4kyu_SumByFactors.js

// 4kyu - Sum by Factors
// Given an array of positive or negative integers

// I= [i1,..,in]

// you have to produce a sorted array P of the form

// [ [p, sum of all ij of I for which p is a prime factor (p positive) 

// of ij] ...]

// P will be sorted by increasing order of the prime numbers. The final 
// result has to be given as a string in Java, C#, C, C++ and as an array 
// of arrays in other languages.

// Example:

// I = [12, 15]; //result = [[2, 12], [3, 27], [5, 15]]
// [2, 3, 5] is the list of all prime factors of the elements of I, hence 
// the result.

// Notes:

// It can happen that a sum is 0 if some numbers are negative!
// Example: I = [15, 30, -45] 5 divides 15, 30 and (-45) so 5 appears in 
// the result, the sum of the numbers for which 5 is a factor is 0 so we 
// have [5, 0] in the result amongst others.

// In Fortran - as in any other language - the returned string is not 
// permitted to contain any redundant trailing whitespace: you can use 
// dynamically allocated character strings.

function sumOfDivided(lst) {
    if(lst.length == 0) return [];
    let m = Math.max.apply(null, lst.map(Math.abs)), primes = [], marked = Array(m+1);

    for(let i = 2; i <= m; ++i) {
        if(marked[i]) continue;
        let sum = 0, isMul = false, fn = function(n) { if(n % i == 0) { sum += n; isMul = true; } };
        lst.forEach(fn);
        if(isMul) primes.push([i, sum]);
        for(let j = 2*i; j <= m; j += i)
            marked[j] = true;
    }

    return primes;
}