Pretty Good Oscillator

// The MIT License (MIT)
// © mihakralj
//@version=6
indicator("Pretty Good Oscillator", "PGO", overlay=false)

//@function Calculate Pretty Good Oscillator (PGO)
//@param source Price data to analyze
//@param period Number of bars for SMA and ATR calculation
//@returns PGO value normalized by ATR
pgo(series float source, simple int period) =>
    if period <= 0
        runtime.error("Period must be greater than 0")
    if period > 5000
        runtime.error("Period exceeds maximum of 5000")
    
    var array<float> sma_buffer = array.new_float(period, na)
    var int sma_head = 0
    var float sma_sum = 0.0
    var int valid_count = 0
    
    float oldest = array.get(sma_buffer, sma_head)
    if not na(oldest)
        sma_sum -= oldest
        valid_count -= 1
    
    if not na(source)
        sma_sum += source
        valid_count += 1
    
    array.set(sma_buffer, sma_head, source)
    sma_head := (sma_head + 1) % period
    
    float sma_value = nz(sma_sum / valid_count, source)
    
    float prevClose = nz(close[1], close)
    float tr1 = high - low
    float tr2 = math.abs(high - prevClose)
    float tr3 = math.abs(low - prevClose)
    float tr = math.max(tr1, math.max(tr2, tr3))
    
    float a = 1.0 / float(period)
    float beta = 1.0 - a
    var bool warmup = true
    var float e = 1.0
    var float ema = 0.0
    var float atr = nz(tr)
    
    ema := a * (nz(tr) - ema) + ema
    
    if warmup
        e *= beta
        float c = 1.0 / (1.0 - e)
        atr := c * ema
        warmup := e > 1e-10
    else
        atr := ema
    
    float pgo_value = atr > 0 ? (source - sma_value) / atr : na
    
    pgo_value

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

// Inputs
i_period = input.int(14, "Period", minval=1, maxval=500, tooltip="Number of bars for SMA and ATR calculation")
i_source = input.source(close, "Source")

// Calculation
result = pgo(i_source, i_period)

// Plot
plot(result, "PGO", color=color.yellow, linewidth=2)
hline(0, "Zero Line", color=color.gray, linestyle=hline.style_solid)
hline(3, "Overbought", color=color.red, linestyle=hline.style_dashed)
hline(-3, "Oversold", color=color.green, linestyle=hline.style_dashed)

// Background coloring for extreme zones
bgcolor(not na(result) and result > 3 ? color.new(color.red, 85) : not na(result) and result < -3 ? color.new(color.green, 85) : na)