Deserialization of simple Lua table stored as string - lua

Question

I'm transfering a lua table literal in a string from a web application in to PICO-8 that I'm trying to deserialize back in to a lua table in PICO-8.
The string is in the form '{"top", {"one", {"one a", "one b"}}, {"two", {"two a", "two b"}}}'
To try and keep things simple I'm only going to include lowercase characters in the strings and only strings are allowed in the nested tables.
I feel like I've got a grasp on parsing the characters, but I don't know how to keep track of where I am in the recreated data, both the depth of the structure and the index.
How is this usually done?
The catch is that as PICO-8 lua doesn't contain load or loadstring the parsing has to be done manually. The following code is using table.insert and string.sub instead of the PICO-8 equivalents because I'm using a lua REPL to help prototype this code.
Here is what I have so far with print statements what I think I need to do where.
Any help would be greatly appreciated.
test_obj = {"top", {"one", {"one a", "one b"}}, {"two", {"two a", "two b"}}}
data_string = '{"top", {"one", {"one a", "one b"}}, {"two", {"two a", "two b"}}}'
data = nil
string = ''
level = 0
while #data_string > 0 do
local d=string.sub(data_string,1,1)
if stringChar(d) then
string = string..d
end
if comma(d) then
print(string)
table.insert(data, string)
string = ''
end
if openBracket(d) then
if data == nil then
data = {}
print('new table')
else
print('insert table')
end
level = level + 1
print('on level', level)
end
if closeBracket(d) then
print('end of table')
level = level - 1
print('up to level', level)
end
data_string=string.sub(data_string,2)
end

Answer 1

Use Lua patterns to avoid parsing each character
local function read_exp_list(s)
local exps, res = {}, {}
local function save(v)
exps[#exps + 1] = v
return ('\0'):rep(#exps)
end
s = s:gsub('%b{}', function(s) return save{read_exp_list(s:sub(2, -2))} end) -- arrays
s = s:gsub('"(.-)"', save) -- strings
s = s:gsub('%-?%d+', function(s) return save(tonumber(s)) end) -- integer numbers
for k in s:gmatch'%z+' do
res[#res + 1] = exps[#k]
end
return (table.unpack or unpack)(res)
end
local data_string = '{-42, "top", {"one", {"one a", "one b"}}, {"two", {"two a", "two b"}}}'
local obj = read_exp_list(data_string)
-- obj == {-42, "top", {"one", {"one a", "one b"}}, {"two", {"two a", "two b"}}}
Strings must be enclosed in " and must not contain characters {}\ inside. String may be empty.
Numbers must be integer in decimal notation with optional minus.
Arrays must contain only strings, numbers and subarrays. Array may be empty.
UPDATE:
The following code only uses functions string.sub, table.insert, tonumber, type
local function is_digit(c)
return c >= '0' and c <= '9'
end
local function read_from_string(input)
if type(input) == 'string' then
local data = input
local pos = 0
function input(undo)
if undo then
pos = pos - 1
else
pos = pos + 1
return string.sub(data, pos, pos)
end
end
end
local c
repeat
c = input()
until c ~= ' ' and c ~= ','
if c == '"' then
local s = ''
repeat
c = input()
if c == '"' then
return s
end
s = s..c
until c == ''
elseif c == '-' or is_digit(c) then
local s = c
repeat
c = input()
local d = is_digit(c)
if d then
s = s..c
end
until not d
input(true)
return tonumber(s)
elseif c == '{' then
local arr = {}
local elem
repeat
elem = read_from_string(input)
table.insert(arr, elem)
until not elem
return arr
end
end
local data_string = '{-42, "top", {"one", {"one a", "one b"}}, {"two", {"two a", "two b"}}}'
local obj = read_from_string(data_string)
-- obj == {-42, "top", {"one", {"one a", "one b"}}, {"two", {"two a", "two b"}}}
Strings must be enclosed in " and must not contain character \ inside. String may be empty.
Numbers must be integer in decimal notation with optional minus.
Arrays must contain only strings, numbers and subarrays. Array may be empty.