local json = {}

local function is_array(t)
  local i = 0
  for _ in pairs(t) do
    i = i + 1
    if t[i] == nil then
      return false
    end
  end
  return true
end

local function escape(s)
  s = s:gsub("\\", "\\\\")
  s = s:gsub("\"", "\\\"")
  s = s:gsub("\n", "\\n")
  s = s:gsub("\r", "\\r")
  s = s:gsub("\t", "\\t")
  return s
end

function json.encode(data)
  local t = type(data)
  if t == "nil" then
    return "null"
  elseif t == "boolean" then
    return data and "true" or "false"
  elseif t == "number" then
    return tostring(data)
  elseif t == "string" then
    return "\"" .. escape(data) .. "\""
  elseif t == "table" then
    local parts = {}
    if is_array(data) then
      for _, v in ipairs(data) do
        table.insert(parts, json.encode(v))
      end
      return "[" .. table.concat(parts, ",") .. "]"
    else
      for k, v in pairs(data) do
        table.insert(parts, "\"" .. escape(k) .. "\":" .. json.encode(v))
      end
      return "{" .. table.concat(parts, ",") .. "}"
    end
  else
    return "null"
  end
end

local function skip_whitespace(s, i)
  while i <= #s and s:find("^%s", i) do
    i = i + 1
  end
  return i
end

local function parse_string(s, i)
  local result = {}
  i = i + 1
  while i <= #s do
    local c = s:sub(i, i)
    if c == "\"" then
      return table.concat(result), i + 1
    elseif c == "\\" and i < #s then
      local next_c = s:sub(i + 1, i + 1)
      if next_c == "n" then
        table.insert(result, "\n")
      elseif next_c == "r" then
        table.insert(result, "\r")
      elseif next_c == "t" then
        table.insert(result, "\t")
      elseif next_c == "\\\"" then
        table.insert(result, "\"")
      elseif next_c == "\\\\" then
        table.insert(result, "\\")
      else
        table.insert(result, next_c)
      end
      i = i + 2
    else
      table.insert(result, c)
      i = i + 1
    end
  end
  return nil, i
end

local function parse_number(s, i)
  local start = i
  if s:sub(i, i) == "-" then
    i = i + 1
  end
  while i <= #s and s:find("^%d", s:sub(i, i)) do
    i = i + 1
  end
  if s:sub(i, i) == "." then
    i = i + 1
    while i <= #s and s:find("^%d", s:sub(i, i)) do
      i = i + 1
    end
  end
  if s:sub(i, i):find("^[eE]") then
    i = i + 1
    if s:sub(i, i):find("^[+-]") then
      i = i + 1
    end
    while i <= #s and s:find("^%d", s:sub(i, i)) do
      i = i + 1
    end
  end
  return tonumber(s:sub(start, i - 1)), i
end

local function parse_value(s, i)
  i = skip_whitespace(s, i)
  if i > #s then
    return nil, i
  end

  local c = s:sub(i, i)

  if c == "{" then
    local obj = {}
    i = i + 1
    while i <= #s do
      i = skip_whitespace(s, i)
      if s:sub(i, i) == "}" then
        return obj, i + 1
      end
      if s:sub(i, i) == "\"" then
        local key
        key, i = parse_string(s, i)
        i = skip_whitespace(s, i)
        if s:sub(i, i) ~= ":" then
          return nil, i
        end
        i = i + 1
        local val
        val, i = parse_value(s, i)
        obj[key] = val
        i = skip_whitespace(s, i)
        if s:sub(i, i) == "}" then
          return obj, i + 1
        end
        if s:sub(i, i) == "," then
          i = i + 1
        end
      else
        return nil, i
      end
    end
    return nil, i
  elseif c == "[" then
    local arr = {}
    i = i + 1
    while i <= #s do
      i = skip_whitespace(s, i)
      if s:sub(i, i) == "]" then
        return arr, i + 1
      end
      local val
      val, i = parse_value(s, i)
      table.insert(arr, val)
      i = skip_whitespace(s, i)
      if s:sub(i, i) == "]" then
        return arr, i + 1
      end
      if s:sub(i, i) == "," then
        i = i + 1
      end
    end
    return nil, i
  elseif c == "\"" then
    return parse_string(s, i)
  elseif s:sub(i, i + 3) == "null" then
    return nil, i + 4
  elseif s:sub(i, i + 3) == "true" then
    return true, i + 4
  elseif s:sub(i, i + 4) == "false" then
    return false, i + 5
  else
    return parse_number(s, i)
  end
end

function json.decode(s)
  if type(s) ~= "string" then
    return nil, "invalid input"
  end
  local data, i = parse_value(s, 1)
  return data, i
end

return json