json_parser.lua

json = {}
json.null = {}

function kind_of(obj)
	if type(obj) ~= "table" then
		return type(obj)
	end
	local i = 1
	for _ in pairs(obj) do
		if obj[i] ~= nil then
			i = i + 1
		else
			return "table"
		end
	end
	if i == 1 then
		return "table"
	else
		return "array"
	end
end

function escape_str(s)
	local in_char = {"\\", '"', "/", "\b", "\f", "\n", "\r", "\t"}
	local out_char = {"\\", '"', "/", "b", "f", "n", "r", "t"}
	for i, c in ipairs(in_char) do
		s = s:gsub(c, "\\" .. out_char[i])
	end
	return s
end

function skip_delim(str, pos, delim, err_if_missing)
	pos = pos + #str:match("^%s*", pos)
	if str:sub(pos, pos) ~= delim then
		if err_if_missing then
			error("Expected " .. delim .. " near position " .. pos)
		end
		return pos, false
	end
	return pos + 1, true
end

function parse_str_val(str, pos, val)
	val = val or ""
	local early_end_error = "End of input found while parsing string."
	if pos > #str then
		error(early_end_error)
	end
	local c = str:sub(pos, pos)
	if c == '"' then
		return val, pos + 1
	end
	if c ~= "\\" then
		return parse_str_val(str, pos + 1, val .. c)
	end
	local esc_map = {b = "\b", f = "\f", n = "\n", r = "\r", t = "\t"}
	local nextc = str:sub(pos + 1, pos + 1)
	if not nextc then
		error(early_end_error)
	end
	return parse_str_val(str, pos + 2, val .. (esc_map[nextc] or nextc))
end

function parse_num_val(str, pos)
	local num_str = str:match("^-?%d+%.?%d*[eE]?[+-]?%d*", pos)
	local val = tonumber(num_str)
	if not val then
		error("Error parsing number at position " .. pos .. ".")
	end
	return val, pos + #num_str
end

function json.stringify(obj, as_key)
	local s = {}
	local kind = kind_of(obj)
	if kind == "array" then
		if as_key then
			error("Can't encode array as key.")
		end
		s[#s + 1] = "["
		for i, val in ipairs(obj) do
			if i > 1 then
				s[#s + 1] = ", "
			end
			s[#s + 1] = json.stringify(val)
		end
		s[#s + 1] = "]"
	elseif kind == "table" then
		if as_key then
			error("Can't encode table as key.")
		end
		s[#s + 1] = "{"
		for k, v in pairs(obj) do
			if #s > 1 then
				s[#s + 1] = ", "
			end
			s[#s + 1] = json.stringify(k, true)
			s[#s + 1] = ":"
			s[#s + 1] = json.stringify(v)
		end
		s[#s + 1] = "}"
	elseif kind == "string" then
		return '"' .. escape_str(obj) .. '"'
	elseif kind == "number" then
		if as_key then
			return '"' .. tostring(obj) .. '"'
		end
		return tostring(obj)
	elseif kind == "boolean" then
		return tostring(obj)
	elseif kind == "nil" then
		return "null"
	else
		error("Unjsonifiable type: " .. kind .. ".")
	end
	return table.concat(s)
end

function json.parse(str, pos, end_delim)
	pos = pos or 1
	if pos > #str then
		error("Reached unexpected end of input.")
	end
	pos = pos + #str:match("^%s*", pos)
	local first = str:sub(pos, pos)
	if first == "{" then
		local obj, key, delim_found = {}, true, true
		pos = pos + 1
		while true do
			key, pos = json.parse(str, pos, "}")
			if key == nil then
				return obj, pos
			end
			if not delim_found then
				error("Comma missing between object items.")
			end
			pos = skip_delim(str, pos, ":", true)
			obj[key], pos = json.parse(str, pos)
			pos, delim_found = skip_delim(str, pos, ",")
		end
	elseif first == "[" then
		local arr, val, delim_found = {}, true, true
		pos = pos + 1
		while true do
			val, pos = json.parse(str, pos, "]")
			if val == nil then
				return arr, pos
			end
			if not delim_found then
				error("Comma missing between array items.")
			end
			arr[#arr + 1] = val
			pos, delim_found = skip_delim(str, pos, ",")
		end
	elseif first == '"' then
		return parse_str_val(str, pos + 1)
	elseif first == "-" or first:match("%d") then
		return parse_num_val(str, pos)
	elseif first == end_delim then
		return nil, pos + 1
	else
		local literals = {["true"] = true, ["false"] = false, ["null"] = json.null}
		for lit_str, lit_val in pairs(literals) do
			local lit_end = pos + #lit_str - 1
			if str:sub(pos, lit_end) == lit_str then
				return lit_val, lit_end + 1
			end
		end
		local pos_info_str = "position " .. pos .. ": " .. str:sub(pos, pos + 10)
		error("Invalid json syntax starting at " .. pos_info_str)
	end
end

function dump(o)
	if type(o) == "table" then
		local s = "{ "
		local first_key = true
		for k, v in pairs(o) do
			if not first_key then
				s = s .. ", "
			end
			first_key = false
			if type(k) == "number" then
				s = s .. "[" .. k .. "]"
			elseif type(k) == "string" and k:match("^[%a_][%w_]*$") and k ~= "function" then
				s = s .. k
			else
				s = s .. '["' .. k .. '"]'
			end
			s = s .. " = " .. dump(v)
		end
		return s .. " }"
	elseif type(o) == "string" then
		if string.find(o, '"') then
			return "'" .. o .. "'"
		else
			return '"' .. o .. '"'
		end
	else
		return tostring(o)
	end
end