This indicator was developed by Roger Altman and was introduced in the February 1993 issue of "Technical Analysis of Stocks & Commodities" magazine. The RSI was developed by J. Welles Wilder and is discussed in his book "New Concepts in Technical Trading".
After I got done coding this, I saw that tradingview.com/u/TheLark/ had already coded the same indicator. Small differences, but same indicator, just want to give credit to a line leader.