// Licensed under the Apache License, Version 2.0
// © mihakralj
//@version=6
indicator("Pearson's Correlation (CORREL)", "CORREL", overlay=false)

//@function Calculates Pearson correlation coefficient using single pass with circular buffer
//@param src1 series float First series to analyze
//@param src2 series float Second series to analyze
//@param len simple int Lookback period for calculation
//@returns float Pearson correlation coefficient between -1 and 1
//@optimized for performance using combined covariance and variance calculation
correlation(series float src1, series float src2, simple int len) =>
    if len <= 0
        runtime.error("Period must be greater than 0")
    var int p = math.max(1, len)
    var array<float> buffer1 = array.new_float(p, na)
    var array<float> buffer2 = array.new_float(p, na)
    var int head = 0, var int count = 0
    var float sum1 = 0.0, var float sum2 = 0.0
    var float sumSq1 = 0.0, var float sumSq2 = 0.0
    var float sumProd = 0.0
    float oldest1 = array.get(buffer1, head)
    float oldest2 = array.get(buffer2, head)
    if not na(oldest1) and not na(oldest2)
        sum1 -= oldest1, sum2 -= oldest2
        sumSq1 -= oldest1 * oldest1, sumSq2 -= oldest2 * oldest2
        sumProd -= oldest1 * oldest2
        count -= 1
    if not na(src1) and not na(src2)
        sum1 += src1, sum2 += src2
        sumSq1 += src1 * src1, sumSq2 += src2 * src2
        sumProd += src1 * src2
        count += 1
        array.set(buffer1, head, src1)
        array.set(buffer2, head, src2)
    else
        array.set(buffer1, head, na)
        array.set(buffer2, head, na)
    head := (head + 1) % p
    if count > 1
        mean1 = sum1 / count, mean2 = sum2 / count
        cov = (sumProd / count) - mean1 * mean2
        var1 = (sumSq1 / count) - mean1 * mean1
        var2 = (sumSq2 / count) - mean2 * mean2
        stddev1 = math.sqrt(math.max(0.0, var1))
        stddev2 = math.sqrt(math.max(0.0, var2))
        denominator = stddev1 * stddev2
        if denominator != 0
            cov / denominator
        else
            na
    else
        na

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

// Inputs
i_source1 = input.source(close, "Source 1")
i_source2_ticker = input.symbol("SPY", "Source 2 Ticker (e.g., SPY, AAPL)")
i_period = input.int(20, "Period", minval=2)

i_source2 = request.security(i_source2_ticker, timeframe.period, close, lookahead=barmerge.lookahead_off)

// Calculation
correlation_value = correlation(i_source1, i_source2, i_period)

// Plot
plot(correlation_value, "Correlation", color=color.yellow, linewidth=2)