// Licensed under the Apache License, Version 2.0
// © mihakralj
//@version=6
indicator("Percentage Change (CHANGE)", "CHANGE", overlay=false, format=format.percent)

//@function Calculates the percentage change of a source series over a specified length using the history referencing operator for efficiency.
//@param source The source series (e.g. close price).
//@param length The lookback period (number of bars). Must be > 0.
//@returns float The percentage change over the specified length. Returns `na` if the historical value is `na` or zero.
//@optimized Uses direct history access `source[length]` instead of array manipulation.
change(float source, int length) =>
    if length <= 0
        runtime.error("Length must be greater than 0")
    float oldValue = source[length]
    if na(oldValue) or oldValue == 0
        na
    else
        (source / oldValue - 1) // Already a percentage, Pine handles plotting format

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

// Inputs
i_source = input.source(close, "Source")
i_length = input.int(1, "Length", minval = 1)

// Calculation
result = change(i_source, i_length)

// Plot
plot(result, "Change %", color=color.yellow, linewidth=2)