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Improve JSON serialization performance on Linux #718

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Merged
merged 3 commits into from
Nov 24, 2016
Merged

Improve JSON serialization performance on Linux #718

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150f071
Improve performance on Linux by using String concatenation and only c…
djones6 Nov 16, 2016
867109e
Add performance test case for JSON serialization
djones6 Nov 17, 2016
887d20c
Revert "Add performance test case for JSON serialization"
djones6 Nov 21, 2016
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13 changes: 10 additions & 3 deletions Foundation/NSJSONSerialization.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -101,14 +101,13 @@ open class JSONSerialization : NSObject {
/* Generate JSON data from a Foundation object. If the object will not produce valid JSON then an exception will be thrown. Setting the NSJSONWritingPrettyPrinted option will generate JSON with whitespace designed to make the output more readable. If that option is not set, the most compact possible JSON will be generated. If an error occurs, the error parameter will be set and the return value will be nil. The resulting data is a encoded in UTF-8.
*/
internal class func _data(withJSONObject value: Any, options opt: WritingOptions, stream: Bool) throws -> Data {
var result = Data()
var jsonStr = String()

var writer = JSONWriter(
pretty: opt.contains(.prettyPrinted),
writer: { (str: String?) in
if let str = str {
let count = str.lengthOfBytes(using: .utf8)
result.append(UnsafeRawPointer(str.cString(using: .utf8)!).bindMemory(to: UInt8.self, capacity: count), count: count)
jsonStr.append(str)
}
}
)
Expand All @@ -131,6 +130,14 @@ open class JSONSerialization : NSObject {
}
}

let count = jsonStr.lengthOfBytes(using: .utf8)
let bufferLength = count+1 // Allow space for null terminator
var utf8: [CChar] = Array<CChar>(repeating: 0, count: bufferLength)
if !jsonStr.getCString(&utf8, maxLength: bufferLength, encoding: .utf8) {
fatalError("Failed to generate a CString from a String")
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@parkera parkera Nov 17, 2016 •
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I realize we had a force-unwrap here in the previous code, but why would getCString fail here? If it did, is there a way to recover instead of asserting?

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Good question. The API documentation for Swift String.getCString does not specify what the return value means, but I gather from the implementation that it calls through to NSString.getCString, the documentation for which says that false would be returned if the buffer is too small, or an encoding error occurs.
In this case, we sized the buffer appropriately, and because UTF8 can represent all Unicode code points, we shouldn't encounter an encoding error. Perhaps it would be safe to ignore the return value in this case?

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If we're sure it should not happen, then asserting on that is the right answer instead of ignoring.

}
let rawBytes = UnsafeRawPointer(UnsafePointer(utf8))
let result = Data(bytes: rawBytes.bindMemory(to: UInt8.self, capacity: count), count: count)
return result
}
open class func data(withJSONObject value: Any, options opt: WritingOptions = []) throws -> Data {
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