Apirine Slow RSI (SRSI)

// Licensed under the Apache License, Version 2.0
// © mihakralj
//@version=6
indicator("Apirine Slow RSI (SRSI)", "SRSI", overlay = false)

//@function Apirine Slow RSI — RSI variant that measures price changes relative
//          to an EMA rather than to the previous close. Uses Wilder's smoothing
//          (RMA) for gain/loss averaging.
//          Pipeline: EMA(Close) → diff = Close - EMA → RMA(posDiff, negDiff) → RSI formula.
//          Output bounded [0, 100].
//@param source Series to analyze
//@param emaLength EMA period for price smoothing (>= 1)
//@param rsiLength Wilder's smoothing period for gain/loss (>= 1)
//@returns SRSI oscillator value (bounded 0-100)
//@reference Apirine, V. (2015). "The Slow Relative Strength Index."
//           Technical Analysis of Stocks & Commodities, April 2015.
//@optimized O(1) per bar — pure scalar state, no buffers
srsi(series float source, simple int emaLength, simple int rsiLength) =>
    if emaLength < 1
        runtime.error("EMA length must be at least 1")
    if rsiLength < 1
        runtime.error("RSI length must be at least 1")

    float price = nz(source)

    // --- EMA of close ---
    float alpha_ema = 2.0 / (emaLength + 1)
    var float ema_val = na
    ema_val := na(ema_val) ? price : alpha_ema * (price - ema_val) + ema_val

    // --- Difference: Close - EMA ---
    float diff = price - ema_val
    float pos_diff = diff > 0 ? diff : 0.0
    float neg_diff = diff < 0 ? -diff : 0.0

    // --- Wilder's smoothing (RMA) for pos/neg differences ---
    float alpha_rma = 1.0 / rsiLength
    var float avg_pos = na
    var float avg_neg = na
    avg_pos := na(avg_pos) ? pos_diff : avg_pos + alpha_rma * (pos_diff - avg_pos)
    avg_neg := na(avg_neg) ? neg_diff : avg_neg + alpha_rma * (neg_diff - avg_neg)

    // --- SRSI = 100 - 100 / (1 + SRS) ---
    float result = avg_neg < 1e-10 ? (avg_pos < 1e-10 ? 50.0 : 100.0) : 100.0 - 100.0 / (1.0 + avg_pos / avg_neg)
    result

// ── Inputs ──
int   p_emaLen = input.int(6, "EMA Length", minval = 1)
int   p_rsiLen = input.int(14, "RSI Length", minval = 1)
float p_src    = input.source(close, "Source")

// ── Calculation ──
float out = srsi(p_src, p_emaLen, p_rsiLen)

// ── Plot ──
plot(out, "SRSI", color.yellow, 2)
hline(80, "Overbought", color.new(color.red, 60))
hline(50, "Middle", color.gray)
hline(20, "Oversold", color.new(color.green, 60))