dkjson.lua

    -- Module options:
    local always_try_using_lpeg = true
    local register_global_module_table = false
    local global_module_name = 'json'


local pairs, type, tostring, tonumber, getmetatable, setmetatable, rawset =
      pairs, type, tostring, tonumber, getmetatable, setmetatable, rawset
local error, require, pcall, select = error, require, pcall, select
local floor, huge = math.floor, math.huge
local strrep, gsub, strsub, strbyte, strchar, strfind, strlen, strformat =
      string.rep, string.gsub, string.sub, string.byte, string.char,
      string.find, string.len, string.format
local strmatch = string.match
local concat = table.concat

local json = { version = "dkjson 2.4" }

if register_global_module_table then
  _G[global_module_name] = json
end

local _ENV = nil -- blocking globals in Lua 5.2

pcall (function()
  -- Enable access to blocked metatables.
  -- Don't worry, this module doesn't change anything in them.
  local debmeta = require "debug".getmetatable
  if debmeta then getmetatable = debmeta end
end)

json.null = setmetatable ({}, {
  __tojson = function () return "null" end
})

local function isarray (tbl)
  local max, n, arraylen = 0, 0, 0
  for k,v in pairs (tbl) do
    if k == 'n' and type(v) == 'number' then
      arraylen = v
      if v > max then
        max = v
      end
    else
      if type(k) ~= 'number' or k < 1 or floor(k) ~= k then
        return false
      end
      if k > max then
        max = k
      end
      n = n + 1
    end
  end
  if max > 10 and max > arraylen and max > n * 2 then
    return false -- don't create an array with too many holes
  end
  return true, max
end

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

local function escapeutf8 (uchar)
  local value = escapecodes[uchar]
  if value then
    return value
  end
  local a, b, c, d = strbyte (uchar, 1, 4)
  a, b, c, d = a or 0, b or 0, c or 0, d or 0
  if a <= 0x7f then
    value = a
  elseif 0xc0 <= a and a <= 0xdf and b >= 0x80 then
    value = (a - 0xc0) * 0x40 + b - 0x80
  elseif 0xe0 <= a and a <= 0xef and b >= 0x80 and c >= 0x80 then
    value = ((a - 0xe0) * 0x40 + b - 0x80) * 0x40 + c - 0x80
  elseif 0xf0 <= a and a <= 0xf7 and b >= 0x80 and c >= 0x80 and d >= 0x80 then
    value = (((a - 0xf0) * 0x40 + b - 0x80) * 0x40 + c - 0x80) * 0x40 + d - 0x80
  else
    return ""
  end
  if value <= 0xffff then
    return strformat ("\\u%.4x", value)
  elseif value <= 0x10ffff then
    -- encode as UTF-16 surrogate pair
    value = value - 0x10000
    local highsur, lowsur = 0xD800 + floor (value/0x400), 0xDC00 + (value % 0x400)
    return strformat ("\\u%.4x\\u%.4x", highsur, lowsur)
  else
    return ""
  end
end

local function fsub (str, pattern, repl)
  -- gsub always builds a new string in a buffer, even when no match
  -- exists. First using find should be more efficient when most strings
  -- don't contain the pattern.
  if strfind (str, pattern) then
    return gsub (str, pattern, repl)
  else
    return str
  end
end

local function quotestring (value)
  -- based on the regexp "escapable" in https://github.com/douglascrockford/JSON-js
  value = fsub (value, "[%z\1-\31\"\\\127]", escapeutf8)
  if strfind (value, "[\194\216\220\225\226\239]") then
    value = fsub (value, "\194[\128-\159\173]", escapeutf8)
    value = fsub (value, "\216[\128-\132]", escapeutf8)
    value = fsub (value, "\220\143", escapeutf8)
    value = fsub (value, "\225\158[\180\181]", escapeutf8)
    value = fsub (value, "\226\128[\140-\143\168-\175]", escapeutf8)
    value = fsub (value, "\226\129[\160-\175]", escapeutf8)
    value = fsub (value, "\239\187\191", escapeutf8)
    value = fsub (value, "\239\191[\176-\191]", escapeutf8)
  end
  return "\"" .. value .. "\""
end
json.quotestring = quotestring

local function replace(str, o, n)
  local i, j = strfind (str, o, 1, true)
  if i then
    return strsub(str, 1, i-1) .. n .. strsub(str, j+1, -1)
  else
    return str
  end
end

-- locale independent num2str and str2num functions
local decpoint, numfilter

local function updatedecpoint ()
  decpoint = strmatch(tostring(0.5), "([^05+])")
  -- build a filter that can be used to remove group separators
  numfilter = "[^0-9%-%+eE" .. gsub(decpoint, "[%^%$%(%)%%%.%[%]%*%+%-%?]", "%%%0") .. "]+"
end

updatedecpoint()

local function num2str (num)
  return replace(fsub(tostring(num), numfilter, ""), decpoint, ".")
end

local function str2num (str)
  local num = tonumber(replace(str, ".", decpoint))
  if not num then
    updatedecpoint()
    num = tonumber(replace(str, ".", decpoint))
  end
  return num
end

local function addnewline2 (level, buffer, buflen)
  buffer[buflen+1] = "\n"
  buffer[buflen+2] = strrep ("  ", level)
  buflen = buflen + 2
  return buflen
end

function json.addnewline (state)
  if state.indent then
    state.bufferlen = addnewline2 (state.level or 0,
                           state.buffer, state.bufferlen or #(state.buffer))
  end
end

local encode2 -- forward declaration

local function addpair (key, value, prev, indent, level, buffer, buflen, tables, globalorder)
  local kt = type (key)
  if kt ~= 'string' and kt ~= 'number' then
    return nil, "type '" .. kt .. "' is not supported as a key by JSON."
  end
  if prev then
    buflen = buflen + 1
    buffer[buflen] = ","
  end
  if indent then
    buflen = addnewline2 (level, buffer, buflen)
  end
  buffer[buflen+1] = quotestring (key)
  buffer[buflen+2] = ":"
  return encode2 (value, indent, level, buffer, buflen + 2, tables, globalorder)
end

encode2 = function (value, indent, level, buffer, buflen, tables, globalorder)
  local valtype = type (value)
  local valmeta = getmetatable (value)
  valmeta = type (valmeta) == 'table' and valmeta -- only tables
  local valtojson = valmeta and valmeta.__tojson
  if valtojson then
    if tables[value] then
      return nil, "reference cycle"
    end
    tables[value] = true
    local state = {
        indent = indent, level = level, buffer = buffer,
        bufferlen = buflen, tables = tables, keyorder = globalorder
    }
    local ret, msg = valtojson (value, state)
    if not ret then return nil, msg end
    tables[value] = nil
    buflen = state.bufferlen
    if type (ret) == 'string' then
      buflen = buflen + 1
      buffer[buflen] = ret
    end
  elseif value == nil then
    buflen = buflen + 1
    buffer[buflen] = "null"
  elseif valtype == 'number' then
    local s
    if value ~= value or value >= huge or -value >= huge then
      -- This is the behaviour of the original JSON implementation.
      s = "null"
    else
      s = num2str (value)
    end
    buflen = buflen + 1
    buffer[buflen] = s
  elseif valtype == 'boolean' then
    buflen = buflen + 1
    buffer[buflen] = value and "true" or "false"
  elseif valtype == 'string' then
    buflen = buflen + 1
    buffer[buflen] = quotestring (value)
  elseif valtype == 'table' then
    if tables[value] then
      return nil, "reference cycle"
    end
    tables[value] = true
    level = level + 1
    local isa, n = isarray (value)
    if n == 0 and valmeta and valmeta.__jsontype == 'object' then
      isa = false
    end
    local msg
    if isa then -- JSON array
      buflen = buflen + 1
      buffer[buflen] = "["
      for i = 1, n do
        buflen, msg = encode2 (value[i], indent, level, buffer, buflen, tables, globalorder)
        if not buflen then return nil, msg end
        if i < n then
          buflen = buflen + 1
          buffer[buflen] = ","
        end
      end
      buflen = buflen + 1
      buffer[buflen] = "]"
    else -- JSON object
      local prev = false
      buflen = buflen + 1
      buffer[buflen] = "{"
      local order = valmeta and valmeta.__jsonorder or globalorder
      if order then
        local used = {}
        n = #order
        for i = 1, n do
          local k = order[i]
          local v = value[k]
          if v then
            used[k] = true
            buflen, msg = addpair (k, v, prev, indent, level, buffer, buflen, tables, globalorder)
            prev = true -- add a seperator before the next element
          end
        end
        for k,v in pairs (value) do
          if not used[k] then
            buflen, msg = addpair (k, v, prev, indent, level, buffer, buflen, tables, globalorder)
            if not buflen then return nil, msg end
            prev = true -- add a seperator before the next element
          end
        end
      else -- unordered
        for k,v in pairs (value) do
          buflen, msg = addpair (k, v, prev, indent, level, buffer, buflen, tables, globalorder)
          if not buflen then return nil, msg end
          prev = true -- add a seperator before the next element
        end
      end
      if indent then
        buflen = addnewline2 (level - 1, buffer, buflen)
      end
      buflen = buflen + 1
      buffer[buflen] = "}"
    end
    tables[value] = nil
  else
    return nil, "type '" .. valtype .. "' is not supported by JSON."
  end
  return buflen
end

function json.encode (value, state)
  state = state or {}
  local oldbuffer = state.buffer
  local buffer = oldbuffer or {}
  updatedecpoint()
  local ret, msg = encode2 (value, state.indent, state.level or 0,
                   buffer, state.bufferlen or 0, state.tables or {}, state.keyorder)
  if not ret then
    error (msg, 2)
  elseif oldbuffer then
    state.bufferlen = ret
    return true
  else
    return concat (buffer)
  end
end

local function loc (str, where)
  local line, pos, linepos = 1, 1, 0
  while true do
    pos = strfind (str, "\n", pos, true)
    if pos and pos < where then
      line = line + 1
      linepos = pos
      pos = pos + 1
    else
      break
    end
  end
  return "line " .. line .. ", column " .. (where - linepos)
end

local function unterminated (str, what, where)
  return nil, strlen (str) + 1, "unterminated " .. what .. " at " .. loc (str, where)
end

local function scanwhite (str, pos)
  while true do
    pos = strfind (str, "%S", pos)
    if not pos then return nil end
    if strsub (str, pos, pos + 2) == "\239\187\191" then
      -- UTF-8 Byte Order Mark
      pos = pos + 3
    else
      return pos
    end
  end
end

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

local function unichar (value)
  if value < 0 then
    return nil
  elseif value <= 0x007f then
    return strchar (value)
  elseif value <= 0x07ff then
    return strchar (0xc0 + floor(value/0x40),
                    0x80 + (floor(value) % 0x40))
  elseif value <= 0xffff then
    return strchar (0xe0 + floor(value/0x1000),
                    0x80 + (floor(value/0x40) % 0x40),
                    0x80 + (floor(value) % 0x40))
  elseif value <= 0x10ffff then
    return strchar (0xf0 + floor(value/0x40000),
                    0x80 + (floor(value/0x1000) % 0x40),
                    0x80 + (floor(value/0x40) % 0x40),
                    0x80 + (floor(value) % 0x40))
  else
    return nil
  end
end

local function scanstring (str, pos)
  local lastpos = pos + 1
  local buffer, n = {}, 0
  while true do
    local nextpos = strfind (str, "[\"\\]", lastpos)
    if not nextpos then
      return unterminated (str, "string", pos)
    end
    if nextpos > lastpos then
      n = n + 1
      buffer[n] = strsub (str, lastpos, nextpos - 1)
    end
    if strsub (str, nextpos, nextpos) == "\"" then
      lastpos = nextpos + 1
      break
    else
      local escchar = strsub (str, nextpos + 1, nextpos + 1)
      local value
      if escchar == "u" then
        value = tonumber (strsub (str, nextpos + 2, nextpos + 5), 16)
        if value then
          local value2
          if 0xD800 <= value and value <= 0xDBff then
            -- we have the high surrogate of UTF-16. Check if there is a
            -- low surrogate escaped nearby to combine them.
            if strsub (str, nextpos + 6, nextpos + 7) == "\\u" then
              value2 = tonumber (strsub (str, nextpos + 8, nextpos + 11), 16)
              if value2 and 0xDC00 <= value2 and value2 <= 0xDFFF then
                value = (value - 0xD800)  * 0x400 + (value2 - 0xDC00) + 0x10000
              else
                value2 = nil -- in case it was out of range for a low surrogate
              end
            end
          end
          value = value and unichar (value)
          if value then
            if value2 then
              lastpos = nextpos + 12
            else
              lastpos = nextpos + 6
            end
          end
        end
      end
      if not value then
        value = escapechars[escchar] or escchar
        lastpos = nextpos + 2
      end
      n = n + 1
      buffer[n] = value
    end
  end
  if n == 1 then
    return buffer[1], lastpos
  elseif n > 1 then
    return concat (buffer), lastpos
  else
    return "", lastpos
  end
end

local scanvalue -- forward declaration

local function scantable (what, closechar, str, startpos, nullval, objectmeta, arraymeta)
  local len = strlen (str)
  local tbl, n = {}, 0
  local pos = startpos + 1
  if what == 'object' then
    setmetatable (tbl, objectmeta)
  else
    setmetatable (tbl, arraymeta)
  end
  while true do
    pos = scanwhite (str, pos)
    if not pos then return unterminated (str, what, startpos) end
    local char = strsub (str, pos, pos)
    if char == closechar then
      return tbl, pos + 1
    end
    local val1, err
    val1, pos, err = scanvalue (str, pos, nullval, objectmeta, arraymeta)
    if err then return nil, pos, err end
    pos = scanwhite (str, pos)
    if not pos then return unterminated (str, what, startpos) end
    char = strsub (str, pos, pos)
    if char == ":" then
      if val1 == nil then
        return nil, pos, "cannot use nil as table index (at " .. loc (str, pos) .. ")"
      end
      pos = scanwhite (str, pos + 1)
      if not pos then return unterminated (str, what, startpos) end
      local val2
      val2, pos, err = scanvalue (str, pos, nullval, objectmeta, arraymeta)
      if err then return nil, pos, err end
      tbl[val1] = val2
      pos = scanwhite (str, pos)
      if not pos then return unterminated (str, what, startpos) end
      char = strsub (str, pos, pos)
    else
      n = n + 1
      tbl[n] = val1
    end
    if char == "," then
      pos = pos + 1
    end
  end
end

scanvalue = function (str, pos, nullval, objectmeta, arraymeta)
  pos = pos or 1
  pos = scanwhite (str, pos)
  if not pos then
    return nil, strlen (str) + 1, "no valid JSON value (reached the end)"
  end
  local char = strsub (str, pos, pos)
  if char == "{" then
    return scantable ('object', "}", str, pos, nullval, objectmeta, arraymeta)
  elseif char == "[" then
    return scantable ('array', "]", str, pos, nullval, objectmeta, arraymeta)
  elseif char == "\"" then
    return scanstring (str, pos)
  else
    local pstart, pend = strfind (str, "^%-?[%d%.]+[eE]?[%+%-]?%d*", pos)
    if pstart then
      local number = str2num (strsub (str, pstart, pend))
      if number then
        return number, pend + 1
      end
    end
    pstart, pend = strfind (str, "^%a%w*", pos)
    if pstart then
      local name = strsub (str, pstart, pend)
      if name == "true" then
        return true, pend + 1
      elseif name == "false" then
        return false, pend + 1
      elseif name == "null" then
        return nullval, pend + 1
      end
    end
    return nil, pos, "no valid JSON value at " .. loc (str, pos)
  end
end

local function optionalmetatables(...)
  if select("#", ...) > 0 then
    return ...
  else
    return {__jsontype = 'object'}, {__jsontype = 'array'}
  end
end

function json.decode (str, pos, nullval, ...)
  local objectmeta, arraymeta = optionalmetatables(...)
  return scanvalue (str, pos, nullval, objectmeta, arraymeta)
end

function json.use_lpeg ()
  local g = require ("lpeg")

  if g.version() == "0.11" then
    error "due to a bug in LPeg 0.11, it cannot be used for JSON matching"
  end

  local pegmatch = g.match
  local P, S, R = g.P, g.S, g.R

  local function ErrorCall (str, pos, msg, state)
    if not state.msg then
      state.msg = msg .. " at " .. loc (str, pos)
      state.pos = pos
    end
    return false
  end

  local function Err (msg)
    return g.Cmt (g.Cc (msg) * g.Carg (2), ErrorCall)
  end

  local Space = (S" \n\r\t" + P"\239\187\191")^0

  local PlainChar = 1 - S"\"\\\n\r"
  local EscapeSequence = (P"\\" * g.C (S"\"\\/bfnrt" + Err "unsupported escape sequence")) / escapechars
  local HexDigit = R("09", "af", "AF")
  local function UTF16Surrogate (match, pos, high, low)
    high, low = tonumber (high, 16), tonumber (low, 16)
    if 0xD800 <= high and high <= 0xDBff and 0xDC00 <= low and low <= 0xDFFF then
      return true, unichar ((high - 0xD800)  * 0x400 + (low - 0xDC00) + 0x10000)
    else
      return false
    end
  end
  local function UTF16BMP (hex)
    return unichar (tonumber (hex, 16))
  end
  local U16Sequence = (P"\\u" * g.C (HexDigit * HexDigit * HexDigit * HexDigit))
  local UnicodeEscape = g.Cmt (U16Sequence * U16Sequence, UTF16Surrogate) + U16Sequence/UTF16BMP
  local Char = UnicodeEscape + EscapeSequence + PlainChar
  local String = P"\"" * g.Cs (Char ^ 0) * (P"\"" + Err "unterminated string")
  local Integer = P"-"^(-1) * (P"0" + (R"19" * R"09"^0))
  local Fractal = P"." * R"09"^0
  local Exponent = (S"eE") * (S"+-")^(-1) * R"09"^1
  local Number = (Integer * Fractal^(-1) * Exponent^(-1))/str2num
  local Constant = P"true" * g.Cc (true) + P"false" * g.Cc (false) + P"null" * g.Carg (1)
  local SimpleValue = Number + String + Constant
  local ArrayContent, ObjectContent

  -- The functions parsearray and parseobject parse only a single value/pair
  -- at a time and store them directly to avoid hitting the LPeg limits.
  local function parsearray (str, pos, nullval, state)
    local obj, cont
    local npos
    local t, nt = {}, 0
    repeat
      obj, cont, npos = pegmatch (ArrayContent, str, pos, nullval, state)
      if not npos then break end
      pos = npos
      nt = nt + 1
      t[nt] = obj
    until cont == 'last'
    return pos, setmetatable (t, state.arraymeta)
  end

  local function parseobject (str, pos, nullval, state)
    local obj, key, cont
    local npos
    local t = {}
    repeat
      key, obj, cont, npos = pegmatch (ObjectContent, str, pos, nullval, state)
      if not npos then break end
      pos = npos
      t[key] = obj
    until cont == 'last'
    return pos, setmetatable (t, state.objectmeta)
  end

  local Array = P"[" * g.Cmt (g.Carg(1) * g.Carg(2), parsearray) * Space * (P"]" + Err "']' expected")
  local Object = P"{" * g.Cmt (g.Carg(1) * g.Carg(2), parseobject) * Space * (P"}" + Err "'}' expected")
  local Value = Space * (Array + Object + SimpleValue)
  local ExpectedValue = Value + Space * Err "value expected"
  ArrayContent = Value * Space * (P"," * g.Cc'cont' + g.Cc'last') * g.Cp()
  local Pair = g.Cg (Space * String * Space * (P":" + Err "colon expected") * ExpectedValue)
  ObjectContent = Pair * Space * (P"," * g.Cc'cont' + g.Cc'last') * g.Cp()
  local DecodeValue = ExpectedValue * g.Cp ()

  function json.decode (str, pos, nullval, ...)
    local state = {}
    state.objectmeta, state.arraymeta = optionalmetatables(...)
    local obj, retpos = pegmatch (DecodeValue, str, pos, nullval, state)
    if state.msg then
      return nil, state.pos, state.msg
    else
      return obj, retpos
    end
  end

  -- use this function only once:
  json.use_lpeg = function () return json end

  json.using_lpeg = true

  return json -- so you can get the module using json = require "dkjson".use_lpeg()
end

if always_try_using_lpeg then
  pcall (json.use_lpeg)
end

return json

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