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Editorial Team
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Asked: 2026-05-14T19:55:24+00:00

in Redis ( http://code.google.com/p/redis ) there are scores associated to elements, in order to

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in Redis (http://code.google.com/p/redis) there are scores associated to elements, in order to take this elements sorted. This scores are doubles, even if many users actually sort by integers (for instance unix times).

When the database is saved we need to write this doubles ok disk. This is what is used currently:

  snprintf((char*)buf+1,sizeof(buf)-1,"%.17g",val);

Additionally infinity and not-a-number conditions are checked in order to also represent this in the final database file.

Unfortunately converting a double into the string representation is pretty slow. While we have a function in Redis that converts an integer into a string representation in a much faster way. So my idea was to check if a double could be casted into an integer without lost of data, and then using the function to turn the integer into a string if this is true.

For this to provide a good speedup of course the test for integer “equivalence” must be fast. So I used a trick that is probably undefined behavior but that worked very well in practice. Something like that:

double x = ... some value ...
if (x == (double)((long long)x))
    use_the_fast_integer_function((long long)x);
else
    use_the_slow_snprintf(x);

In my reasoning the double casting above converts the double into a long, and then back into an integer. If the range fits, and there is no decimal part, the number will survive the conversion and will be exactly the same as the initial number.

As I wanted to make sure this will not break things in some system, I joined #c on freenode and I got a lot of insults 😉 So I’m now trying here.

Is there a standard way to do what I’m trying to do without going outside ANSI C? Otherwise, is the above code supposed to work in all the Posix systems that currently Redis targets? That is, archs where Linux / Mac OS X / *BSD / Solaris are running nowaday?

What I can add in order to make the code saner is an explicit check for the range of the double before trying the cast at all.

Thank you for any help.

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1 Answer

  1. Editorial Team

    Perhaps some old fashion fixed point math could help you out. If you converted your double to a fixed point value, you still get decimal precision and converting to a string is as easy as with ints with the addition of a single shift function.

    Another thought would be to roll your own snprintf() function. Doing the conversion from double to int is natively supported by many FPU units so that should be lightning fast. Converting that to a string is simple as well.

    Just a few random ideas for you.

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