// Licensed under the Apache License, Version 2.0
// © mihakralj
//@version=6
indicator("Standard Deviation (STDDEV)", "STDDEV", overlay=false)

//@function Calculates the standard deviation using a single pass with a circular buffer.
//@param src {series float} Source series.
//@param len {int} Lookback length. `len` > 0.
//@returns {series float} Standard deviation of `src` for `len` bars back. Returns `na` if not enough data.
stddev(series float src, int len) =>
    if len <= 0
        runtime.error("Period must be greater than 0")
    var int p = math.max(1, len)
    var array<float> buffer = array.new_float(p, na)
    var int head = 0,  var int count = 0
    var float sum = 0.0, var float sumSq = 0.0
    float oldest = array.get(buffer, head)
    if not na(oldest)
        sum -= oldest
        sumSq -= oldest * oldest
        count -= 1
    float val = nz(src)
    sum += val
    sumSq += val * val
    count += 1
    array.set(buffer, head, val)
    head := (head + 1) % p
    count > 1 ? math.sqrt(math.max(0.0, (sumSq / count) - math.pow(sum / count, 2))) : 0.0


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

// Inputs
i_period = input.int(14, "Period", minval=1) // Default period 14
i_source = input.source(close, "Source")

// Calculation
stddev_value = stddev(i_source, i_period)

// Plot
plot(stddev_value, "StdDev", color=color.yellow, linewidth=2) // Changed color