1 files changed, 20 insertions, 0 deletions
diff --git a/src/prelude/array.ts b/src/prelude/array.ts
index 54f7081712..8536e486d6 100644
--- a/src/prelude/array.ts
+++ b/src/prelude/array.ts
@@ -18,6 +18,10 @@ export function erase<T>(x: T, xs: T[]): T[] {
 	return xs.filter(y => x !== y);
 }
 
+export function setDifference<T>(xs: T[], ys: T[]): T[] {
+	return xs.filter(x => !ys.includes(x));
+}
+
 export function unique<T>(xs: T[]): T[] {
 	return [...new Set(xs)];
 }
@@ -25,3 +29,19 @@ export function unique<T>(xs: T[]): T[] {
 export function sum(xs: number[]): number {
 	return xs.reduce((a, b) => a + b, 0);
 }
+
+export function groupBy<T>(f: (x: T, y: T) => boolean, xs: T[]): T[][] {
+	const groups = [] as T[][];
+	for (const x of xs) {
+		if (groups.length !== 0 && f(groups[groups.length - 1][0], x)) {
+			groups[groups.length - 1].push(x);
+		} else {
+			groups.push([x]);
+		}
+	}
+	return groups;
+}
+
+export function groupOn<T, S>(f: (x: T) => S, xs: T[]): T[][] {
+	return groupBy((a, b) => f(a) === f(b), xs);
+}