Positive Volume Index (PVI)

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//  Copyright by HPotter v1.0 11/06/2014
// The theory behind the indexes is as follows: On days of increasing volume, 
// you can expect prices to increase, and on days of decreasing volume, you can 
// expect prices to decrease. This goes with the idea of the market being in-gear 
// and out-of-gear. Both PVI and NVI work in similar fashions: Both are a running 
// cumulative of values, which means you either keep adding or subtracting price 
// rate of change each day to the previous day`s sum. In the case of PVI, if today`s 
// volume is less than yesterday`s, don`t add anything; if today`s volume is greater, 
// then add today`s price rate of change. For NVI, add today`s price rate of change 
// only if today`s volume is less than yesterday`s.
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study(title="Positive Volume Index", shorttitle="Positive Volume Index")
EMA_Len = input(255, minval=1)
xROC = roc(close, 1)
nRes = iff(volume > volume[1], nz(nRes[1], 0) + xROC, nz(nRes[1], 0))
nResEMA = ema(nRes, EMA_Len)
plot(nRes, color=red, title="PVI")
plot(nResEMA, color=blue, title="EMA")