improve comments

Author: jangorecki

R/frollapply.R

@@ -295,7 +295,7 @@ frollapply = function(X, N, FUN, ..., by.column=TRUE, fill=NA, align=c("right","
             setDTthreads(1L)       ## disable nested parallelism
             lapply(ii[[th]],       ## loops over indexes for that thread
                    FUN = tight,    ## handles adaptive and by.column
-                   dest = cpy(w),  ## allocate own window for each thread
+                   dest = cpy(w),  ## allocate own window for each thread, if we would not copy here, then copy would be handled later on by fork's copy-on-write
                    src = thisx,    ## full input
                    n = thisn)      ## scalar or in adaptive case a vector
             # nocov end

src/frollapply.c

@@ -85,7 +85,7 @@ SEXP memcpyDTadaptive(SEXP dest, SEXP src, SEXP offset, SEXP size) {
 SEXP setgrowable(SEXP x) {
   if (!isNewList(x)) {
     SET_GROWABLE_BIT(x);
-    SET_TRUELENGTH(x, LENGTH(x)); // we just reset length, doesn't seems to be necessary but for future compatibility to R it is more safe to set it up
+    SET_TRUELENGTH(x, LENGTH(x)); // important because gc() uses TRUELENGTH to keep counts
   } else {
     // # nocov start ## does not seem to be reported to codecov most likely due to running in a fork, I manually debugged that it is being called when running froll.Rraw
     for (int i=0; i<LENGTH(x); ++i) {