Encoding runs in worst-case quadratic time

[appgurueu]

Consider the following benchmark which encodes the deeply nested structure [1000, [999, [998, [...]]]]:

local json = require"json"
for _ = 1, 1e3 do
	local t = {}
	for i = 1, 1e3 do
		t = {i, t}
	end
	json.encode(t)
end

this should run in a fraction of a second. Instead, it takes ~2 seconds on my device. This is because string concatenations / recursive table.concat are internally used to encode the JSON:

return "[" .. table.concat(res, ",") .. "]"

This means that the outer loop will have to add the [ and ] brackets in time linear in table.concat(res, ","), which in our example will be just marginally shorter, which in turn will only differ in length by a constant. Thus you do a linear time operation linearly often, resulting in worst-case quadratic encoding runtime in the depth of the nested structure.

The proper fix is to use a shared rope and have all recursive calls write to the same rope. You then only concatenate strings once at the end. Building the string then runs in linear time. You can find an example of this approach in my JSON implementation.

[flamendless]

Perhaps you could make a PR implementing that solution :)

[Vurv78]

If you want a fast implementation: https://github.com/Vurv78/qjson.lua

Runs thousands of times faster than this repo

[appgurueu]

If you want a fast implementation: https://github.com/Vurv78/qjson.lua

Runs thousands of times faster than this repo

Refer to this comment by me.

TL;DR: Your JSON parser is broken and has a time complexity issue as well. Your performance claims in their current form can't be taken seriously.

[Vurv78]

If you want a fast implementation: https://github.com/Vurv78/qjson.lua
Runs thousands of times faster than this repo

Refer to this comment by me.

TL;DR: Your JSON parser is broken and has a time complexity issue as well. Your performance claims in their current form can't be taken seriously.

Both of these have been addressed. Thanks for bringing it to my attention (also see my other comment)