// Licensed under the Apache License, Version 2.0
// © mihakralj
//@version=6
indicator("Elder Ray Index (ERI)", "ERI", overlay=false)

//@function Calculates Elder Ray Index (Bull Power and Bear Power)
//@param period EMA period for the centerline
//@returns tuple [bullPower, bearPower] measuring buying/selling pressure
eri(simple int period) =>
    if period <= 0
        runtime.error("Period must be greater than 0")

    float alpha = 2.0 / (period + 1.0)
    float beta = 1.0 - alpha

    var float ema = 0.0
    var float e = 1.0
    var bool warmup = true
    var bool first = true

    float src = nz(close)

    if first
        ema := src
        first := false
    else
        ema := alpha * src + beta * ema

    float emaVal = src
    if warmup
        e *= beta
        float c = e > 1e-10 ? 1.0 / (1.0 - e) : 1.0
        emaVal := ema * c
        if e <= 1e-10
            warmup := false
    else
        emaVal := ema

    float bullPower = high - emaVal
    float bearPower = low - emaVal

    [bullPower, bearPower]

// ---------- Main loop ----------

// Inputs
i_period = input.int(13, "Period", minval=1, maxval=500)

// Calculation
[bull, bear] = eri(i_period)

// Plot
plot(bull, "Bull Power", color.new(color.green, 0), 2)
plot(bear, "Bear Power", color.new(color.red, 0), 2)
hline(0, "Zero", color=color.gray, linestyle=hline.style_dotted)