平台:x86_64
结论:Zset的最大分数不要超过18014398509481982(17位数字,54位二进制),否则不会得到期望的值。
REdis:5.0.4
Zset采用double存储分数值(score),而incrbyfloat和hincrbyfloat采用的是long double存储数值。
double本身是可以存储比18014398509481982(17位数字,54位二进制)大的值,比如18014398509481983(也是17位数字,54位二进制)。但REdis在返回结果时,调用strtod将值转成double类型。
取值源代码:
double zzlGetScore(unsigned char *sptr) { // t_zset.c:722
unsigned char *vstr;
unsigned int vlen;
long long vlong;
char buf[128];
double score;
serverAssert(sptr != NULL);
serverAssert(ziplistGet(sptr,&vstr,&vlen,&vlong));
if (vstr) {
memcpy(buf,vstr,vlen);
buf[vlen] = '\0';
// 调用库函数strtod,
// 如果值小于18014398509481983,能得到预期的值,
// 否则总是返回18014398509481984(2的54次方)。
score = strtod(buf,NULL); // 字符串转double
} else {
score = vlong;
}
return score;
}
以字符串形式返回:
/* Add a double as a bulk reply */
void addReplyDouble(client *c, double d) { // networking.c:471
char dbuf[128], sbuf[128];
int dlen, slen;
if (isinf(d)) {
/* Libc in odd systems (Hi Solaris!) will format infinite in a
* different way, so better to handle it in an explicit way. */
addReplyBulkCString(c, d > 0 ? "inf" : "-inf");
} else {
dlen = snprintf(dbuf,sizeof(dbuf),"%.17g",d); // double转字符串
slen = snprintf(sbuf,sizeof(sbuf),"$%d\r\n%s\r\n",dlen,dbuf);
addReplyString(c,sbuf,slen);
}
}
x86_64上的测试:
127.0.0.1:6379> del k1
(integer) 1
127.0.0.1:6379> zadd k1 18014398509481982 m1
(integer) 1
127.0.0.1:6379> zrange k1 0 -1 WITHSCORES
1) "m1"
2) "18014398509481982" 预期的值
127.0.0.1:6379> del k1
(integer) 1
127.0.0.1:6379> zadd k1 18014398509481983 m1
(integer) 1
127.0.0.1:6379> zrange k1 0 -1 WITHSCORES
1) "m1"
2) "18014398509481984" 非预期的值(和strtod相关)
127.0.0.1:6379> del k1
(integer) 1
127.0.0.1:6379> zadd k1 18014398509481982 m1
(integer) 1
127.0.0.1:6379> zrange k1 0 -1 WITHSCORES
1) "m1"
2) "18014398509481982" 预期的值
127.0.0.1:6379> zincrby k1 1 m1
"18014398509481984" 非预期的值
127.0.0.1:6379> zrange k1 0 -1 WITHSCORES
1) "m1"
2) "18014398509481984" 非预期的值
附:C/C++浮点知识图谱