Choppiness Index

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

//@function Calculates Choppiness Index to measure market trendiness
//@param length Lookback period for calculation
//@returns CHOP value between 0 and 100 (lower=trending, higher=choppy)
//@references E.W. Dreiss, Australian commodity trader
//@optimized O(n) with circular buffers for TR sum and high/low tracking
chop(simple int length) =>
    if length <= 1
        runtime.error("Length must be > 1")
    var float sum_tr = 0.0
    var int head = 0
    var int filled = 0
    var array<float> atr_buf = array.new_float(length, na)
    var array<float> high_buf = array.new_float(length, na)
    var array<float> low_buf = array.new_float(length, na)
    float prevClose = nz(close[1], close)
    float tr = math.max(high - low, math.max(math.abs(high - prevClose), math.abs(low - prevClose)))
    float out = array.get(atr_buf, head)
    if not na(out)
        sum_tr -= out
    else
        filled += 1
    array.set(atr_buf, head, tr)
    array.set(high_buf, head, high)
    array.set(low_buf, head, low)
    sum_tr += tr
    int win = math.min(filled, length)
    float hhv = -1e100
    float llv = 1e100
    for k = 0 to win - 1
        int idx = (head - k + length) % length
        float h = array.get(high_buf, idx)
        float l = array.get(low_buf, idx)
        if not na(h)
            hhv := math.max(hhv, h)
        if not na(l)
            llv := math.min(llv, l)
    head := (head + 1) % length
    float price_range = hhv - llv
    float chop_value = na
    if win >= 2 and price_range > 0
        float log_ratio = math.log(sum_tr / price_range) / math.log(10)
        float log_len = math.log(win) / math.log(10)
        chop_value := 100.0 * log_ratio / log_len
        chop_value := math.max(0.0, math.min(100.0, chop_value))
    chop_value

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

// Inputs
i_length = input.int(14, "Length", minval=2)

// Calculation
chop_value = chop(i_length)

// Plot
plot(chop_value, "CHOP", color=color.yellow, linewidth=2)
hline(61.8, "High Threshold", color=color.new(color.red, 50), linestyle=hline.style_dashed)
hline(38.2, "Low Threshold", color=color.new(color.green, 50), linestyle=hline.style_dashed)
hline(50, "Midline", color=color.new(color.gray, 70), linestyle=hline.style_dotted)