-- Lua implementation of LEAP (LLSD Event API Plugin) protocol
--
-- This module supports Lua scripts run by the Second Life viewer.
--
-- LEAP protocol passes LLSD objects, converted to/from Lua tables, in both
-- directions. A typical LLSD object is a map containing keys 'pump' and
-- 'data'.
--
-- The viewer's Lua post_to(pump, data) function posts 'data' to the
-- LLEventPump 'pump'. This is typically used to engage an LLEventAPI method.
--
-- Similarly, the viewer gives each Lua script its own LLEventPump with a
-- unique name. That name is returned by get_event_pumps(). Every event
-- received on that LLEventPump is queued for retrieval by get_event_next(),
-- which returns (pump, data): the name of the LLEventPump on which the event
-- was received and the received event data. When the queue is empty,
-- get_event_next() blocks the calling Lua script until the next event is
-- received.
--
-- Usage:
-- 1. Launch some number of Lua coroutines. The code in each coroutine may
--    call leap.send(), leap.request() or leap.generate(). leap.send() returns
--    immediately ("fire and forget"). leap.request() blocks the calling
--    coroutine until it receives and returns the viewer's response to its
--    request. leap.generate() expects an arbitrary number of responses to the
--    original request.
-- 2. To handle events from the viewer other than direct responses to
--    requests, instantiate a leap.WaitFor object with a filter(pump, data)
--    override method that returns non-nil for desired events. A coroutine may
--    call wait() on any such WaitFor.
-- 3. Once the coroutines have been launched, call leap.process() on the main
--    coroutine. process() retrieves incoming events from the viewer and
--    dispatches them to waiting request() or generate() calls, or to
--    appropriate WaitFor instances. process() returns when either
--    get_event_next() raises an error or the viewer posts nil to the script's
--    reply pump to indicate it's done.
-- 4. Alternatively, a running coroutine may call leap.done() to break out of
--    leap.process(). process() won't notice until the next event from the
--    viewer, though.

local ErrorQueue = require('ErrorQueue')

local leap = {}

-- _reply: string name of reply LLEventPump. Any events the viewer posts to
-- this pump will be queued for get_event_next(). We usually specify it as the
-- reply pump for requests to internal viewer services.
-- _command: string name of command LLEventPump. post_to(_command, ...)
-- engages LLLeapListener operations such as listening on a specified other
-- LLEventPump, etc.
leap._reply, leap._command = get_event_pumps()
-- Dict of features added to the LEAP protocol since baseline implementation.
-- Before engaging a new feature that might break an older viewer, we can
-- check for the presence of that feature key. This table is solely about the
-- LEAP protocol itself, the way we communicate with the viewer. To discover
-- whether a given listener exists, or supports a particular operation, use
-- _command's "getAPI" operation.
-- For Lua, _command's "getFeatures" operation suffices?
-- leap._features = {}

-- Each outstanding request() or generate() call has a corresponding
-- WaitForReqid object (later in this module) to handle the
-- response(s). If an incoming event contains an echoed ["reqid"] key,
-- we can look up the appropriate WaitForReqid object more efficiently
-- in a dict than by tossing such objects into the usual waitfors list.
-- Note: the ["reqid"] must be unique, otherwise we could end up
-- replacing an earlier WaitForReqid object in self.pending with a
-- later one. That means that no incoming event will ever be given to
-- the old WaitForReqid object. Any coroutine waiting on the discarded
-- WaitForReqid object would therefore wait forever.
leap._pending = {}
-- Our consumer will instantiate some number of WaitFor subclass objects.
-- As these are traversed in descending priority order, we must keep
-- them in a list.
leap._waitfors = {}
-- It has been suggested that we should use UUIDs as ["reqid"] values,
-- since UUIDs are guaranteed unique. However, as the "namespace" for
-- ["reqid"] values is our very own _reply pump, we can get away with
-- an integer.
leap._reqid = 0
-- break leap.process() loop
leap._done = false

-- get the name of the reply pump
function leap.replypump()
    return leap._reply
end

-- get the name of the command pump
function leap.cmdpump()
    return leap._command
end

-- Fire and forget. Send the specified request LLSD, expecting no reply.
-- In fact, should the request produce an eventual reply, it will be
-- treated as an unsolicited event.
-- 
-- See also request(), generate().
function leap.send(pump, data, reqid)
    local data = data
    if type(data) == 'table' then
        data = table.clone(data)
        data['reply'] = leap._reply
        if reqid ~= nil then
            data['reqid'] = reqid
        end
    end
    post_to(pump, data)
end

-- Send the specified request LLSD, expecting exactly one reply. Block
-- the calling coroutine until we receive that reply.
-- 
-- Every request() (or generate()) LLSD block we send will get stamped
-- with a distinct ["reqid"] value. The requested event API must echo the
-- same ["reqid"] field in each reply associated with that request. This way
-- we can correctly dispatch interleaved replies from different requests.
-- 
-- If the desired event API doesn't support the ["reqid"] echo convention,
-- you should use send() instead -- since request() or generate() would
-- wait forever for a reply stamped with that ["reqid"] -- and intercept
-- any replies using WaitFor.
-- 
-- Unless the request data already contains a ["reply"] key, we insert
-- reply=self.replypump to try to ensure that the expected reply will be
-- returned over the socket.
--
-- See also send(), generate().
function leap.request(pump, data)
    local reqid = leap._requestSetup(pump, data)
    local ok, response = pcall(leap._pending[reqid].wait)
    -- kill off temporary WaitForReqid object, even if error
    leap._pending[reqid] = nil
    if ok then
        return response
    else
        error(response)
    end
end

-- common setup code shared by request() and generate()
function leap._requestSetup(pump, data)
    -- invent a new, unique reqid
    local reqid = leap._reqid
    leap._reqid += 1
    -- Instantiate a new WaitForReqid object. The priority is irrelevant
    -- because, unlike the WaitFor base class, WaitForReqid does not
    -- self-register on our leap._waitfors list. Instead, capture the new
    -- WaitForReqid object in leap._pending so _dispatch() can find it.
    leap._pending[reqid] = leap.WaitForReqid:new(reqid)
    -- Pass reqid to send() to stamp it into (a copy of) the request data.
    leap.send(pump, data, reqid)
    return reqid
end

-- Send the specified request LLSD, expecting an arbitrary number of replies.
-- Each one is yielded on receipt. If you omit checklast, this is an infinite
-- generator; it's up to the caller to recognize when the last reply has been
-- received, and stop resuming for more.
-- 
-- If you pass checklast=<callable accepting(event)>, each response event is
-- passed to that callable (after the yield). When the callable returns
-- True, the generator terminates in the usual way.
-- 
-- See request() remarks about ["reqid"].
function leap.generate(pump, data, checklast)
    -- Invent a new, unique reqid. Arrange to handle incoming events
    -- bearing that reqid. Stamp the outbound request with that reqid, and
    -- send it.
    local reqid = leap._requestSetup(pump, data)
    local ok, response
    repeat
        ok, response = pcall(leap._pending[reqid].wait)
        if not ok then
            break
        end
        coroutine.yield(response)
    until checklast and checklast(response)
    -- If we break the above loop, whether or not due to error, clean up.
    leap._pending[reqid] = nil
    if not ok then
        error(response)
    end
end

-- Kick off response processing. The calling script must create and resume one
-- or more coroutines to perform viewer requests using send(), request() or
-- generate() before calling this function to handle responses.
--
-- While waiting for responses from the viewer, the C++ coroutine running the
-- calling Lua script is blocked: no other Lua coroutine is running.
function leap.process()
    leap._done = false
    local ok, pump, data
    while not leap._done do
        ok, pump, data = pcall(get_event_next)
        print_debug('leap.process() got', ok, pump, data)
        -- ok false means get_event_next() raised a Lua error
        -- data nil means get_event_next() returned (pump, LLSD()) to indicate done
        if not (ok and data) then
            print_debug('leap.process() done')
            break
        end
        leap._dispatch(pump, data)
    end
    -- we're done: clean up all pending coroutines
    -- if ok, then we're just done.
    -- if not ok, then 'pump' is actually the error message.
    message = if ok then 'done' else pump
    for i, waitfor in pairs(leap._pending) do
        waitfor:_exception(message)
    end
    for i, waitfor in pairs(leap._waitfors) do
        waitfor:_exception(message)
    end
    -- now that we're done with cleanup, propagate the error we caught above
    if not ok then
        error(pump)
    end
end

function leap.done()
    leap._done = true
end

-- Route incoming (pump, data) event to the appropriate waiting coroutine.
function leap._dispatch(pump, data)
    local reqid = data['reqid']
    -- if the response has no 'reqid', it's not from request() or generate()
    if reqid == nil then
        return leap._unsolicited(pump, data)
    end
    -- have reqid; do we have a WaitForReqid?
    local waitfor = leap._pending[reqid]
    if waitfor == nil then
        return leap._unsolicited(pump, data)
    end
    -- found the right WaitForReqid object, let it handle the event
    data['reqid'] = nil
    waitfor:_handle(pump, data)
end

-- Handle an incoming (pump, data) event with no recognizable ['reqid']
function leap._unsolicited(pump, data)
    -- we maintain waitfors in descending priority order, so the first waitfor
    -- to claim this event is the one with the highest priority
    for i, waitfor in pairs(leap._waitfors) do
        if waitfor:_handle(pump, data) then
            return
        end
    end
    print_debug('_unsolicited(', pump, ', ', data, ') discarding unclaimed event')
end

-- called by WaitFor.enable()
function leap._registerWaitFor(waitfor)
    table.insert(leap._waitfors, waitfor)
    -- keep waitfors sorted in descending order of specified priority
    table.sort(leap._waitfors,
               function (lhs, rhs) return lhs.priority > rhs.priority end)
end

-- called by WaitFor.disable()
function leap._unregisterWaitFor(waitfor)
    for i, w in pairs(leap._waitfors) do
        if w == waitfor then
            leap._waitfors[i] = nil
            break
        end
    end
end

-- ******************************************************************************
-- WaitFor and friends
-- ******************************************************************************

-- An unsolicited event is handled by the highest-priority WaitFor subclass
-- object willing to accept it. If no such object is found, the unsolicited
-- event is discarded.
-- 
-- * First, instantiate a WaitFor subclass object to register its interest in
--   some incoming event(s). WaitFor instances are self-registering; merely
--   instantiating the object suffices.
-- * Any coroutine may call a given WaitFor object's wait() method. This blocks
--   the calling coroutine until a suitable event arrives.
-- * WaitFor's constructor accepts a float priority. Every incoming event
--   (other than those claimed by request() or generate()) is passed to each
--   extant WaitFor.filter() method in descending priority order. The first
--   such filter() to return nontrivial data claims that event.
-- * At that point, the blocked wait() call on that WaitFor object returns the
--   item returned by filter().
-- * WaitFor contains a queue. Multiple arriving events claimed by that WaitFor
--   object's filter() method are added to the queue. Naturally, until the
--   queue is empty, calling wait() immediately returns the front entry.
-- 
-- It's reasonable to instantiate a WaitFor subclass whose filter() method
-- unconditionally returns the incoming event, and whose priority places it
-- last in the list. This object will enqueue every unsolicited event left
-- unclaimed by other WaitFor subclass objects.
-- 
-- It's not strictly necessary to associate a WaitFor object with exactly one
-- coroutine. You might have multiple "worker" coroutines drawing from the same
-- WaitFor object, useful if the work being done per event might itself involve
-- "blocking" operations. Or a given coroutine might sample a number of WaitFor
-- objects in round-robin fashion... etc. etc. Nonetheless, it's
-- straightforward to designate one coroutine for each WaitFor object.

-- --------------------------------- WaitFor ---------------------------------
leap.WaitFor = { _id=0 }

function leap.WaitFor:new(priority, name)
    local obj = setmetatable({}, self)
    self.__index = self

    obj.priority = priority
    if name then
        obj.name = name
    else
        self._id += 1
        obj.name = 'WaitFor' .. self._id
    end
    obj._queue = ErrorQueue:new()
    obj._registered = false
    -- if no priority, then don't enable() - remember 0 is truthy
    if priority then
        obj:enable()
    end

    return obj
end

function leap.WaitFor.tostring(self)
    -- Lua (sub)classes have no name; can't prefix with that
    return self.name
end

-- Re-enable a disable()d WaitFor object. New WaitFor objects are
-- enable()d by default.
function leap.WaitFor:enable()
    if not self._registered then
        leap._registerWaitFor(self)
        self._registered = true
    end
end

-- Disable an enable()d WaitFor object.
function leap.WaitFor:disable()
    if self._registered then
        leap._unregisterWaitFor(self)
        self._registered = false
    end
end

-- Block the calling coroutine until a suitable unsolicited event (one
-- for which filter() returns the event) arrives.
function leap.WaitFor:wait()
    print_debug(self.name .. ' about to wait')
    item = self._queue:Dequeue()
    print_debug(self.name .. ' got ', item)
    return item
end

-- Loop over wait() calls.
function leap.WaitFor:iterate()
    -- on each iteration, call self.wait(self)
    return self.wait, self, nil
end

-- Override filter() to examine the incoming event in whatever way
-- makes sense.
-- 
-- Return nil to ignore this event.
-- 
-- To claim the event, return the item you want placed in the queue.
-- Typically you'd write:
-- return data
-- or perhaps
-- return {pump=pump, data=data}
-- or some variation.
function leap.WaitFor:filter(pump, data)
    error('You must override the WaitFor.filter() method')
end

-- called by leap._unsolicited() for each WaitFor in leap._waitfors
function leap.WaitFor:_handle(pump, data)
    item = self:filter(pump, data)
    -- if this item doesn't pass the filter, we're not interested
    if not item then
        return false
    end
    -- okay, filter() claims this event
    self:process(item)
    return true
end

-- called by WaitFor:_handle() for an accepted event
function leap.WaitFor:process(item)
    self._queue:Enqueue(item)
end

-- called by leap.process() when get_event_next() raises an error
function leap.WaitFor:_exception(message)
    print_warning(self.name .. ' error: ' .. message)
    self._queue:Error(message)
end

-- ------------------------------ WaitForReqid -------------------------------
leap.WaitForReqid = leap.WaitFor:new()

function leap.WaitForReqid:new(reqid)
    -- priority is meaningless, since this object won't be added to the
    -- priority-sorted ViewerClient.waitfors list. Use the reqid as the
    -- debugging name string.
    local obj = leap.WaitFor:new(nil, 'WaitForReqid(' .. reqid .. ')')
    setmetatable(obj, self)
    self.__index = self

    return obj
end

function leap.WaitForReqid:filter(pump, data)
    -- Because we expect to directly look up the WaitForReqid object of
    -- interest based on the incoming ["reqid"] value, it's not necessary
    -- to test the event again. Accept every such event.
    return data
end

return leap