LUA--Open Computers json converter

--
-- json.lua
--
-- Copyright (c) 2018 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 = {}

-------------------------------------------------------------------------------
-- 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 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(3, 6),  16 )
   local n2 = tonumber( s:sub(9, 12), 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 has_unicode_escape = false
   local has_surrogate_escape = false
   local has_escape = false
   local last
   for j = i + 1, #str do
      local x = str:byte(j)

      if x < 32 then
         decode_error(str, j, "control character in string")
      end

      if last == 92 then -- "\\" (escape char)
         if x == 117 then -- "u" (unicode escape sequence)
            local hex = str:sub(j + 1, j + 5)
            if not hex:find("%x%x%x%x") then
               decode_error(str, j, "invalid unicode escape in string")
            end
            if hex:find("^[dD][89aAbB]") then
               has_surrogate_escape = true
            else
               has_unicode_escape = true
            end
         else
            local c = string.char(x)
            if not escape_chars[c] then
               decode_error(str, j, "invalid escape char '" .. c .. "' in string")
            end
            has_escape = true
         end
         last = nil

      elseif x == 34 then -- '"' (end of string)
         local s = str:sub(i + 1, j - 1)
         if has_surrogate_escape then
            s = s:gsub("\\u[dD][89aAbB]..\\u....", parse_unicode_escape)
         end
         if has_unicode_escape then
            s = s:gsub("\\u....", parse_unicode_escape)
         end
         if has_escape then
            s = s:gsub("\\.", escape_char_map_inv)
         end
         return s, j + 1

      else
         last = x
      end
   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