json.lua

--
-- 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