Price Channel (PCHANNEL)

// The MIT License (MIT)
// © mihakralj
//@version=6
indicator("Price Channel (PCHANNEL)", "PCHANNEL", overlay=true)

//@function Calculates Price Channel
//@param length_param Lookback period for determining the highest high and lowest low
//@returns tuple [upperChannel, middleChannel, lowerChannel]
//@optimized Uses monotonic deque for O(1) amortized complexity per bar
pchannel(simple int length_param) =>
    if length_param <= 0
        runtime.error("Length must be greater than 0")
    var deque_hi = array.new_int(0)
    var src_buffer_hi = array.new_float(0, na)
    var int current_index_hi = 0
    var deque_lo = array.new_int(0)
    var src_buffer_lo = array.new_float(0, na)
    var int current_index_lo = 0
    if array.size(src_buffer_hi) != length_param
        src_buffer_hi := array.new_float(length_param, na)
        current_index_hi := 0
        array.clear(deque_hi)
        src_buffer_lo := array.new_float(length_param, na)
        current_index_lo := 0
        array.clear(deque_lo)
    float cv_hi = nz(high)
    array.set(src_buffer_hi, current_index_hi, cv_hi)
    float cv_lo = nz(low)
    array.set(src_buffer_lo, current_index_lo, cv_lo)
    while array.size(deque_hi) > 0 and array.get(deque_hi, 0) <= bar_index - length_param
        array.shift(deque_hi)
    while array.size(deque_lo) > 0 and array.get(deque_lo, 0) <= bar_index - length_param
        array.shift(deque_lo)
    while array.size(deque_hi) > 0
        if array.get(src_buffer_hi, array.get(deque_hi, array.size(deque_hi) - 1) % length_param) <= cv_hi
            array.pop(deque_hi)
        else
            break
    array.push(deque_hi, bar_index)
    while array.size(deque_lo) > 0
        if array.get(src_buffer_lo, array.get(deque_lo, array.size(deque_lo) - 1) % length_param) >= cv_lo
            array.pop(deque_lo)
        else
            break
    array.push(deque_lo, bar_index)
    float highestHigh = array.get(src_buffer_hi, array.get(deque_hi, 0) % length_param)
    current_index_hi := (current_index_hi + 1) % length_param
    float lowestLow = array.get(src_buffer_lo, array.get(deque_lo, 0) % length_param)
    current_index_lo := (current_index_lo + 1) % length_param
    [highestHigh, (highestHigh + lowestLow) / 2.0, lowestLow]

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

// Inputs
i_length = input.int(20, "Length", minval=1)

// Calculation
[upperCh, middleCh, lowerCh] = pchannel(i_length)

// Plot
plot(middleCh, "Middle Channel", color=color.yellow, linewidth=2)
p1 = plot(upperCh, "Upper Channel", color=color.yellow, linewidth=2)
p2 = plot(lowerCh, "Lower Channel", color=color.yellow, linewidth=2)
fill(p1, p2, color=color.new(color.blue, 90), title="Band Fill")