Project Euler 14

ProjectEuler14.js

// Project Euler 14, but in JS (longest Collatz chain for starting n < 1e6)
// Take a cup of ☕, this is going to take a while... (because it's slow)

function* collatzSeq(n) {
    yield n;
    if (n<=1) return;
    yield* collatzSeq(n%2==0 ? n/2 : 3*n+1);
}

function seqLength(s) {
    let i=0;
    for (const _ of s) ++i;
    return i;
}

function* range(start, end) {
    for (let i=start; i != end; yield(i++));
}

function* map(s, f) {
    for (const x of s) yield f(x);
}

function maxBy(s, by) {
    let maxValue = Number.MIN_VALUE;
    let res = undefined;
    
    for (const x of s) {
        const cmpValue = by(x);
        if (cmpValue > maxValue) {
            maxValue = cmpValue;
            res = x;
        }
    }
    return res;
}

let [length, n] = maxBy(map(range(1,1000000), n => [seqLength(collatzSeq(n)), n]), x=>x[0])
console.log(`seqLength(collatzSeq(${n})) == ${length}`)