fix: zscore return 0 when double value is too small

[zsh1995]

I use miniredis to zadd 0.000000000000000000000000000000000000000000000000000000000000000025339988685347402, then zscore command return a 0, while using real redis server can correctly execute the zadd and zscore command. I found this issuse: https://github.com/alicebob/miniredis/pull/17 already trying to fix this problem, but didn't pass integration test. I read redis's source code https://github.com/redis/redis/blob/unstable/deps/fpconv/fpconv_dtoa.c#L349 , I believe my approach is basically consistent with it.

[alicebob]

Hi! Thanks for the PR. That was already painful to get working back then :( Currently the tests fail with the normal floating point problems...

[zsh1995]

Thanks Reply! I try this on redis:

127.0.0.1:7079> zadd a_zset 1.2 one
(integer) 0
127.0.0.1:7079> zadd a_zset incr 1.2 one
"2.3999999999999999"
127.0.0.1:7079> zadd a_zset incr 1.2 one
"3.5999999999999996"
127.0.0.1:7079> zadd a_zset incr 1.2 one
"4.7999999999999998"

It seems that if we want to be consistent with redis, we need to adjust the test cases

[alicebob]

On Wed, Sep 20, 2023 at 08:13:32PM -0700, zsh1995 wrote:

Thanks Reply! I try this on redis:

127.0.0.1:7079> zadd a_zset 1.2 one
(integer) 0
127.0.0.1:7079> zadd a_zset incr 1.2 one
"2.3999999999999999"
127.0.0.1:7079> zadd a_zset incr 1.2 one
"3.5999999999999996"
127.0.0.1:7079> zadd a_zset incr 1.2 one
"4.7999999999999998"

It seems that if we want to be consistent with redis, we need to adjust the test cases

Oh, that's interesting. I'll have a proper look next week.

[alicebob]

I'm getting different results, with redis 7.2 on an AMD64 machine:

~/src/miniredis/integration/redis_src$ ./redis-cli 
127.0.0.1:6379> zadd a_zset 1.2 one
(integer) 0
127.0.0.1:6379> zadd a_zset incr 1.2 one
"2.4"
127.0.0.1:6379> zadd a_zset incr 1.2 one
"3.5999999999999996"
127.0.0.1:6379> zadd a_zset incr 1.2 one
"4.8"
127.0.0.1:6379> 

[alicebob]

Redis uses "grisu2" from pdf linked in https://florian.loitsch.com/publications --> https://drive.google.com/file/d/0BwvYOx00EwKmcTZUM0Q4VW1DTHc/view?resourcekey=0-gW-moqRGyWpqFsyhiBI_-w

Go does not do exactly the same thing, but short of implementing grisu2 in Go (sounds fun!), I think this is Good Enough. You were absolutely right in that it should use the "g" option to convert the float (meaning: switch to scientific if needed).

[alicebob]

short of implementing grisu2 in Go

I did just that (#347), but it needs a bit more work still.

[alicebob]

Thanks, merged! I'll merge the grisu pr over this as well, so the formatting in real and mini redis should be identical.