join_by_copy_strands.patch

diff --git a/src/strings/ops.c b/src/strings/ops.c
index 080951f7..9c2e9d08 100644
--- a/src/strings/ops.c
+++ b/src/strings/ops.c
@@ -1616,6 +1616,7 @@ MVMString * MVM_string_join(MVMThreadContext *tc, MVMString *separator, MVMObjec
     MVMuint16   sstrands, total_strands;
     MVMint32    concats_stable = 1;
     size_t      bytes;
+    int         all_strands_of_same_type, strand_type;
 
     MVM_string_check_arg(tc, separator, "join separator");
     if (!IS_CONCRETE(input))
@@ -1637,6 +1638,10 @@ MVMString * MVM_string_join(MVMThreadContext *tc, MVMString *separator, MVMObjec
     });
     });
 
+    all_strands_of_same_type = separator->body.storage_type == MVM_STRING_STRAND;
+    if (all_strands_of_same_type)
+        strand_type = separator->body.storage.strands->blob_string->body.storage_type;
+
     /* Take a first pass through the string, counting up length and the total
      * number of strands we encounter as well as building a flat array of the
      * strings (to we only have to do the indirect calls once). */
@@ -1667,6 +1672,10 @@ MVMString * MVM_string_join(MVMThreadContext *tc, MVMString *separator, MVMObjec
             piece = MVM_repr_get_str(tc, item);
         }
 
+        if (all_strands_of_same_type)
+            all_strands_of_same_type = piece->body.storage_type == MVM_STRING_STRAND &&
+                                       strand_type == piece->body.storage.strands->blob_string->body.storage_type;
+
         /* If it wasn't the first piece, add separator here. */
         if (num_pieces) {
             total_strands += sstrands;
@@ -1695,11 +1704,22 @@ MVMString * MVM_string_join(MVMThreadContext *tc, MVMString *separator, MVMObjec
 
     /* If we just collect all the things as strands, are we within bounds, and
      * will be come out ahead? */
-    if (total_strands < MVM_STRING_MAX_STRANDS && 16 <= total_graphs / total_strands) {
-        /* XXX TODO: Implement this, conditionalize branch thing below. */
+    if (total_strands < MVM_STRING_MAX_STRANDS && all_strands_of_same_type) {
+        size_t offset = 0;
+        result->body.storage_type    = MVM_STRING_STRAND;
+        result->body.storage.strands = allocate_strands(tc, total_strands);
+        result->body.num_strands     = total_strands;
+        for (i = 0; i < num_pieces; i++) {
+                       MVMString *piece = pieces[i];
+            if (0 < i) {
+                copy_strands(tc, separator, 0, result, offset, separator->body.num_strands);
+                offset += separator->body.num_strands;
+            }
+            copy_strands(tc, piece, 0, result, offset, piece->body.num_strands);
+            offset += piece->body.num_strands;
+        }
     }
-    /*else {*/
-    if (1) {
+    else {
         /* We'll produce a single, flat string. */
         MVMint64        position = 0;
         MVMGraphemeIter gi;