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Improved table encoding code to handle sparse arrays. #28

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ad6ac0f
Improved table encoding code to handle sparse arrays. Floating point …
aryajur Jul 19, 2020
32a59c8
Updated code to allow array specifications using table.pack function.…
aryajur Aug 19, 2020
f521e4a
nLen initialization fixed
aryajur Aug 23, 2020
75127d2
Added check to prevent sparse arrays > 100 getting through
aryajur Nov 25, 2023
0e6634a
Improved indentation by using tab. Added module variable SPARSELIMIT …
aryajur Nov 28, 2023
8ce4842
Fixed bug for array encoding
aryajur Aug 27, 2025
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90 changes: 51 additions & 39 deletions json.lua
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Original file line number Diff line number Diff line change
Expand Up @@ -57,44 +57,56 @@ end


local function encode_table(val, stack)
local res = {}
stack = stack or {}

-- Circular reference?
if stack[val] then error("circular reference") end

stack[val] = true

if rawget(val, 1) ~= nil or next(val) == nil then
-- Treat as array -- check keys are valid and it is not sparse
local n = 0
for k in pairs(val) do
if type(k) ~= "number" then
error("invalid table: mixed or invalid key types")
end
n = n + 1
end
if n ~= #val then
error("invalid table: sparse array")
end
-- Encode
for i, v in ipairs(val) do
table.insert(res, encode(v, stack))
end
stack[val] = nil
return "[" .. table.concat(res, ",") .. "]"

else
-- Treat as an object
for k, v in pairs(val) do
if type(k) ~= "string" then
error("invalid table: mixed or invalid key types")
end
table.insert(res, encode(k, stack) .. ":" .. encode(v, stack))
end
stack[val] = nil
return "{" .. table.concat(res, ",") .. "}"
end
local res = {}
stack = stack or {}

-- Circular reference?
if stack[val] then error("circular reference") end

stack[val] = true
-- Check whether to treat as a array or object
local array = true
local length = 0
local nLen = 0
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@appgurueu appgurueu Dec 17, 2022

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Have you considered the worst-case this might create: What if I serialize {[1e9] = true}?

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@aryajur aryajur Nov 25, 2023

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Missed all these. Revisiting after a long time. Thanks for catching that. Had not considered it at all. Been using it, did not have issues. Fixed it in the next commit by preventing sparse arrays greater than 100 (arbitrarility) length getting through. Seems like other json modules also handle sparse arrays in a undefined way. 100 should be good for many cases I suppose.

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@appgurueu appgurueu Nov 25, 2023

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Yeah, it's a tricky issue, and behavior varies wildly. I believe ideally it should be left up to the user. Perhaps use something like a "density" factor as a threshold?

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@aryajur aryajur Nov 28, 2023

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I have added a variable called SPARSELIMIT in the module that the user can change after requiring the module to change it. Default set it to 100.

local count = 0
for k,v in pairs(val) do
if (type(k) ~= "number" or k<=0) and not (k == "n" and type(v) == "number") then
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@appgurueu appgurueu Nov 25, 2023

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What about fractional keys, like 4.2? I think you also need a check like k % 1 == 0 or math.floor(k) == k to check that k is (not) an integer.

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@aryajur aryajur Nov 28, 2023

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Thank you, added k%1 ~= 0 in the condition.

array = nil
break -- Treat as object
else
if k > length then
length = k
end
if k == "n" and type(v) == "number" then
nLen = v
end
count = count + 1
end
end

if nLen > length then
length = nLen
end
if array and not (length > 100 and count ~= length) then -- Check Array detected but sparse > 100 length then treat as object
-- Encode
for i=1,length do
table.insert(res, encode(val[i], stack))
end
stack[val] = nil
return "[" .. table.concat(res, ",") .. "]"
else
-- Treat as an object
for k, v in pairs(val) do
--[[
if type(k) ~= "string" then
error("invalid table: mixed or invalid key types")
end
]]
table.insert(res, encode(k, stack) .. ":" .. encode(v, stack))
end
stack[val] = nil
return "{" .. table.concat(res, ",") .. "}"
end
end


Expand All @@ -108,7 +120,7 @@ local function encode_number(val)
if val ~= val or val <= -math.huge or val >= math.huge then
error("unexpected number value '" .. tostring(val) .. "'")
end
return string.format("%.14g", val)
return tostring(val)
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@appgurueu appgurueu Dec 17, 2022

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Unreliable.

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@aryajur aryajur Nov 25, 2023

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Don't understand. Perhaps a more useful/productive comment? Thanks.

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@appgurueu appgurueu Nov 25, 2023

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Sorry, that was indeed too short to be useful. The documentation of tostring says:

Receives an argument of any type and converts it to a string in a reasonable format. For complete control of how numbers are converted, use string.format.

It doesn't guarantee anything. It could output the number with arbitrarily poor precision, or it could use an arbitrary format (which need not be valid JSON). (In practice, it does indeed behave quite reasonable, but you shouldn't rely on this; it should also be noted that it is not "lossless" in practice, it rounds (albeit neither is the current method - 14 digits aren't enough).)

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@aryajur aryajur Nov 28, 2023

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Thank you for explaining it. I see. I remember I had changed it to make a certain case work properly when I was using it in the oauth2 module. Unfortunately I did not document why I changed it. I think I switched it back in the repository for now.

end


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