Adaptive Price Channel

// Licensed under the Apache License, Version 2.0
// © mihakralj
//@version=6
indicator("Adaptive Price Channel (APCHANNEL)", "APCHANNEL", overlay=true)

//@function Calculates the Adaptive Price Channel using dual EMA on highs and lows.
//@doc The channel applies exponential smoothing to price highs and lows independently,
//     creating a dynamic envelope that "remembers" significant extremes while gradually
//     fading their influence. Unlike fixed-window Donchian channels that drop extremes
//     abruptly, APCHANNEL decays them smoothly (leaky integration).
//@param alpha Smoothing factor (0 < alpha <= 1). Higher = faster decay, shorter memory.
//@returns tuple of [middle, upper, lower] band values
//@optimized O(1) per bar via EMA recursion; uses FMA pattern: decay*prev + alpha*new
apchannel(simple float alpha) =>
    if alpha <= 0.0 or alpha > 1.0
        runtime.error("Alpha must be > 0 and <= 1")
        [float(na), float(na), float(na)]

    float decay = 1.0 - alpha

    // EMA of highs (upper band)
    var float high_ema = na
    float valid_high = nz(high, nz(high_ema, 0.0))
    if na(high_ema)
        high_ema := valid_high
    else
        high_ema := decay * high_ema + alpha * valid_high

    // EMA of lows (lower band)
    var float low_ema = na
    float valid_low = nz(low, nz(low_ema, 0.0))
    if na(low_ema)
        low_ema := valid_low
    else
        low_ema := decay * low_ema + alpha * valid_low

    // Midpoint
    float mid = (high_ema + low_ema) / 2.0

    [mid, high_ema, low_ema]

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

// Inputs
i_alpha = input.float(0.2, "Alpha (smoothing factor)", minval=0.01, maxval=1.0, step=0.01)

// Calculation
[middle, upper, lower] = apchannel(i_alpha)

// Plot
plot(middle, "Middle", color=color.yellow, linewidth=2)
p1 = plot(upper, "Upper", color=color.new(color.blue, 50), linewidth=1)
p2 = plot(lower, "Lower", color=color.new(color.blue, 50), linewidth=1)
fill(p1, p2, color=color.new(color.blue, 90))