copySharedColumns(): hash instead of TRUELENGTH

src/utils.c

@@ -261,36 +261,22 @@ void copySharedColumns(SEXP x) {
   const int ncol = length(x);
   if (!isNewList(x) || ncol==1) return;
   bool *shared = (bool *)R_alloc(ncol, sizeof(bool)); // on R heap in case alloc fails
-  int *savetl = (int *)R_alloc(ncol, sizeof(int));  // on R heap for convenience but could be a calloc
+  hashtab * marks = hash_create(ncol);
   const SEXP *xp = SEXPPTR_RO(x);
-  // first save the truelength, which may be negative on specials in dogroups, and set to zero; test 2157
-  // the savetl() function elsewhere is for CHARSXP. Here, we are using truelength on atomic vectors.
-  for (int i=0; i<ncol; ++i) {
-    const SEXP thiscol = xp[i];
-    savetl[i] = ALTREP(thiscol) ? 0 : TRUELENGTH(thiscol);
-    SET_TRUELENGTH(thiscol, 0);
-  }
   int nShared=0;
   for (int i=0; i<ncol; ++i) {
     SEXP thiscol = xp[i];
-    if (ALTREP(thiscol) || TRUELENGTH(thiscol)<0) {
+    if (ALTREP(thiscol) || hash_lookup(marks, thiscol, 0)<0) {
       shared[i] = true;  // we mark ALTREP as 'shared' too, whereas 'tocopy' would be better word to use for ALTREP
       nShared++;
-      // do not copyAsPlain() here yet, as its alloc might fail. Must restore tl first to all columns before attempting any copies.
     } else {
-      shared[i] = false;              // so the first column will never be shared (unless it is an altrep) even it is shared
+      shared[i] = false;              // so the first column will never be shared (unless it is an altrep) even if is shared
                                       // 'shared' means a later column shares an earlier column
-      SET_TRUELENGTH(thiscol, -i-1);  // -i-1 so that if, for example, column 3 shares column 1, in iteration 3 we'll know not
+      hash_set(marks, thiscol, -i-1);  // -i-1 so that if, for example, column 3 shares column 1, in iteration 3 we'll know not
                                       // only that the 3rd column is shared with an earlier column, but which one too. Although
                                       // we don't use that information currently, we could do in future.
     }
   }
-  // now we know nShared and which ones they are (if any), restore original tl back to the unique set of columns
-  for (int i=0; i<ncol; ++i) {
-    if (!shared[i]) SET_TRUELENGTH(xp[i], savetl[i]);
-    //  ^^^^^^^^^^ important because if there are shared columns, the dup will have savetl==0 but we want the first restore to stand
-  }
-  // now that truelength has been restored for all columns, we can finally call copyAsPlain()
   if (nShared) {
     for (int i=0; i<ncol; ++i) {
       if (shared[i])