--
-- 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("%.30g", 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
