R help - Mar 2014 - What formula does the acf() function use?

Dear list,
I am curious about how R computes the autocorrelation?  What formula does
it use?  Theoretical or sample ACF?

I have tried to type acf in the R console, but I don't understand what it
is actually doing besides the basic if...else... statement.  Could someone
help me read this output?
> acf
function (x, lag.max = NULL, type = c("correlation",
"covariance",
    "partial"), plot = TRUE, na.action = na.fail, demean = TRUE,
    ...)
{
    type <- match.arg(type)
    if (type == "partial") {
        m <- match.call()
        m[[1L]] <- quote(stats::pacf)
        m$type <- NULL
        return(eval(m, parent.frame()))
    }
    series <- deparse(substitute(x))
    x <- na.action(as.ts(x))
    x.freq <- frequency(x)
    x <- as.matrix(x)
    if (!is.numeric(x))
        stop("'x' must be numeric")
    sampleT <- as.integer(nrow(x))
    nser <- as.integer(ncol(x))
    if (is.na(sampleT) || is.na(nser))
        stop("'sampleT' and 'nser' must be integer")
    if (is.null(lag.max))
        lag.max <- floor(10 * (log10(sampleT) - log10(nser)))
    lag.max <- as.integer(min(lag.max, sampleT - 1L))
    if (is.na(lag.max) || lag.max < 0)
        stop("'lag.max' must be at least 0")
    if (demean)
        x <- sweep(x, 2, colMeans(x, na.rm = TRUE), check.margin = FALSE)
    lag <- matrix(1, nser, nser)
    lag[lower.tri(lag)] <- -1
    acf <- .Call(C_acf, x, lag.max, type == "correlation")
    lag <- outer(0:lag.max, lag/x.freq)
    acf.out <- structure(list(acf = acf, type = type, n.used = sampleT,
        lag = lag, series = series, snames = colnames(x)), class =
"acf")
    if (plot) {
        plot.acf(acf.out, ...)
        invisible(acf.out)
    }
    else acf.out
}
<bytecode: 0x1020badf0>
<environment: namespace:stats>


Thanks in advance,

Mike

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