add diff-arrays utility for efficient array diffs

1 files changed, 56 insertions, 0 deletions

diff --git a/packages/backend/src/misc/diff-arrays.ts b/packages/backend/src/misc/diff-arrays.ts
new file mode 100644
index 0000000000..1f6820481b
--- /dev/null
+++ b/packages/backend/src/misc/diff-arrays.ts
@@ -0,0 +1,56 @@
+/*
+ * SPDX-FileCopyrightText: hazelnoot and other Sharkey contributors
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+export interface DiffResult<T> {
+	added: T[];
+	removed: T[];
+}
+
+/**
+ * Calculates the difference between two snapshots of data.
+ * Null, undefined, and empty arrays are supported, and values do not have to be unique.
+ * Result sets are de-duplicated, and will be empty if no data was added or removed (respectively).
+ * @param dataBefore Array containing data before the change
+ * @param dataAfter Array containing data after the change
+ */
+export function diffArrays<T>(dataBefore: T[] | null | undefined, dataAfter: T[] | null | undefined): DiffResult<T> {
+	const before = dataBefore ? new Set(dataBefore) : null;
+	const after = dataAfter ? new Set(dataAfter) : null;
+
+	// data before AND after => changed
+	if (before?.size && after?.size) {
+		const added: T[] = [];
+		const removed: T[] = [];
+
+		for (const host of before) {
+			// before and NOT after => removed
+			if (!after.has(host)) {
+				removed.push(host);
+			}
+		}
+
+		for (const host of after) {
+			// after and NOT before => added
+			if (!before.has(host)) {
+				added.push(host);
+			}
+		}
+
+		return { added, removed };
+	}
+
+	// data ONLY before => all removed
+	if (before?.size) {
+		return { added: [], removed: Array.from(before) };
+	}
+
+	// data ONLY after => all added
+	if (after?.size) {
+		return { added: Array.from(after), removed: [] };
+	}
+
+	// data NEITHER before nor after => no change
+	return { added: [], removed: [] };
+}