Speed up JSON Parsing

[argon]

I ran 2 benchmarks against the current JSON parser implementation on master, comparing it to Darwin Foundation for large inputs and found it was 5-10x slower. (72s vs 8s for a single decode). This new code reduces this to a factor for 2-4x (17s)

While JSON is a text-based serialization, all control characters are ASCII characters. The data can be most efficiently parsed as a stream of bytes instead of the initial overhead of converting the bytes to a String.

The byte -> string conversion in the current code only accounts for 3s of time but parsing numbers is incredibly expensive (Double.init?(_ text: String) converts the string back to bytes to use strtod()). This technique uses strtod directly on the byte array. There is a penalty for non-UTF-8 encoded data which is inline with the Darwin implementation

The most efficient encoding to use for parsing is UTF-8, so if you have a choice in encoding the data passed to this method, use UTF-8.

Another modification has been made to remove the intermediate parser values as the heap allocations for each intermediate parser was adding significant overhead.

This also includes a commit to implement .AllowFragments.

[phausler]

from a cursory glance this looks like a reasonable change; builds pass on ubuntu.

Do you happen to have the performance benchmarks handy? that might be something interesting to check in so that we could potentially build up a performance test suite.

Additionally what other hotspots are there in Foundation that you found by this?

[argon]

The performance "benchmark" wasn't in any way automated unfortunately. I simply created a main.swift to read the JSON file from disk then decode it, then I profiled it using instruments.

The main hotspots in this code now relate to

• Array and Dictionary size increases, leading to a large number of copies
• Bridging from NSString -> String in parseString

In general String parsing for the sf-city-lots-json bears a huge time cost ~(5000ms vs 250ms) when compared with Darwin Foundation.