contracts: initial commit

All draft stuff here for now, but this is *relatively* un-naive
as it's based on a lot of recent research and discussion.
But none of this code is evaluated yet!!

1 files changed, 17 insertions, 0 deletions

diff --git a/src/content/contracts/adjacentAlbumsInGroup.js b/src/content/contracts/adjacentAlbumsInGroup.js
new file mode 100644
index 00000000..e982fa5b
--- /dev/null
+++ b/src/content/contracts/adjacentAlbumsInGroup.js
@@ -0,0 +1,17 @@
+export default {
+  hook(contract, [album, group]) {
+    contract.provide({
+      group,
+      album,
+      albums: contract.selectProperty(group, 'albums'),
+    });
+  },
+
+  compute({group, album, albums}) {
+    const datedAlbums = albums.filter(album => album.date);
+    const index = datedAlbums.indexOf(album);
+    const previousAlbum = (index > 0) && datedAlbums[index - 1];
+    const nextAlbum = (index < datedAlbums.length - 1) && datedAlbums[index + 1];
+    return {previousAlbum, nextAlbum};
+  },
+};