feat: paginate and merge frequent activities (#274)

* feat: paginate and merge frequent activities

* feat: due date activities

* fix: requested changes fixes

* refactor: slight reshuffle and improve types

* chore: translations

---------

Co-authored-by: Henry <henry_ball@hotmail.co.uk>

packages/api/src/utils/activities.ts

@@ -0,0 +1,109 @@
+import type { PaginatedActivitiesResult } from "@kan/db/repository/cardActivity.repo";
+import type { ActivityType } from "@kan/db/schema";
+
+type ActivityBase = PaginatedActivitiesResult["activities"][number];
+
+interface Activity extends ActivityBase {
+  mergeCount?: number; // number of activities merged into this one
+  mergedLabels?: string[]; // list of label names when merging label activities
+}
+
+// types that can be merged with simple count
+const MERGEABLE_COUNT_TYPES: readonly ActivityType[] = [
+  "card.updated.description",
+];
+
+// types that merge with a list of items
+const MERGEABLE_LIST_TYPES: readonly ActivityType[] = [
+  "card.updated.label.added",
+  "card.updated.label.removed",
+];
+
+const MERGE_TIME_WINDOW_MS = 5 * 60 * 1000; // 5 minutes window for merging activities
+
+export function mergeActivities(activities: Activity[]): Activity[] {
+  if (activities.length === 0) return [];
+
+  const merged: Activity[] = [];
+  let i = 0;
+
+  while (i < activities.length) {
+    const current = activities[i];
+    if (!current) {
+      i++;
+      continue;
+    }
+
+    const currentType = current.type;
+
+    const isCountMergeable = MERGEABLE_COUNT_TYPES.includes(currentType);
+    const isListMergeable = MERGEABLE_LIST_TYPES.includes(currentType);
+    const canMerge = (isCountMergeable || isListMergeable) && current.user?.id;
+
+    if (!canMerge) {
+      merged.push(current);
+      i++;
+      continue;
+    }
+
+    const group: Activity[] = [current];
+    let j = i + 1;
+
+    while (j < activities.length) {
+      const next = activities[j];
+      if (!next) break;
+
+      const timeDiff =
+        new Date(next.createdAt).getTime() -
+        new Date(current.createdAt).getTime();
+
+      if (
+        next.type === current.type &&
+        next.user?.id === current.user?.id &&
+        timeDiff >= 0 && // next is newer or same time
+        timeDiff <= MERGE_TIME_WINDOW_MS
+      ) {
+        group.push(next);
+        j++;
+      } else {
+        break;
+      }
+    }
+
+    if (group.length > 1) {
+      const oldest = group[0];
+      const latest = group[group.length - 1];
+
+      if (oldest && latest) {
+        if (isListMergeable) {
+          const labelNames = group
+            .map((a) => a.label?.name)
+            .filter((name): name is string => !!name);
+
+          merged.push({
+            ...oldest,
+            createdAt: oldest.createdAt,
+            mergeCount: group.length,
+            mergedLabels: labelNames,
+          });
+        } else {
+          merged.push({
+            ...oldest,
+            createdAt: oldest.createdAt,
+            toDescription: latest.toDescription,
+            fromDescription: oldest.fromDescription,
+            mergeCount: group.length,
+          });
+        }
+      } else {
+        merged.push(current);
+      }
+    } else {
+      merged.push(current);
+    }
+
+    i = j;
+  }
+
+  return merged;
+}