import { EndoRelation, Predicate } from './relation'; /** * Count the number of elements that satisfy the predicate */ export function countIf(f: Predicate, xs: T[]): number { return xs.filter(f).length; } /** * Count the number of elements that is equal to the element */ export function count(a: T, xs: T[]): number { return countIf(x => x === a, xs); } /** * Concatenate an array of arrays */ export function concat(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(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(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(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(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(f: EndoRelation, 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(f: (x: T) => S, xs: T[]): T[][] { return groupBy((a, b) => f(a) === f(b), xs); } export function groupByX(collections: T[], keySelector: (x: T) => string) { return collections.reduce((obj: Record, 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(f: Predicate, 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(x: T | T[] | undefined): T[] { return Array.isArray(x) ? x : x != null ? [x] : []; } export function toSingle(x: T | T[] | undefined): T | undefined { return Array.isArray(x) ? x[0] : x; }