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