/**
 * Count the number of elements that satisfy the predicate
 */ export function countIf(f, xs) {
    return xs.filter(f).length;
}
/**
 * Count the number of elements that is equal to the element
 */ export function count(a, xs) {
    return countIf((x)=>x === a, xs);
}
/**
 * Concatenate an array of arrays
 */ export function concat(xss) {
    return [].concat(...xss);
}
/**
 * Intersperse the element between the elements of the array
 * @param sep The element to be interspersed
 */ export function intersperse(sep, xs) {
    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, xs) {
    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, ys) {
    return xs.filter((x)=>!ys.includes(x));
}
/**
 * Remove all but the first element from every group of equivalent elements
 */ export function unique(xs) {
    return [
        ...new Set(xs)
    ];
}
export function uniqBy(a, key) {
    const seen = new Set();
    return a.filter(function(item) {
        const k = key(item);
        return seen.has(k) ? false : seen.add(k);
    });
}
export function sum(xs) {
    return xs.reduce((a, b)=>a + b, 0);
}
export function maximum(xs) {
    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, xs) {
    const groups = [];
    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, xs) {
    return groupBy((a, b)=>f(a) === f(b), xs);
}
export function groupByX(collections, keySelector) {
    return collections.reduce((obj, item)=>{
        const key = keySelector(item);
        if (!Object.prototype.hasOwnProperty.call(obj, key)) {
            obj[key] = [];
        }
        obj[key].push(item);
        return obj;
    }, {});
}
/**
 * Compare two arrays by lexicographical order
 */ export function lessThan(xs, ys) {
    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, xs) {
    const ys = [];
    for (const x of xs){
        if (f(x)) {
            ys.push(x);
        } else {
            break;
        }
    }
    return ys;
}
export function cumulativeSum(xs) {
    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) {
    return Array.isArray(x) ? x : x != null ? [
        x
    ] : [];
}
export function toSingle(x) {
    return Array.isArray(x) ? x[0] : x;
}
export function toSingleLast(x) {
    return Array.isArray(x) ? x.at(-1) : x;
}