// Licensed under the Apache License, Version 2.0
// © mihakralj
//@version=6
indicator("Inertia Oscillator (INERTIA)", "INERTIA", overlay=false)

//@function Calculates Inertia oscillator measuring trend strength based on distance from linear regression
//@param source Source series to calculate Inertia for
//@param length Period for linear regression calculation
//@returns Inertia value measuring trend strength
inertia(series float source, simple int length) =>
    if length <= 0
        runtime.error("Length must be positive")
    if na(source)
        na
    else
        available_bars = bar_index + 1
        effective_length = math.min(length, available_bars)
        sum_x = 0.0, sum_y = 0.0, sum_xy = 0.0, sum_x2 = 0.0
        for i = 0 to effective_length - 1
            x = effective_length - 1 - i
            y = nz(source[i])
            sum_x += x, sum_y += y
            sum_xy += x * y, sum_x2 += x * x
        n = effective_length
        denominator = n * sum_x2 - sum_x * sum_x
        if denominator == 0
            0.0 
        else
            slope = (n * sum_xy - sum_x * sum_y) / denominator
            intercept = (sum_y - slope * sum_x) / n
            regression_value = slope * (effective_length - 1) + intercept
            source - regression_value

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

// Inputs
i_length = input.int(20, "Length", minval=1, maxval=500, tooltip="Period for linear regression calculation")
i_source = input.source(close, "Source", tooltip="Price series to analyze")

// Calculation
inertia_value = inertia(i_source, i_length)

// Plots
plot(inertia_value, "Inertia", color=color.yellow, linewidth=2)