Bollinger band using STD Dev and MA for Amibroker (AFL)

_SECTION_BEGIN("Price");
SetChartOptions(0,chartShowArrows|chartShowDates);
_N(Title = StrFormat("{{NAME}} - {{INTERVAL}} {{DATE}} Open %g, Hi %g, Lo %g, Close %g (%.1f%%) {{VALUES}}", O, H, L, C, SelectedValue( ROC( C, 1 ) ) ));

Plot( C, "Current Price: ", IIf( O < C, colorBrightGreen, colorRed ), styleCandle );
_SECTION_END();


_SECTION_BEGIN("Middle Band");

P = ParamField("Price Field", -1);
Period = Param("Period", 20, 2, 300, 1);

MiddleBand = MA(P, Period);

Plot(MiddleBand,"", ParamColor("Color", colorYellow), ParamStyle("Style")|styleNoRescale);
_SECTION_END();

_SECTION_BEGIN("Uppar Band");

P = ParamField("Price Field", -1);
Period = Param("Period", 20, 2, 300, 1);

UpparBand = MA(P, Period) + StDev(P, Period)*2;

Plot(UpparBand, "", ParamColor("Color", colorGreen), ParamStyle("Style")|styleNoRescale);

_SECTION_END();

_SECTION_BEGIN("Lower Band");

P = ParamField("Price Field", -1);
Period = Param("Period", 20, 2, 300, 1);

LowerBand = MA(P, Period) - StDev(P, Period)*2;

Plot(LowerBand, "", ParamColor("Color", colorRed), ParamStyle("Style")|styleNoRescale);

_SECTION_END();

_SECTION_BEGIN("Buy Sell calculation");

PositionSize = 5000;
Buy =  Ref(L < LowerBand, -2) AND Ref(C > LowerBand, -1);
Short = Ref(H > UpparBand, -2)  AND Ref(C < UpparBand, -1);

Buyst=ValueWhen(Buy,O,1);
Shortst=ValueWhen(Short,O,1);
 
Sell=  Cross(Buyst*0.995,C) ;
Cover=  Cross(C,Shortst*1.005);
 
Buy=ExRem(Buy,Short) OR ExRem(Buy,Sell) ;
Short=ExRem(Short,Buy) OR ExRem(Short,Cover);
Sell=ExRem(Sell,Buy);
Cover=ExRem(Cover,Short);
SellPrice = ValueWhen( Sell, C, 1 );

BuyPrice = ValueWhen( Buy, C, 1 );

FirstVisibleBar = Status( "FirstVisibleBar" );
Lastvisiblebar = Status("LastVisibleBar");
for( b = Firstvisiblebar; b <= Lastvisiblebar AND b < BarCount; b++)
{
if( Buy[b] ) PlotText("\n Buy\N "+NumToStr(BuyPrice[b],1.2),b,BuyPrice[b],colorBrightGreen);
else if( Sell[b] ) PlotText(" \nSell \N"+NumToStr(SellPrice[b],1.2),b,SellPrice[b],colorRed);
}

PlotShapes(shapeUpArrow*Buy, colorGreen, 0, L, Offset=-45);
PlotShapes(shapeDownArrow*Sell, colorRed, 0, H, Offset=-45);


_SECTION_END();