feat: group all-account rows by instrument and day (WealthLedger/main#47) (#126)

src/shared/lib/group-items.ts

@@ -0,0 +1,47 @@
+type GroupedItem<T>
+	= | { item: T; type: "single" }
+		| { items: [T, T, ...T[]]; key: string; type: "group" };
+
+function groupItems<T>(
+	items: readonly T[],
+	keyOf: (item: T) => null | string,
+): GroupedItem<T>[] {
+	const result: GroupedItem<T>[] = [];
+	const indices = new Map<string, number>();
+
+	for (const item of items) {
+		const key = keyOf(item);
+
+		if (key === null) {
+			result.push({ item, type: "single" });
+			continue;
+		}
+
+		const index = indices.get(key);
+
+		if (index === undefined) {
+			indices.set(key, result.length);
+			result.push({ item, type: "single" });
+			continue;
+		}
+
+		// eslint-disable-next-line security/detect-object-injection -- Map-owned index.
+		const existing = result[index];
+
+		if (existing?.type === "single") {
+			// eslint-disable-next-line security/detect-object-injection -- Map-owned index.
+			result[index] = {
+				items: [existing.item, item],
+				key,
+				type: "group",
+			};
+		}
+		else if (existing) {
+			existing.items.push(item);
+		}
+	}
+
+	return result;
+}
+
+export { groupItems };