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

//@function Calculates Force Index — EMA-smoothed price change × volume
//@param period EMA smoothing period for the raw force
//@returns smoothed Force Index value
fi(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
    var float prevClose = na

    float src = nz(close)
    float vol = nz(volume)

    float rawForce = not na(prevClose) ? (src - prevClose) * vol : 0.0
    prevClose := src

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

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

    result

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

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

// Calculation
float forceIndex = fi(i_period)

// Plot
plot(forceIndex, "Force Index", color.yellow, 2)
hline(0, "Zero", color=color.gray, linestyle=hline.style_dotted)