126 lines
3.3 KiB
JavaScript
126 lines
3.3 KiB
JavaScript
/**
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* Count the number of elements that satisfy the predicate
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*/ export function countIf(f, xs) {
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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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*/ export function count(a, xs) {
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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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*/ export function concat(xss) {
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return [].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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*/ export function intersperse(sep, xs) {
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return concat(xs.map((x)=>[
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sep,
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x
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])).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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*/ export function erase(a, xs) {
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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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*/ export function difference(xs, ys) {
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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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*/ export function unique(xs) {
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return [
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...new Set(xs)
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];
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}
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export function uniqBy(a, key) {
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const seen = new Set();
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return a.filter(function(item) {
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const k = key(item);
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return seen.has(k) ? false : seen.add(k);
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});
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}
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export function sum(xs) {
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return xs.reduce((a, b)=>a + b, 0);
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}
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export function maximum(xs) {
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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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*/ export function groupBy(f, xs) {
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const groups = [];
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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([
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x
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]);
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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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*/ export function groupOn(f, xs) {
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return groupBy((a, b)=>f(a) === f(b), xs);
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}
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export function groupByX(collections, keySelector) {
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return collections.reduce((obj, item)=>{
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const key = keySelector(item);
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if (!Object.prototype.hasOwnProperty.call(obj, key)) {
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obj[key] = [];
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}
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obj[key].push(item);
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return obj;
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}, {});
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}
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/**
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* Compare two arrays by lexicographical order
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*/ export function lessThan(xs, ys) {
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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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*/ export function takeWhile(f, xs) {
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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) {
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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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export function toArray(x) {
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return Array.isArray(x) ? x : x != null ? [
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x
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] : [];
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}
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export function toSingle(x) {
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return Array.isArray(x) ? x[0] : x;
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}
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export function toSingleLast(x) {
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return Array.isArray(x) ? x.at(-1) : x;
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}
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