getChronologyRelations: filter out duplicate artists

Only within the same call, so an artist who is both composer
and cover artist will still be listed in the chronology section
twice ("45th track by rj!" + "15th cover art by rj!").
This commit includes a documentation block about that.

1 files changed, 23 insertions, 0 deletions

diff --git a/src/content/util/getChronologyRelations.js b/src/content/util/getChronologyRelations.js
index 4b7d9ce..11281e7 100644
--- a/src/content/util/getChronologyRelations.js
+++ b/src/content/util/getChronologyRelations.js
@@ -4,6 +4,29 @@ export default function getChronologyRelations(thing, {
   linkThing,
   getThings,
 }) {
+  // One call to getChronologyRelations is considered "lumping" together all
+  // contributions as carrying equivalent meaning (for example, "artist"
+  // contributions and "contributor" contributions are bunched together in
+  // one call to getChronologyRelations, while "cover artist" contributions
+  // are a separate call). getChronologyRelations prevents duplicates that
+  // carry the same meaning by only using the first instance of each artist
+  // in the contributions array passed to it. It's expected that the string
+  // identifying which kind of contribution ("track" or "cover art") is
+  // shared and applied to all contributions, as providing them together
+  // in one call to getChronologyRelations implies they carry the same
+  // meaning.
+
+  const artistsSoFar = new Set();
+
+  contributions = contributions.filter(({who}) => {
+    if (artistsSoFar.has(who)) {
+      return false;
+    } else {
+      artistsSoFar.add(who);
+      return true;
+    }
+  });
+
   return contributions.map(({who}) => {
     const things = Array.from(new Set(getThings(who)));
     const index = things.indexOf(thing);