-- Description from the http://www.investopedia.com/articles/trading/03/022603.asp

-- initializes the indicator
function Init()
    indicator:name("Elder-Ray");
    indicator:description("")
    indicator:requiredSource(core.Bar);
    indicator:type(core.Oscillator);

    indicator.parameters:addInteger("N", "Number of periods for smoothing", "", 13);
    indicator.parameters:addColor("BullC", "Color of the bull power line", "", core.rgb(255, 0, 0));
    indicator.parameters:addColor("BearC", "Color of the bear power line", "", core.rgb(0, 255, 0));
end

local source;
local EMA;
local Bull;
local Bear;

-- process parameters and prepare for calculations
function Prepare()
    source = instance.source;
    EMA = core.indicators:create("EMA", source.close, instance.parameters.N, core.rgb(0, 0, 0));
    local name = profile:id() .. "(" .. source:name() .. ", " .. instance.parameters.N .. ")";
    instance:name(name);
    Bull = instance:addStream("Bull", core.Line, name .. ".Bull", "Bull", instance.parameters.BullC, EMA.DATA:first());
    Bear = instance:addStream("Bear", core.Line, name .. ".Bear", "Bear", instance.parameters.BearC, EMA.DATA:first());
    Bull:addLevel(0);
end

-- Indicator calculation routine
function Update(period, mode)
    EMA:update(mode);

    if (period >= EMA.DATA:first()) then
        Bull[period] = source.high[period] - EMA.DATA[period];
        Bear[period] = source.low[period] - EMA.DATA[period];
    end
end