refactoring

Resolve #7779

packages/backend/src/prelude/array.ts

@@ -0,0 +1,138 @@
+import { EndoRelation, Predicate } from './relation';
+
+/**
+ * Count the number of elements that satisfy the predicate
+ */
+
+export function countIf<T>(f: Predicate<T>, xs: T[]): number {
+	return xs.filter(f).length;
+}
+
+/**
+ * Count the number of elements that is equal to the element
+ */
+export function count<T>(a: T, xs: T[]): number {
+	return countIf(x => x === a, xs);
+}
+
+/**
+ * Concatenate an array of arrays
+ */
+export function concat<T>(xss: T[][]): T[] {
+	return ([] as T[]).concat(...xss);
+}
+
+/**
+ * Intersperse the element between the elements of the array
+ * @param sep The element to be interspersed
+ */
+export function intersperse<T>(sep: T, xs: T[]): T[] {
+	return concat(xs.map(x => [sep, x])).slice(1);
+}
+
+/**
+ * Returns the array of elements that is not equal to the element
+ */
+export function erase<T>(a: T, xs: T[]): T[] {
+	return xs.filter(x => x !== a);
+}
+
+/**
+ * Finds the array of all elements in the first array not contained in the second array.
+ * The order of result values are determined by the first array.
+ */
+export function difference<T>(xs: T[], ys: T[]): T[] {
+	return xs.filter(x => !ys.includes(x));
+}
+
+/**
+ * Remove all but the first element from every group of equivalent elements
+ */
+export function unique<T>(xs: T[]): T[] {
+	return [...new Set(xs)];
+}
+
+export function sum(xs: number[]): number {
+	return xs.reduce((a, b) => a + b, 0);
+}
+
+export function maximum(xs: number[]): number {
+	return Math.max(...xs);
+}
+
+/**
+ * Splits an array based on the equivalence relation.
+ * The concatenation of the result is equal to the argument.
+ */
+export function groupBy<T>(f: EndoRelation<T>, 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;
+}
+
+/**
+ * Splits an array based on the equivalence relation induced by the function.
+ * The concatenation of the result is equal to the argument.
+ */
+export function groupOn<T, S>(f: (x: T) => S, xs: T[]): T[][] {
+	return groupBy((a, b) => f(a) === f(b), xs);
+}
+
+export function groupByX<T>(collections: T[], keySelector: (x: T) => string) {
+	return collections.reduce((obj: Record<string, T[]>, item: T) => {
+		const key = keySelector(item);
+		if (!obj.hasOwnProperty(key)) {
+			obj[key] = [];
+		}
+
+		obj[key].push(item);
+
+		return obj;
+	}, {});
+}
+
+/**
+ * Compare two arrays by lexicographical order
+ */
+export function lessThan(xs: number[], ys: number[]): boolean {
+	for (let i = 0; i < Math.min(xs.length, ys.length); i++) {
+		if (xs[i] < ys[i]) return true;
+		if (xs[i] > ys[i]) return false;
+	}
+	return xs.length < ys.length;
+}
+
+/**
+ * Returns the longest prefix of elements that satisfy the predicate
+ */
+export function takeWhile<T>(f: Predicate<T>, xs: T[]): T[] {
+	const ys = [];
+	for (const x of xs) {
+		if (f(x)) {
+			ys.push(x);
+		} else {
+			break;
+		}
+	}
+	return ys;
+}
+
+export function cumulativeSum(xs: number[]): number[] {
+	const ys = Array.from(xs); // deep copy
+	for (let i = 1; i < ys.length; i++) ys[i] += ys[i - 1];
+	return ys;
+}
+
+export function toArray<T>(x: T | T[] | undefined): T[] {
+	return Array.isArray(x) ? x : x != null ? [x] : [];
+}
+
+export function toSingle<T>(x: T | T[] | undefined): T | undefined {
+	return Array.isArray(x) ? x[0] : x;
+}