// The MIT License (MIT)
// © mihakralj
//@version=6
indicator("Acceleration (Slope of Slope) (ACCEL)", "ACCEL", overlay=false, precision=8)

//@function Calculates acceleration (slope of slope)
//@doc https://github.com/mihakralj/pinescript/blob/main/indicators/numerics/accel.md
//@param src Source series to calculate slope from
//@param len Lookback period for calculation
//@returns acceleration
accel(series float src, simple int len1) =>
    if len1 <= 1
        runtime.error("Length 1 for slope calculation must be greater than 1")
    var float sumX1 = 0.0, var float sumY1 = 0.0, var float sumXY1 = 0.0, var float sumX21 = 0.0
    var int   validCount1 = 0
    var array<float> x_values1 = array.new_float(len1)
    var array<float> y_values1 = array.new_float(len1)
    var int   head1 = 0
    var int internal_time_counter1 = 0
    if internal_time_counter1 >= len1
        float oldX1 = array.get(x_values1, head1)
        float oldY1 = array.get(y_values1, head1)
        if not na(oldY1)
            sumX1 := sumX1 - oldX1, sumY1 := sumY1 - oldY1
            sumXY1 := sumXY1 - oldX1 * oldY1, sumX21 := sumX21 - oldX1 * oldX1
            validCount1 := validCount1 - 1
    float currentX1 = internal_time_counter1
    float currentY1 = src
    array.set(x_values1, head1, currentX1)
    array.set(y_values1, head1, currentY1)
    if not na(currentY1)
        sumX1 := sumX1 + currentX1, sumY1 := sumY1 + currentY1
        sumXY1 := sumXY1 + currentX1 * currentY1, sumX21 := sumX21 + currentX1 * currentX1
        validCount1 := validCount1 + 1
    head1 := (head1 + 1) % len1
    internal_time_counter1 := internal_time_counter1 + 1
    float current_slope = na
    if validCount1 >= 2
        float n1 = validCount1
        float divisor1 = n1 * sumX21 - sumX1 * sumX1
        if divisor1 != 0.0
            current_slope := (n1 * sumXY1 - sumX1 * sumY1) / divisor1
    var float sumX2 = 0.0, var float sumY2 = 0.0, var float sumXY2 = 0.0, var float sumX22 = 0.0
    var int   validCount2 = 0
    var array<float> x_values2 = array.new_float(len1)
    var array<float> y_values2 = array.new_float(len1)
    var int   head2 = 0
    var int internal_time_counter2 = 0
    if internal_time_counter2 >= len1
        float oldX2 = array.get(x_values2, head2)
        float oldY2 = array.get(y_values2, head2)
        if not na(oldY2)
            sumX2 := sumX2 - oldX2, sumY2 := sumY2 - oldY2
            sumXY2 := sumXY2 - oldX2 * oldY2, sumX22 := sumX22 - oldX2 * oldX2
            validCount2 := validCount2 - 1
    float currentX2 = internal_time_counter2
    float currentY2 = current_slope
    array.set(x_values2, head2, currentX2)
    array.set(y_values2, head2, currentY2)
    if not na(currentY2)
        sumX2 := sumX2 + currentX2, sumY2 := sumY2 + currentY2
        sumXY2 := sumXY2 + currentX2 * currentY2, sumX22 := sumX22 + currentX2 * currentX2
        validCount2 := validCount2 + 1
    head2 := (head2 + 1) % len1
    internal_time_counter2 := internal_time_counter2 + 1
    float calculatedAccel = na
    if validCount2 >= 2
        float n2 = validCount2
        float divisor2 = n2 * sumX22 - sumX2 * sumX2
        if divisor2 != 0.0
            calculatedAccel := (n2 * sumXY2 - sumX2 * sumY2) / divisor2
    calculatedAccel

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

// Inputs
i_period = input.int(14, "Period", minval=2)
i_source = input.source(close, "Source")

// Calculation
a = accel(i_source, i_period)

// Plot
plot(a, "Accel", color=color.yellow, linewidth=2)