);
    decrRefCount(newobj);
}

5. 长度统计

//hlen命令
void hlenCommand(client *c) {
    robj *o;

    if ((o = lookupKeyReadOrReply(c,c->argv[1],shared.czero)) == NULL ||
        checkType(c,o,OBJ_HASH)) return;

    addReplyLongLong(c,hashTypeLength(o));
}

//hstrlen命令
void hstrlenCommand(client *c) {
    robj *o;

    if ((o = lookupKeyReadOrReply(c,c->argv[1],shared.czero)) == NULL ||
        checkType(c,o,OBJ_HASH)) return;
    addReplyLongLong(c,hashTypeGetValueLength(o,c->argv[2]->ptr));
}

6. 获取所有

//hkeys命令 OBJ_HASH_KEY
void hkeysCommand(client *c) {
    genericHgetallCommand(c,OBJ_HASH_KEY);
}

//hvals命令 OBJ_HASH_VALUE
void hvalsCommand(client *c) {
    genericHgetallCommand(c,OBJ_HASH_VALUE);
}

//hgetall命令 OBJ_HASH_KEY|OBJ_HASH_VALUE
void hgetallCommand(client *c) {
    genericHgetallCommand(c,OBJ_HASH_KEY|OBJ_HASH_VALUE);
}

void genericHgetallCommand(client *c, int flags) {
    robj *o;
    hashTypeIterator *hi;
    int multiplier = 0;
    int length, count = 0;

    if ((o = lookupKeyReadOrReply(c,c->argv[1],shared.emptymultibulk)) == NULL
        || checkType(c,o,OBJ_HASH)) return;

    if (flags & OBJ_HASH_KEY) multiplier++;
    if (flags & OBJ_HASH_VALUE) multiplier++;

    length = hashTypeLength(o) * multiplier;
    addReplyMultiBulkLen(c, length);

    hi = hashTypeInitIterator(o);
    //遍历dict
    while (hashTypeNext(hi) != C_ERR) {
        if (flags & OBJ_HASH_KEY) {
            addHashIteratorCursorToReply(c, hi, OBJ_HASH_KEY);
            count++;
        }
        if (flags & OBJ_HASH_VALUE) {
            addHashIteratorCursorToReply(c, hi, OBJ_HASH_VALUE);
            count++;
        }
    }

    hashTypeReleaseIterator(hi);
    serverAssert(count == length);
}