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export function countIf<T>(f: (x: T) => boolean, xs: T[]): number {
return xs.filter(f).length;
}
export function count<T>(x: T, xs: T[]): number {
return countIf(y => x === y, xs);
}
export function concat<T>(xss: T[][]): T[] {
return ([] as T[]).concat(...xss);
}
export function intersperse<T>(sep: T, xs: T[]): T[] {
return concat(xs.map(x => [sep, x])).slice(1);
}
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)];
}
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);
}
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