// Licensed under the Apache License, Version 2.0
// © mihakralj
//@version=6
indicator("Absolute Price Oscillator (APO)", "APO", overlay=false)

//@function Calculates Absolute Price Oscillator (APO) as difference between fast and slow EMAs
//@param source Series to calculate APO from
//@param fastLength Period for fast EMA
//@param slowLength Period for slow EMA
//@returns APO value (fast EMA - slow EMA)
apo(series float source, simple int fastLength, simple int slowLength) =>
    if fastLength <= 0 or slowLength <= 0
        runtime.error("Lengths must be greater than 0")
    if fastLength >= slowLength
        runtime.error("Fast length must be less than slow length")
    float alphaFast = 2.0 / (fastLength + 1.0)
    float alphaSlow = 2.0 / (slowLength + 1.0)
    var float emaFast = na var float emaSlow = na, var float e = 1.0
    var bool warmup = true, var float result = na
    if not na(source)
        if na(emaFast)
            emaFast := source, emaSlow := source, result := 0
        else
            emaFast := alphaFast * (source - emaFast) + emaFast
            emaSlow := alphaSlow * (source - emaSlow) + emaSlow
            if warmup
                e *= (1.0 - alphaSlow)
                float c = e > 1e-10 ? 1.0 / (1.0 - e) : 1.0
                float aFast = emaFast * c
                float aSlow = emaSlow * c
                result := aFast - aSlow
                if e <= 1e-10
                    warmup := false
            else
                result := emaFast - emaSlow
    result

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

// Inputs
i_source = input.source(close, "Source")
i_fastLength = input.int(12, "Fast Length", minval=1)
i_slowLength = input.int(26, "Slow Length", minval=1)

// Calculation
apo_value = apo(i_source, i_fastLength, i_slowLength)

// Plot
plot(apo_value, "APO", color.new(color.yellow, 0), 2)