Luacontroller JSON parser

luacontroller_json.lua

-- JSON parser code for the Minetest Mesecons Luacontroller
-- License: CC0 1.0 Universal (public domain)

-- Based on https://gist.github.com/tylerneylon/59f4bcf316be525b30ab
-- Rewritten by OgelGames to work in the restricted Luacontroller sandbox
-- Designed to be copy-pasted into a standalone Luacontroller

if event.type ~= "digiline" or event.channel ~= "json" then
  return
elseif type(event.msg) ~= "string" then
  digiline_send("json", "Cannot parse non-string value!")
end

local sub, find, concat, tonum = string.sub, string.find, table.concat, tonumber

local esc_map = {b = '\b', f = '\f', n = '\n', r = '\r', t = '\t'}

local whitespace = {
  [' '] = true, ['\b'] = true, ['\f'] = true, ['\n'] = true,
  ['\r'] = true, ['\t'] = true, ['\v'] = true
}

local digits = {
  ['1'] = true, ['2'] = true, ['3'] = true, ['4'] = true, ['5'] = true,
  ['6'] = true, ['7'] = true, ['8'] = true, ['9'] = true, ['0'] = true,
  ['-'] = true, ['+'] = true, ['e'] = true, ['E'] = true, ['.'] = true
}

local function skip_chars(str, pos, chars)
  local c ; while pos <= #str do
    c = sub(str, pos, pos)
    if not chars[c] then return pos, c end
    pos = pos + 1
  end
end

local function check_delim(str, pos, delim, err)
  if not pos then return end
  local c ; pos, c = skip_chars(str, pos, whitespace)
  if c ~= delim then
    if err then return end ; return pos, false
  end
  return pos + 1, true
end

local function find_str_end(str, pos)
  local a = find(str, '"', pos, true)
  local b = find(str, '\\', pos, true)
  if not a or not b then return a or b, (a and '"') or (b and '\\')
  elseif a < b then return a, '"' end ; return b, '\\'
end

local function parse_json(str, pos, end_delim)
  pos = pos or 1 ; local first
  if pos > #str then return end
  pos, first = skip_chars(str, pos, whitespace)
  if first == end_delim then
    return nil, pos + 1
  elseif first == '{' then
    local t, f , k = {}, true ; pos = pos + 1
    while true do
      k, pos = parse_json(str, pos, '}') ; if not pos then return end
      if k == nil then return t, pos end ; if not f then return end
      pos = check_delim(str, pos, ':', true) ; if not pos then return end
      t[k], pos = parse_json(str, pos) ; if not pos then return end
      pos, f = check_delim(str, pos, ',')
    end
  elseif first == '[' then
    local a, f, v = {}, true ; pos = pos + 1
    while true do
      v, pos = parse_json(str, pos, ']') ; if not pos then return end
      if v == nil then return a, pos end ; if not f then return end
      a[#a + 1] = v ; pos, f = check_delim(str, pos, ',')
    end
  elseif first == '"' then
    local p, i, c = {} ; pos = pos + 1
    while true do
      i, c = find_str_end(str, pos)
      if not c then return end
      p[#p + 1] = sub(str, pos, i-1) ; i = i + 1
      if c == '"' then return concat(p), i end
      local nc = sub(str, i, i)
      p[#p + 1] = esc_map[nc] or nc ; pos = i + 1
    end
  elseif digits[first] then
    local i = skip_chars(str, pos, digits)
    local num = tonum(sub(str, pos, i - 1))
    if not num then return end ; return num, i
  else
    local s = sub(str, pos, pos + 3)
    if s == 'true' then return true, pos + 4 end
    if s == 'null' then return {}, pos + 4 end
    s = sub(str, pos, pos + 4)
    if s == 'false' then return false, pos + 5 end
  end
end

digiline_send("json", parse_json(event.msg) or "Error parsing json!")