Simplify flow action engine compilation1.22.0

Instead of the added complexity of discarding flow nodes
we can simplify the compilation process by just retaining
them and jumping over them in further iteration and graph
and tree runtime usage.

This change moves toward a model that does just this, which
makes it also easier to in the future use the newly added
flow graph nodes to do meaningful things (like use them as
a point to change which flow_detail is used).

Change-Id: Icb1695f4b995a0392f940837514774768f222db4

1 files changed, 13 insertions, 0 deletions

diff --git a/taskflow/utils/iter_utils.py b/taskflow/utils/iter_utils.py
index 68810e8..a96e9cf 100644
--- a/taskflow/utils/iter_utils.py
+++ b/taskflow/utils/iter_utils.py
@@ -16,12 +16,25 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import itertools
+
 
 def count(it):
     """Returns how many values in the iterator (depletes the iterator)."""
     return sum(1 for _value in it)
 
 
+def unique_seen(it, *its):
+    """Yields unique values from iterator(s) (and retains order)."""
+    seen = set()
+    for value in itertools.chain(it, *its):
+        if value in seen:
+            continue
+        else:
+            yield value
+            seen.add(value)
+
+
 def find_first_match(it, matcher, not_found_value=None):
     """Searches iterator for first value that matcher callback returns true."""
     for value in it: