local function insertNonNil(t, v)
  if v then
    v = tostring(v)
    if #v > 0 then
      table.insert(t, v)
    end
  end
end

local function backtrace(levelStart, shift)
  c = c or coroutine.running()

  for level = levelStart, math.huge do
    local info = debug.getinfo(c, level, "ufnSl")

    if not info then
      break
    end

    local funcType, name, args, exec, defined = {}, nil, {}, "", {}

    insertNonNil(funcType, info.what)
    insertNonNil(funcType, info.namewhat)
    table.insert(funcType, "function")

    name = info.name or "<anon>"

    if info.nparams then
      for an = 1, info.nparams do
        local argName, argValue = debug.getlocal(c, level, an)

        if argValue ~= nil then
          argName = argName .. "=" .. tostring(argValue)
        end

        table.insert(args, argName)
      end
    end

    if info.isvararg then
      table.insert(args, "...")
    end

    if info.currentline and info.currentline ~= -1 then
      exec = ":" .. tostring(info.currentline)
    end

    insertNonNil(defined, info.short_src)

    if info.linedefined and info.linedefined ~= -1 then
      table.insert(defined, info.linedefined)
    end

    funcType = table.concat(funcType, " ")
    args = table.concat(args, ", ")
    defined = (defined[1] and (" in " .. defined[1] .. "") or "") ..
              (defined[2] and (" at L" .. defined[2]) or "")

    -- local Lua function <anon>(a, b, c, ...):33 (defined in blah.lua at 33)

    local line = ("#%2d: %s %s%s%s%s"):format(
      level - levelStart + shift,
      funcType,
      name,
      #args > 0 and ("(" .. args .. ")") or "",
      exec,
      #defined > 0 and (" (defined" .. defined .. ")") or ""
    )

    print(line)
  end

  return message
end

local function getLevel(f)
  for i = 0, math.huge, 1 do
    local frame = debug.getinfo(i, "f")

    if not frame then
      break
    end

    if frame.func == f then
      return i
    end
  end
end

local function dbg(...)
  if select("#", ...) > 0 and not (...) then
    return
  end

  if os.getenv("DEBUG") == "off" then
    return
  end

  local level = 0

  local environment = setmetatable({}, {
    __index = function(_, var)
      local dbgLevel = getLevel(dbg)

      -- locals
      for i = 1, math.huge, 1 do
        local name, value = debug.getlocal(level + dbgLevel, i)

        if not name then
          break
        end

        if name == var then
          return value
        end
      end

      -- arguments
      for i = -1, -math.huge, -1 do
        local name, value = debug.getlocal(level + dbgLevel, i)

        if not name then
          break
        end

        if name == var then
          return value
        end
      end

      -- upvalues
      local frame = debug.getinfo(level + dbgLevel, "f")

      if not frame then
        return
      end

      local env

      for i = 1, math.huge do
        local name, value = debug.getupvalue(frame.func, i)

        if not name then
          break
        end

        if name == var then
          return value
        end

        if name == "_ENV" then
          env = value
        end
      end

      if env then
        return env[var]
      end
    end,

    __newindex = function(_, var, value)
      local dbgLevel = getLevel(dbg)

      -- locals
      for i = 1, math.huge, 1 do
        local name = debug.getlocal(level + dbgLevel, i)

        if not name then
          break
        end

        if name == var then
          debug.setlocal(level + dbgLevel, i, value)
          return
        end
      end

      -- arguments
      for i = -1, -math.huge, -1 do
        local name = debug.getlocal(level + dbgLevel, i)

        if not name then
          break
        end

        if name == var then
          debug.setlocal(level + dbgLevel, i, value)
          return
        end
      end

      -- environment
      local frame = debug.getinfo(level + dbgLevel, "f")

      if not frame then
        return
      end

      local env

      for i = 1, math.huge, 1 do
        local name, upvalue = debug.getupvalue(frame.func, i)

        if not name then
          break
        end

        if name == var then
          debug.setupvalue(frame.func, i, value)
          return
        end

        if name == "_ENV" then
          env = upvalue
        end
      end

      if env then
        env[value] = value
      end
    end,
  })

  while true do
    local frameInfo = debug.getinfo(getLevel(dbg) + level, "nSl")
    local name = frameInfo.name or ""
    io.write(("dbg(%d: %s:%d)> "):format(level, name, frameInfo.currentline))

    local input = io.read("*l")

    if not input then
      print()
      break
    end

    if input:sub(1, 1) == ":" then
      local params = {}

      for param in input:sub(2):gmatch("%S+") do
        table.insert(params, param)
      end

      if params[1] == "up" then
        if debug.getinfo(level + getLevel(dbg) + 1, "") then
          level = level + 1
        else
          print("Reached the top of the stack")
        end
      elseif params[1] == "down" then
        if debug.getinfo(level + getLevel(dbg) - 1, "") then
          level = level - 1
        else
          print("Reached the bottom of the stack")
        end
      elseif params[1] == "frame" and tonumber(params[2]) then
        if debug.getinfo(getLevel(dbg) + tonumber(params[2]), "") then
          level = tonumber(params[2])
        else
          print("No such frame")
        end
      elseif params[1] == "bt" then
        backtrace(level + getLevel(dbg) + 1, level)
      end
    else
      local ok, err = load("return " .. input, "=debug", "t", environment)

      if not ok then
        ok, err = load(input, "=debug", "t", environment)
      end

      local values

      if ok then
        values = table.pack(xpcall(ok, debug.traceback))
        ok = table.remove(values, 1)
        values.n = values.n - 1
        err = values[1]
      end

      if not ok then
        io.stderr:write(("%s\n"):format(err))
      else
        for i = 1, values.n, 1 do
          if type(values[i]) == "string" then
            values[i] = ("%q"):format(values[i]):gsub("\\\n", "\\n")
          else
            values[i] = ("%s"):format(values[i])
          end
        end

        if values.n > 0 then
          print(table.concat(values, "\t", 1, values.n))
        end
      end
    end
  end
end

return dbg