// Licensed under the Apache License, Version 2.0
// © mihakralj
//@version=6
indicator("Fractal Chaos Bands (FCB)", "FCB", overlay=true)

//@function Calculates Fractal Chaos Bands based on fractal highs and lows
//@param period Lookback period for highest/lowest calculation
//@returns [upper_band, lower_band] Fractal Chaos Band values
//@optimized Uses monotonic deque for O(1) amortized complexity
fcb(simple int period) =>
    if period <= 0
        runtime.error("Period must be greater than 0")
    bool is_fractal_high = high[1] > high[2] and high[1] > high[0]
    bool is_fractal_low = low[1] < low[2] and low[1] < low[0]
    var float hi_fractal = high
    var float lo_fractal = low
    if is_fractal_high
        hi_fractal := high[1]
    if is_fractal_low
        lo_fractal := low[1]
    var hi_deque = array.new_int(0)
    var hi_buffer = array.new_float(period, na)
    var int hi_current_index = 0
    float hi_current_val = nz(hi_fractal)
    array.set(hi_buffer, hi_current_index, hi_current_val)
    while array.size(hi_deque) > 0 and array.get(hi_deque, 0) <= bar_index - period
        array.shift(hi_deque)
    while array.size(hi_deque) > 0
        int last_index = array.get(hi_deque, array.size(hi_deque) - 1)
        int buffer_lookup = last_index % period
        if array.get(hi_buffer, buffer_lookup) <= hi_current_val
            array.pop(hi_deque)
        else
            break
    array.push(hi_deque, bar_index)
    int highest_index = array.get(hi_deque, 0)
    int highest_buffer_index = highest_index % period
    float upper_band = array.get(hi_buffer, highest_buffer_index)
    hi_current_index := (hi_current_index + 1) % period
    var lo_deque = array.new_int(0)
    var lo_buffer = array.new_float(period, na)
    var int lo_current_index = 0
    float lo_current_val = nz(lo_fractal)
    array.set(lo_buffer, lo_current_index, lo_current_val)
    while array.size(lo_deque) > 0 and array.get(lo_deque, 0) <= bar_index - period
        array.shift(lo_deque)
    while array.size(lo_deque) > 0
        int last_index = array.get(lo_deque, array.size(lo_deque) - 1)
        int buffer_lookup = last_index % period
        if array.get(lo_buffer, buffer_lookup) >= lo_current_val
            array.pop(lo_deque)
        else
            break
    array.push(lo_deque, bar_index)
    int lowest_index = array.get(lo_deque, 0)
    int lowest_buffer_index = lowest_index % period
    float lower_band = array.get(lo_buffer, lowest_buffer_index)
    lo_current_index := (lo_current_index + 1) % period
    [upper_band, lower_band]

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

// Inputs
i_period = input.int(20, "Period", minval=1, maxval=999)
plot_fractal_points = input.bool(false, "Show Fractal Points")

// Calculation
[upper_band, lower_band] = fcb(i_period)

// Plots
p_upper = plot(upper_band, "Upper Band", color=color.yellow, linewidth=2)
p_lower = plot(lower_band, "Lower Band", color=color.yellow, linewidth=2)
fill(p_upper, p_lower, color=color.new(color.blue, 90), title="Band Fill")