Small error when parsing long float value

[Mark-Simulacrum]

JavaScript's JSON.parse doesn't lose precision from my tests.

Test case below:

#[test]
fn large_precision() {
    assert_eq!(parse("0.004750000000000001").unwrap().as_f64().unwrap(), 0.004750000000000001);
}

[Mark-Simulacrum]

If it helps, I have quite a few cases similar to this one. Let me know if they'd be helpful.

[dtolnay]

This error comes from the fact that in floating point arithmetic, 4750000000000001f64 * 10e-19f64 is 0.004750000000000002. @maciejhirsz Serde uses 4750000000000001f64 / 10e19f64 because it is more accurate.

[maciejhirsz]

Thanks for the tip @dtolnay.

@Mark-Simulacrum sure, I'd gladly include any edge cases in my tests.

[Mark-Simulacrum]

Warning: the files are large; so wget vs opening in browser is a good idea (Chrome, at least, was unable to do anything beneficial with the file: no display and lots of CPU/memory usage).

Some of these may be false (actually correct serialization), but this is a sort | uniq | diff -u of the output of the program that "found" this error: https://gist.githubusercontent.com/Mark-Simulacrum/280d5b4530ae68f958b3b3542bb613e1/raw/120c3ba89daa9a526225496673e5e19f396f0de3/rust-json.diff. I'm not sure how helpful this diff is, but it's a start.

Also, I've found that I'm not 100% sure that Serde's implementation is equivalent to the JS version either; here's the same type of file for the Serde impl: https://gist.githubusercontent.com/Mark-Simulacrum/05431fd1e1048ee1924879cdde2e0fb9/raw/462391ec7e17acb1ae4a32ae852bd45bbda4d2ab/serde.diff. Serde seems to have less errors, but there do appear to still be errors.

[maciejhirsz]

Thanks for that, very helpful!

[Mark-Simulacrum]

@maciejhirsz I'm thinking we should either reopen this or I can move this into #85; still seeing errors with 0.10.1. Diff included (12M) this time I'm nearly 97% certain on the errors since I included more information in the output: https://gist.githubusercontent.com/Mark-Simulacrum/0a26fadc129dc99c441a3df0b42ae9d7/raw/18c0e5bd85939841446706614173cec6c2be51e5/erroring.diff

[Mark-Simulacrum]

I can also provide the raw JSON files (they're actually already on Github (see rust-lang-nursery/rustc-perf and the instructions there as to how to get the "data" directory) if that helps, although from the 1-2 values I've checked everything matches (JS processed the number correctly).

[Mark-Simulacrum]

@dtolnay I reran my serde_json version and diffed that with the latest diff link, and Serde now has exactly the same problems as json-rust. If you'd like I can file an issue with the diff/errors on serde_json's repo.

[maciejhirsz]

@Mark-Simulacrum not sure if I'm reading this right, did the error rate went up with 0.10.1? The first diff is a fraction of this one.