json.lua

--
-- JSON
--
-- Internal functions.
json = {}

local 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

local 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

-- Returns pos, did_find; there are two cases:
-- 1. Delimiter found: pos = pos after leading space + delim; did_find = true.
-- 2. Delimiter not found: pos = pos after leading space;     did_find = false.
-- This throws an error if err_if_missing is true and the delim is not found.
local 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

-- Expects the given pos to be the first character after the opening quote.
-- Returns val, pos; the returned pos is after the closing quote character.
local 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
	-- We must have a \ character.
	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

-- Returns val, pos; the returned pos is after the number's final character.
local 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

json.null = {} -- This is a one-off table to represent the null value.

function json.parse(str, pos, end_delim)
	pos = pos or 1
	if pos > #str then error('Reached unexpected end of input.' .. str) end
	local pos = pos + #str:match('^%s*', pos) -- Skip whitespace.
	local first = str:sub(pos, pos)
	if first == '{' then                   -- Parse an object.
		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) -- true -> error if missing.
			obj[key], pos = json.parse(str, pos)
			pos, delim_found = skip_delim(str, pos, ',')
		end
	elseif first == '[' then -- Parse an array.
		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                   -- Parse a string.
		return parse_str_val(str, pos + 1)
	elseif first == '-' or first:match('%d') then -- Parse a number.
		return parse_num_val(str, pos)
	elseif first == end_delim then             -- End of an object or array.
		return nil, pos + 1
	else                                       -- Parse true, false, or null.
		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 .. ': ' .. str)
	end
end