day09.js

const fs = require('fs');

const lines = fs.readFileSync('day09.txt', {encoding: 'utf-8'}).split('\n').filter(x => x).map(x => parseInt(x));

class Xmas {
    constructor(preamble) {
        this.array = preamble;
    }

    isValid(int) {
        for (let i = 0; i < this.array.length; i++) {
            for (let j = i+1; j < this.array.length; j++) {
                if(this.array[i] + this.array[j] === int) {
                    return true;
                }
            }
        }
        return false;
    }

    push(int) {
        this.array.push(int);
        this.array.shift();
    }
}

const preambleLength = 25;

const preamble = lines.slice(0, preambleLength);

const xmas = new Xmas(preamble);

let invalid;

for (let i = preambleLength; i < lines.length; i++) {
    const element = lines[i];
    if(xmas.isValid(element)) {
        xmas.push(element)
    } else {
        invalid = element;
        break;
    }
}

console.log(invalid);

let weakness;

for (let l = 2; l < lines.length; l++) {
    for (let i = 0; i < lines.length - l + 1; i++) {
        let sum = 0;
        for (let j = 0; j < l; j++) {
            sum += lines[i+j];
        }
        if(sum === invalid) {
            const set = lines.slice(i, i+l);
            weakness = Math.min(...set) + Math.max(...set);
        }
    }
}

console.log(weakness);