--
-- json.lua
--
-- Copyright (c) 2020 rxi
--
-- Permission is hereby granted, free of charge, to any person obtaining a copy of
-- this software and associated documentation files (the "Software"), to deal in
-- the Software without restriction, including without limitation the rights to
-- use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
-- of the Software, and to permit persons to whom the Software is furnished to do
-- so, subject to the following conditions:
--
-- The above copyright notice and this permission notice shall be included in all
-- copies or substantial portions of the Software.
--
-- THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-- IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-- FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-- AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-- LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-- OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-- SOFTWARE.
--

local json = { _version = "0.1.2" }

-------------------------------------------------------------------------------
-- Encode
-------------------------------------------------------------------------------

local encode

local escape_char_map = {
	[ "\\" ] = "\\",
	[ "\"" ] = "\"",
	[ "\b" ] = "b",
	[ "\f" ] = "f",
	[ "\n" ] = "n",
	[ "\r" ] = "r",
	[ "\t" ] = "t",
}

local escape_char_map_inv = { [ "/" ] = "/" }
for k, v in pairs(escape_char_map) do
	escape_char_map_inv[v] = k
end


local function escape_char(c)
	return "\\" .. (escape_char_map[c] or string.format("u%04x", c:byte()))
end


local function encode_nil(val)
	return "null"
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
end


local function encode_string(val)
	return '"' .. val:gsub('[%z\1-\31\\"]', escape_char) .. '"'
end


local function encode_number(val)
	-- Check for NaN, -inf and inf
	if val ~= val or val <= -math.huge or val >= math.huge then
		error("unexpected number value '" .. tostring(val) .. "'")
	end
	return string.format("%.14g", val)
end


local type_func_map = {
	[ "nil"     ] = encode_nil,
	[ "table"   ] = encode_table,
	[ "string"  ] = encode_string,
	[ "number"  ] = encode_number,
	[ "boolean" ] = tostring,
}


encode = function(val, stack)
	local t = type(val)
	local f = type_func_map[t]
	if f then
		return f(val, stack)
	end
	error("unexpected type '" .. t .. "'")
end


function json.encode(val)
	return ( encode(val) )
end


-------------------------------------------------------------------------------
-- Decode
-------------------------------------------------------------------------------

local parse

local function create_set(...)
	local res = {}
	for i = 1, select("#", ...) do
		res[ select(i, ...) ] = true
	end
	return res
end

local space_chars   = create_set(" ", "\t", "\r", "\n")
local delim_chars   = create_set(" ", "\t", "\r", "\n", "]", "}", ",")
local escape_chars  = create_set("\\", "/", '"', "b", "f", "n", "r", "t", "u")
local literals      = create_set("true", "false", "null")

local literal_map = {
	[ "true"  ] = true,
	[ "false" ] = false,
	[ "null"  ] = nil,
}


local function next_char(str, idx, set, negate)
	for i = idx, #str do
		if set[str:sub(i, i)] ~= negate then
			return i
		end
	end
	return #str + 1
end


local function decode_error(str, idx, msg)
	local line_count = 1
	local col_count = 1
	for i = 1, idx - 1 do
		col_count = col_count + 1
		if str:sub(i, i) == "\n" then
			line_count = line_count + 1
			col_count = 1
		end
	end
	error( string.format("%s at line %d col %d", msg, line_count, col_count) )
end


local function codepoint_to_utf8(n)
	-- http://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&id=iws-appendixa
	local f = math.floor
	if n <= 0x7f then
		return string.char(n)
	elseif n <= 0x7ff then
		return string.char(f(n / 64) + 192, n % 64 + 128)
	elseif n <= 0xffff then
		return string.char(f(n / 4096) + 224, f(n % 4096 / 64) + 128, n % 64 + 128)
	elseif n <= 0x10ffff then
		return string.char(f(n / 262144) + 240, f(n % 262144 / 4096) + 128,
				f(n % 4096 / 64) + 128, n % 64 + 128)
	end
	error( string.format("invalid unicode codepoint '%x'", n) )
end


local function parse_unicode_escape(s)
	local n1 = tonumber( s:sub(1, 4),  16 )
	local n2 = tonumber( s:sub(7, 10), 16 )
	-- Surrogate pair?
	if n2 then
		return codepoint_to_utf8((n1 - 0xd800) * 0x400 + (n2 - 0xdc00) + 0x10000)
	else
		return codepoint_to_utf8(n1)
	end
end


local function parse_string(str, i)
	local res = ""
	local j = i + 1
	local k = j
	
	while j <= #str do
		local x = str:byte(j)
		
		if x < 32 then
			decode_error(str, j, "control character in string")
		
		elseif x == 92 then -- `\`: Escape
			res = res .. str:sub(k, j - 1)
			j = j + 1
			local c = str:sub(j, j)
			if c == "u" then
				local hex = str:match("^[dD][89aAbB]%x%x\\u%x%x%x%x", j + 1)
						or str:match("^%x%x%x%x", j + 1)
						or decode_error(str, j - 1, "invalid unicode escape in string")
				res = res .. parse_unicode_escape(hex)
				j = j + #hex
			else
				if not escape_chars[c] then
					decode_error(str, j - 1, "invalid escape char '" .. c .. "' in string")
				end
				res = res .. escape_char_map_inv[c]
			end
			k = j + 1
		
		elseif x == 34 then -- `"`: End of string
			res = res .. str:sub(k, j - 1)
			return res, j + 1
		end
		
		j = j + 1
	end
	
	decode_error(str, i, "expected closing quote for string")
end


local function parse_number(str, i)
	local x = next_char(str, i, delim_chars)
	local s = str:sub(i, x - 1)
	local n = tonumber(s)
	if not n then
		decode_error(str, i, "invalid number '" .. s .. "'")
	end
	return n, x
end


local function parse_literal(str, i)
	local x = next_char(str, i, delim_chars)
	local word = str:sub(i, x - 1)
	if not literals[word] then
		decode_error(str, i, "invalid literal '" .. word .. "'")
	end
	return literal_map[word], x
end


local function parse_array(str, i)
	local res = {}
	local n = 1
	i = i + 1
	while 1 do
		local x
		i = next_char(str, i, space_chars, true)
		-- Empty / end of array?
		if str:sub(i, i) == "]" then
			i = i + 1
			break
		end
		-- Read token
		x, i = parse(str, i)
		res[n] = x
		n = n + 1
		-- Next token
		i = next_char(str, i, space_chars, true)
		local chr = str:sub(i, i)
		i = i + 1
		if chr == "]" then break end
		if chr ~= "," then decode_error(str, i, "expected ']' or ','") end
	end
	return res, i
end


local function parse_object(str, i)
	local res = {}
	i = i + 1
	while 1 do
		local key, val
		i = next_char(str, i, space_chars, true)
		-- Empty / end of object?
		if str:sub(i, i) == "}" then
			i = i + 1
			break
		end
		-- Read key
		if str:sub(i, i) ~= '"' then
			decode_error(str, i, "expected string for key")
		end
		key, i = parse(str, i)
		-- Read ':' delimiter
		i = next_char(str, i, space_chars, true)
		if str:sub(i, i) ~= ":" then
			decode_error(str, i, "expected ':' after key")
		end
		i = next_char(str, i + 1, space_chars, true)
		-- Read value
		val, i = parse(str, i)
		-- Set
		res[key] = val
		-- Next token
		i = next_char(str, i, space_chars, true)
		local chr = str:sub(i, i)
		i = i + 1
		if chr == "}" then break end
		if chr ~= "," then decode_error(str, i, "expected '}' or ','") end
	end
	return res, i
end


local char_func_map = {
	[ '"' ] = parse_string,
	[ "0" ] = parse_number,
	[ "1" ] = parse_number,
	[ "2" ] = parse_number,
	[ "3" ] = parse_number,
	[ "4" ] = parse_number,
	[ "5" ] = parse_number,
	[ "6" ] = parse_number,
	[ "7" ] = parse_number,
	[ "8" ] = parse_number,
	[ "9" ] = parse_number,
	[ "-" ] = parse_number,
	[ "t" ] = parse_literal,
	[ "f" ] = parse_literal,
	[ "n" ] = parse_literal,
	[ "[" ] = parse_array,
	[ "{" ] = parse_object,
}


parse = function(str, idx)
	local chr = str:sub(idx, idx)
	local f = char_func_map[chr]
	if f then
		return f(str, idx)
	end
	decode_error(str, idx, "unexpected character '" .. chr .. "'")
end


function json.decode(str)
	if type(str) ~= "string" then
		error("expected argument of type string, got " .. type(str))
	end
	local res, idx = parse(str, next_char(str, 1, space_chars, true))
	idx = next_char(str, idx, space_chars, true)
	if idx <= #str then
		decode_error(str, idx, "trailing garbage")
	end
	return res
end


return json