1 files changed, 102 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..b50ca1d4f7
--- /dev/null
+++ b/packages/backend/src/misc/diff-arrays.ts
@@ -0,0 +1,102 @@
+/*
+ * 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 duplicate values are ignored.
+ * Result sets are de-duplicated, and will be empty if no data was added or removed (respectively).
+ * The inputs are treated as un-ordered, so a re-ordering of the same data will NOT be considered a change.
+ * @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
+			// delete operation removes duplicates to speed up the "after" loop
+			if (!after.delete(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: [] };
+}
+
+/**
+ * Checks for any difference between two snapshots of data.
+ * Null, undefined, and empty arrays are supported, and duplicate values are ignored.
+ * The inputs are treated as un-ordered, so a re-ordering of the same data will NOT be considered a change.
+ * @param dataBefore Array containing data before the change
+ * @param dataAfter Array containing data after the change
+ */
+export function diffArraysSimple<T>(dataBefore: T[] | null | undefined, dataAfter: T[] | null | undefined): boolean {
+	const before = dataBefore ? new Set(dataBefore) : null;
+	const after = dataAfter ? new Set(dataAfter) : null;
+
+	if (before?.size && after?.size) {
+		// different size => changed
+		if (before.size !== after.size) return true;
+
+		// removed => changed
+		for (const host of before) {
+			// delete operation removes duplicates to speed up the "after" loop
+			if (!after.delete(host)) {
+				return true;
+			}
+		}
+
+		// added => changed
+		for (const host of after) {
+			if (!before.has(host)) {
+				return true;
+			}
+		}
+
+		// identical values => no change
+		return false;
+	}
+
+	// before and NOT after => change
+	if (before?.size) return true;
+
+	// after and NOT before => change
+	if (after?.size) return true;
+
+	// NEITHER before nor after => no change
+	return false;
+}