ardour {
	["type"]    = "dsp",
	name        = "a-b Switch",
	category    = "Amplifier",
	license     = "MIT",
	author      = "Ardour Team",
	description = [[Auotomatable A/B Input Select. Channels are mapped in pairs: In 1/2 -> Out 1, In 3/4 -> Out 2, etc.]]
}

function dsp_ioconfig ()
	return {
		-- in theory any combination with N_in = 2 * N_out is possible
		-- (and we should inform the host to override strict-i/o
		-- connect_all_audio_outputs = true)
		{ audio_in = 2, audio_out = 1},
		{ audio_in = 4, audio_out = 2},
		{ audio_in = 8, audio_out = 4},
	}
end

function dsp_params ()
	return { { ["type"] = "input", name = "A/B", min = 0, max = 1, default = 0, toggled = true } }
end

local sr = 48000
local cur_a = 0.0
local cur_b = 0.0

function dsp_init (rate)
	sr = rate
end

function dsp_configure (ins, outs)
	n_ainp = ins:n_audio ()
	n_aout = outs:n_audio ()
	assert (n_aout * 2 == n_ainp)
end

-- the DSP callback function
function dsp_runmap (bufs, in_map, out_map, n_samples, offset)
	local ctrl = CtrlPorts:array() -- get control port array
	local target_A = ctrl[1] > 0 and 0.0 or 1.0
	local target_B = 1 - target_A

	local gA = cur_a
	local gB = cur_b

	for c = 1, n_aout do
		local o = out_map:get (ARDOUR.DataType ("audio"), c - 1)
		if o == ARDOUR.ChanMapping.Invalid then
			goto next
		end

		local in_a = c * 2 - 1
		local in_b = c * 2
		local ia = in_map:get (ARDOUR.DataType ("audio"), in_a - 1)
		local ib = in_map:get (ARDOUR.DataType ("audio"), in_b - 1)

		local buf_aout = bufs:get_audio(o)

		-- optimize fixed gain case
		if cur_a == target_A and cur_b == target_B then
			if target_A == 1.0 then
				-- the first (and only first) channel may be in-place
				if buf_aout ~= bufs:get_audio(ia) then
					buf_aout:read_from (bufs:get_audio(ia):data (0), n_samples, offset, offset)
				end
			else
				assert (buf_aout ~= bufs:get_audio(ib))
				buf_aout:read_from (bufs:get_audio(ib):data (0), n_samples, offset, offset)
			end
			goto next
		end

		-- apply gain to each input channel
		if ia ~= ARDOUR.ChanMapping.Invalid and ia ~= ib then
			cur_a = ARDOUR.Amp.apply_gain (bufs:get_audio(ia), sr, n_samples, gA, target_A, offset)
		end
		if ib ~= ARDOUR.ChanMapping.Invalid and ia ~= ib then
			cur_b = ARDOUR.Amp.apply_gain (bufs:get_audio(ib), sr, n_samples, gB, target_B, offset)
		end

		-- channels are supposed to be in linear order
		assert (buf_aout ~= bufs:get_audio(ib))

		-- copy input to output if needed (first channel may be in-place)
		if buf_aout ~= bufs:get_audio(ia) then
			buf_aout:read_from (bufs:get_audio(ia):data (0), n_samples, offset, offset)
		end

		-- add the second buffer
		ARDOUR.DSP.mix_buffers_no_gain (buf_aout:data (offset), bufs:get_audio(ib):data (offset), n_samples)

		::next::
	end
end