fsPlugins

-- fsPlugins created by airminer96

------------
-- Fields --
------------

-- Debug option
local debug = false

-- The functions to be replaced from the fs API
local keys = {"list","exists","isDir","isReadOnly","getDrive","getSize","getFreeSpace","makeDir","delete","open"}

-- The original values from the fs API
local fsRaw = {}

-- The original value of os.unloadAPI
local unloadRaw

-- The list of the registered handlers
local handlers = {}

----------------
-- Public API --
----------------

--[[
  Register a handler
  parameters: handler
    A handler is a function, which is passed the following paramters:
      path: the path of the file being resolved
      handle: the table, from which the fs API methods should be grabbed from
      action: the name of the fs API method being used
    A handler must return the following values:
      path: the overridden path of the file being resolved
      handle: the overridden table, from which the fs API methods should be grabbed from
]]
function registerHandler(handler)
  if type(handler) == "function" then
    table.insert(handlers,handler)
  else
    error("Registered handler is not a function",2)
  end
end

--[[
  Removes a handler
  parameters: handler
]]
function removeHandler(handler)
  for k,v in ipairs(handlers) do
    if v == handler then
      table.remove(handlers,k)
    end
  end
end

---------------
-- Internals --
---------------

-- Checks if the response from a handler is valid
local function responseValid(path, handle)
  if (type(path) == "string" or type(path) == "nil") and (type(handle) == "table" or type(handle) == "nil") then
    for k,v in ipairs(keys) do
      if type(handle[v]) ~= "function" then
        return false
      end
    end
    return true
  end
  return false
end

-- Calls a handler, and removes it if it's response isn't valid
local function callHandler(handler, path, handle, action)
  p,h = handler(path, handle, action)
  if responseValid(p,h) then
    return p,h
  else
    removeHandler(handler)
    return path,handle
  end
end

-- Resolves a path
local function resolve(path, action)
  -- Quickfix for infinite looping
  loop = 0
  p = path
  h = fsRaw
  trace = {}
  activeHandlers = {unpack(handlers)}
  while #activeHandlers > 0  and loop < 100 do
    loop = loop + 1
    p1,h1 = callHandler(activeHandlers[1],p,h,action)
    if (p ~= p1) or (h ~= h1) then
      activeHandlers = {unpack(handlers)}
      p = p1
      h = h1
    else
      table.remove(activeHandlers,1)
    end
  end
  if debug then
    print(path.." --> "..p)
  end
  return p,h
end

-- Creates a wrapper function for the fs API function with the name of <action>
local function funcFactory(action)
  return function(path, ...)
    p,h = resolve(path, action)
    return h[action](p, ...)
  end
end

-- The hook which overrides os.unloadAPI
local function unload(name)
  if name == "fsPlugins" and fsPlugins then
    -- Restore fs API
    for k,v in pairs(fsRaw) do
      fs[k] = v
    end
  end
  if (name == "fsPlugins" or fsPlugins == nil) and os.unloadAPI == unload then
      -- Restore os.unloadAPI
      os.unloadAPI = unloadRaw
  end
  unloadRaw(name)
end

-- os.loadAPI sequence
if shell == nil then
  if fsPlugins then
    -- Unload API if already loaded
    os.unloadAPI("fsPlugins")
  end
  -- Override os.unloadAPI
  unloadRaw = os.unloadAPI
  os.unloadAPI = unload
  -- Override fs API
  for k,v in ipairs(keys) do
    fsRaw[v] = fs[v]
    fs[v] = funcFactory(v)
  end
-- Shell program sequence
else
  tArgs = {...}
  if tArgs[1] == "load" then
    -- Load API
    os.loadAPI(shell.getRunningProgram())
  elseif tArgs[1] == "unload" then
    -- Unload API
    os.unloadAPI("fsPlugins")
  else
    -- Print Usages
    print("Usages:\nfsPlugins help\nfsPlugins api\nfsPlugins load\nfsPlugins unload")
  end
end