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[SR-9641] JSONSerialization inconsistent floating-point conversions #3559

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swift-ci opened this issue Jan 11, 2019 · 5 comments
Open

[SR-9641] JSONSerialization inconsistent floating-point conversions #3559

swift-ci opened this issue Jan 11, 2019 · 5 comments
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@swift-ci
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Previous ID SR-9641
Radar rdar://problem/47255325
Original Reporter dbplunkett (JIRA User)
Type Bug
Environment

Xcode 10.1 build 10B61
Swift 4.2.1

Additional Detail from JIRA
Votes 0
Component/s Foundation
Labels Bug
Assignee bendjones (JIRA)
Priority Medium

md5: e796154a189d9afb62080e631baa45e4

Issue Description:

Some floats change when decoded by JSONSerialization. I believe JSONSerialization is supposed to losslessly round-trip all single- and double-precision floats. The problem seems to be in the decoding, because instantiating a number directly from the encoded string doesn't exhibit this problem.

Three example values are below: One that always changes when decoded, one that changes ONLY when decoded as a root value via `allowFragments`, and one that becomes NaN when decoded (therefore failing to decode at all).

let number = NSNumber(value: 0.0003) // changes when decoded
//let number = NSNumber(value: 0.000001) // changes when decoded as a root value
//let number = NSNumber(value: Double.greatestFiniteMagnitude) // changes to NaN when decoded

let encodedArray = try! JSONSerialization.data(withJSONObject: [number])
let encodedNumber = encodedArray.dropFirst().dropLast()

NSNumber(value: Double(String(data: encodedNumber, encoding: .utf8)!)!) == number // this always succeeds

let decodedArray = try! JSONSerialization.jsonObject(with: encodedArray) as! [NSNumber]
let decodedNumber = try! JSONSerialization.jsonObject(with: encodedNumber, options: .allowFragments) as! NSNumber

decodedArray[0] == number
decodedNumber == number
@belkadan
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0.0003 can't be exactly represented as a float value in the first place, but I guess it shouldn't actually change. cc @phausler and @itaiferber to link with other float serialization bug reports.

@itaiferber
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These are issues with {{NSJSONSerialization}} that we should fix from the Objective-C side. (I can reproduce with transliterated Obj-C code.)

@swift-ci
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Comment by Ben D. Jones (JIRA)

Thanks for the radar Itai. I’ll take a look today while a have a bit of time.

@swift-ci
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Comment by David Plunkett (JIRA)

@belkadan I debated how explicitly I should call out that it doesn't seem like a misunderstanding of float representation on my part is the problem - but yeah, I realize the literals in my code are not exactly representable. I left them that way for brevity.

@belkadan
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:-) We've seen enough people skip that first step that I had to say it, but your use of == demonstrates that you're not falling in that trap.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
@shahmishal shahmishal transferred this issue from apple/swift May 5, 2022
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@itaiferber @belkadan @swift-ci