title: Akka stream conditional zip
body: <p>data streamed comes ordered by id from IMDB tsv files :
  <a href="https://www.imdb.com/interfaces/" rel="nofollow noreferrer">https://www.imdb.com/interfaces/</a></p>
  <ul>
  <li>The movies: <a href="https://datasets.imdbws.com/title.basics.tsv.gz" rel="nofollow noreferrer">https://datasets.imdbws.com/title.basics.tsv.gz</a></li>
  </ul>
  <pre><code>...
  tt0000119   short   Rescue on the River, Pt. 2  Sauvetage en rivière, 1er   0   1896    \N  \N  Documentary,Short
  tt0000120   short   Rescue on the River, Pt. 1  Sauvetage en rivière, 2e    0   1896    \N  \N  Documentary,Short
  tt0000121   short   The Soldier's Courtship The Soldier's Courtship 0   1896    \N  1   Comedy,Short
  tt0000123   short   Conjuring   Séance de prestidigitation  0   1896    \N  \N  Short
  ...
  </code></pre>
  <ul>
  <li>The actors: <a href="https://datasets.imdbws.com/title.principals.tsv.gz" rel="nofollow noreferrer">https://datasets.imdbws.com/title.principals.tsv.gz</a></li>
  </ul>
  <pre><code>...
  tt0000121   1   nm0832461   actor   \N  [&quot;The Soldier&quot;]
  tt0000121   2   nm0780534   actress \N  [&quot;The Girl&quot;]
  tt0000121   3   nm5718242   actress \N  [&quot;The Woman&quot;]
  tt0000121   4   nm0609678   director    \N  \N
  tt0000121   5   nm0666972   producer    producer    \N
  tt0000122   1   nm0617588   director    \N  \N
  tt0000123   1   nm0617588   self    \N  [&quot;Self (stage magician)&quot;]
  ...
  </code></pre>
  <p>I'm trying to figure out a way to stream the actors of an input movie name.
  Basically joining the principals data on the <code>tconst</code> movie id.</p>
  <p>I first thought of grouping the principals by tconst first then zipping the movies data but unfortunately some ids might be missing thus corrupting the zip.</p>
  <p>FlapMatConcat is too slow obviously.</p>
  <p>Any ideas on how I could zip the movies data and skip an actors group if not matched?</p>