loops.lua

-- Observation: the natural push loop is slow at dropping packets. When a
-- packet doesn’t follow the happy path (either because it doesn’t meet the
-- test condition, or because the output link is full), the loop head for the
-- happy path is “replayed” for every “non-happy” packet because of the
-- side-traces created for error situations.

function A:push ()
   local input, output = self.input.input, self.output.output
   while not link.empty(input) do
      local p = link.receive(input)
      if test(p) then
         -- do stuff with p
         link.transmit(output, p)
      else
         -- error, discard p
         packet.free(p)
      end
   end
end

-- Below are some alternative formulations of the loop that intend to force
-- different, more efficient traces.

function B:push ()
   local input, output = self.input.input, self.output.output
   repeat
      -- Trace for happy path
      while test(link.front(input)) do
         local p = link.receive(input)
         -- do stuff with p
         link.transmit(output, p)
      end
      -- Trace for error path
      while not test(link.front(input)) do
         local p = link.receive(input)
         -- error, discard p
         packet.free(p)
      end
   until link.empty(input)
end

function C:push ()
   local input, output = self.input.input, self.output.output
   -- Trace for happy path
   while not link.empty(input) do
      if test(link.front(input)) then
         local p = link.receive(input)
         -- do stuff with p
         link.transmit(output, p)
      else
         -- Trace for error path
         repeat -- error, discard p
            local p = link.receive(input)
            packet.free(p)
         until test(link.front(input))
      end
   end
end

function D:push ()
   local input, output = self.input.input, self.output.output
   while not link.empty(input) and not link.full(output) do
      -- Trace for happy path
      if test(link.front(input)) then
         local p = link.receive(input)
         -- do stuff with p
         link.transmit(output, p)
      else
         -- Trace for error path
         repeat -- error, discard p
            local p = link.receive(input)
            packet.free(p)
         until test(link.front(input))
      end
   end
   -- Trace for congestion path
   while not link.empty(input) do
      link.transmit(output, link.receive(input))
   end
end