COOLDOWN_FACTOR = 0.9995
OVERDRIVE = 1.70
EXPONENT = 1.5

M = 1.7   -- mass
D = 1   -- spring strength
DAMPING = {} -- filled in init()

num_modules = 128
center_module = 64

num_masses = math.floor(num_modules/2)
excitement_pos = 1

-- maximum energy values for each band
maxRedEnergy = 1
maxGreenEnergy = 1
maxBlueEnergy = 1

-- spring-mass-grid values
pos_r = {}
pos_g = {}
pos_b = {}

vel_r = {}
vel_g = {}
vel_b = {}

acc_r = {}
acc_g = {}
acc_b = {}

-- output color buffers
red = {}
green = {}
blue = {}

r_tmp = {}
g_tmp = {}
b_tmp = {}

function limit(val)
	if val > 1 then
		return 1
	elseif val < 0 then
		return 0
	else
		return val
	end
end

function periodic()
	local redEnergy = get_energy_in_band(0, 400);
	local greenEnergy = get_energy_in_band(400, 4000);
	local blueEnergy = get_energy_in_band(4000, 22000);
	local centerIndex = 2 * center_module + 1;

	maxRedEnergy = maxRedEnergy * COOLDOWN_FACTOR
	if redEnergy > maxRedEnergy then
		maxRedEnergy = redEnergy
	end

	maxGreenEnergy = maxGreenEnergy * COOLDOWN_FACTOR
	if greenEnergy > maxGreenEnergy then
		maxGreenEnergy = greenEnergy
	end

	maxBlueEnergy = maxBlueEnergy * COOLDOWN_FACTOR
	if blueEnergy > maxBlueEnergy then
		maxBlueEnergy = blueEnergy
	end

	-- update the spring-mass string

	-- the outside masses are special, as they are auto-returned to 0 position
	--         { spring-mass pendulum                       }  { friction   }
	--acc_r[1] = (-pos_r[1] + (pos_r[2] - pos_r[1])) * D / M
	--acc_g[1] = (-pos_g[1] + (pos_g[2] - pos_g[1])) * D / M
	--acc_b[1] = (-pos_b[1] + (pos_b[2] - pos_b[1])) * D / M

	acc_r[num_masses] = (-pos_r[num_masses] + (pos_r[num_masses-1] - pos_r[num_masses])) * D / M
	acc_g[num_masses] = (-pos_g[num_masses] + (pos_g[num_masses-1] - pos_g[num_masses])) * D / M
	acc_b[num_masses] = (-pos_b[num_masses] + (pos_b[num_masses-1] - pos_b[num_masses])) * D / M

	-- inside masses are only influenced by their neighbors
	for i = 2,num_masses-1 do
		acc_r[i] = (pos_r[i-1] + pos_r[i+1] - 2 * pos_r[i]) * D / M
		acc_g[i] = (pos_g[i-1] + pos_g[i+1] - 2 * pos_g[i]) * D / M
		acc_b[i] = (pos_b[i-1] + pos_b[i+1] - 2 * pos_b[i]) * D / M
	end

	-- update velocity and position
	for i = 1,num_masses do
		vel_r[i] = DAMPING[i] * (vel_r[i] + acc_r[i])
		vel_g[i] = DAMPING[i] * (vel_g[i] + acc_g[i])
		vel_b[i] = DAMPING[i] * (vel_b[i] + acc_b[i])

		pos_r[i] = pos_r[i] + vel_r[i]
		pos_g[i] = pos_g[i] + vel_g[i]
		pos_b[i] = pos_b[i] + vel_b[i]
	end

	-- set the new position for the center module
	newRed = redEnergy / maxRedEnergy
	pos_r[excitement_pos] = newRed
	vel_r[excitement_pos] = 0
	acc_r[excitement_pos] = 0

	newGreen = greenEnergy / maxGreenEnergy
	pos_g[excitement_pos] = newGreen
	vel_b[excitement_pos] = 0
	acc_b[excitement_pos] = 0

	newBlue = blueEnergy / maxBlueEnergy
	pos_b[excitement_pos] = newBlue
	vel_b[excitement_pos] = 0
	acc_b[excitement_pos] = 0

	-- map to LED modules
	for i = 1,num_masses do
		r_tmp[i] = pos_r[i]
		g_tmp[i] = pos_g[i]
		b_tmp[i] = pos_b[i]

		r_tmp[num_modules-i+1] = pos_r[i]
		g_tmp[num_modules-i+1] = pos_g[i]
		b_tmp[num_modules-i+1] = pos_b[i]

		--print(i, pos_r[i])
	end

	-- make colors more exciting
	for i = 1,num_modules do
		red[i]   = limit(OVERDRIVE * math.pow(r_tmp[i], EXPONENT))
		green[i] = limit(OVERDRIVE * math.pow(g_tmp[i], EXPONENT))
		blue[i]  = limit(OVERDRIVE * math.pow(b_tmp[i], EXPONENT))
	end

	-- return the 3 color arrays
	return red, green, blue
end

function init(nmod, cmod)
	num_modules = nmod
	center_module = cmod

	num_masses = math.floor(nmod/2)
	excitement_pos = 1

	for i = 1,nmod do
		red[i] = 0
		green[i] = 0
		blue[i] = 0
	end

	for i = 1,num_masses do
		pos_r[i] = 0
		pos_g[i] = 0
		pos_b[i] = 0

		vel_r[i] = 0
		vel_g[i] = 0
		vel_b[i] = 0

		acc_r[i] = 0
		acc_g[i] = 0
		acc_b[i] = 0

		DAMPING[i] = 1 - 0.08 * math.pow(math.abs((i - excitement_pos) / num_masses), 2)
	end

	-- don't use fading
	return 0
end