1 files changed, 65 insertions, 0 deletions
diff --git a/app/Issue/GroupBy.hs b/app/Issue/GroupBy.hs
new file mode 100644
index 0000000..0e8941f
--- /dev/null
+++ b/app/Issue/GroupBy.hs
@@ -0,0 +1,65 @@
+module Issue.GroupBy
+  ( groupByArg,
+    groupIssuesBy,
+  )
+where
+
+import Data.Map (Map)
+import Data.Map qualified as M
+import Data.Maybe (mapMaybe)
+import Data.Text qualified as T
+import Issue (Issue (..))
+import Issue.Tag (Tag (..))
+import Issue.Tag qualified as I
+import Options.Applicative qualified as O
+
+
+-- FIXME Parse group by using @topic syntax
+
+-- TODO Display number of issues within one group
+--
+-- ```
+-- Issue D
+-- Issue E
+--
+-- @topic tags  (2 issues)
+-- Issue A
+-- Issue B @topic ids
+--
+-- @topic ids  (2 issues)
+-- Issue B @topic tags
+-- Issue C
+-- ```
+
+-- TODO Add issues marker as internal tags
+--
+-- The internal makers `TODO`, `FIXME`, etc. should be available via the
+-- internal tag @type
+
+-- TODO Add author and editor as internal tags
+
+groupByArg :: O.Parser (Maybe T.Text)
+groupByArg =
+  O.optional
+    ( O.strOption
+        ( O.long "group-by"
+            <> O.metavar "TAG"
+            <> O.help "Group selected issues."
+        )
+    )
+
+groupIssuesBy :: T.Text -> [Issue] -> Map T.Text [Issue]
+groupIssuesBy groupBy issues =
+  foldl
+    ( \collected issue ->
+        foldl
+          (flip $ M.alter (Just . maybe [issue] (issue :)))
+          collected
+          (groupsOfIssue groupBy issue)
+    )
+    M.empty
+    issues
+  where
+    groupsOfIssue group issue =
+      mapMaybe I.tagValue $
+        filter (\(Tag key _) -> key == group) (issue.tags ++ issue.internalTags)