fix diff method (breaking)

R/smoothness.R

@@ -2,7 +2,7 @@
 #'
 #' Functions to quantify the smoothness of a vector, which can be used in some cases
 #' as an index of "linearity". A smooth series is one that does not have abrupt changes in
-#' its values. The smoothness of a series can be measured in different ways, such
+#' its values. The smoothness of a series can be approximated in different ways, such
 #' as the standard deviation of the standardized differences or the lag-one
 #' autocorrelation.
 #'
@@ -25,6 +25,10 @@
 #' # When perfectly linear, the "smoothness" is 1
 #' smoothness(1:10)
 #'
+#' # And closer to zero for random
+#' smoothness(rnorm(1000))
+#' smoothness(rnorm(1000), method = "diff")
+#'
 #' @return Value of smoothness.
 #' @references https://stats.stackexchange.com/questions/24607/how-to-measure-smoothness-of-a-time-series-in-r
 #'
@@ -54,7 +58,8 @@ smoothness.numeric <- function(x,
   if (method == "cor") {
     smooth <- stats::cor(utils::head(x, length(x) - lag), utils::tail(x, length(x) - lag))
   } else {
-    smooth <- stats::sd(diff(x, lag = lag)) / abs(mean(diff(x, lag = lag)))
+    diff <- standardize(diff(x))
+    smooth <- 1 - mean((diff(diff) ** 2) / 4)  # Note the reversal to match the other method
   }
 
   if (!is.null(iterations)) {