Module dtoa 

Redis’s double printer, which is Grisu2 by way of fpconv.

Redis 7 stopped writing doubles with %.17g and started calling fpconv_dtoa, a small Grisu2 implementation it vendors under deps/fpconv. Every double a client reads comes out of it: a sorted set score, an INCRBYFLOAT reply, a geo distance, a RESP3 double. So the exact bytes it produces are part of the protocol, and there are two reasons a shortest round trip printer from somewhere else does not reproduce them.

The first is presentation. fpconv switches to an exponent for a large or a small magnitude and Rust’s Display never does, so 1e+30 and 1000000000000000000000000000000 are the same number written two ways and a client comparing bytes sees two different answers. That part could have been fixed by reshaping Rust’s digits.

The second is the digits themselves, and it could not. Grisu2 is not always shortest. It finds the shortest representation for about nineteen values in twenty and emits one extra digit for the rest, and which values fall in which set is a property of the algorithm rather than of the number. Rust’s printer is always shortest. So no amount of reformatting closes the gap, and the only way to answer what Redis answers is to run what Redis runs.

This is that port, kept deliberately line for line with the C so that the next time Redis changes it the diff is readable. The one thing that could not be carried across literally is unsigned overflow: C leaves it wrapping and Rust panics on it in a debug build, so every place the original relies on wrapping is spelled out with a wrapping_ call.

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Constants

MAX

Room for anything dtoa can write.

Functions

dtoa

Writes d into dest exactly as Redis’s fpconv_dtoa would, and answers how many bytes it wrote.