How to add a value in an array to the values before and after it

Question

I am trying to turn an array of numbers into steps of the value of the Non-Zero integer element i.e

spread([0,0,n,0,0] returns => 
[0 + n-2, 0 + n-1, n, 0 +         


        

Answer 1

You could reduce the array by using a copy of the array and go left and right with a recursive function which checks the value to spread and the index.

function spread(array) {
    return array.reduce((r, v, i, a) => {
        const
            iter = (v, i, d) => {
                if (v < 1 || !(i in a)) return;
                r[i] += v;
                iter(v - 1, i + d, d);
            };
        iter(v - 1, i - 1, -1);
        iter(v - 1, i + 1, 1);
        return r;
    }, array.slice());
}

console.log(...spread([0, 0, 0, 4, 0, 0, 0])); // [1, 2, 3, 4, 3, 2, 1]
console.log(...spread([0, 0, 0, 3, 0, 2, 0])); // [0, 1, 2, 3, 3, 3, 1]
console.log(...spread([3, 0, 0, 0]));          // [3, 2, 1, 0]

A different approach by moving arrays and adding them.

function spread(array) {
    const dec = v => Math.max(v - 1, 0);

    var result = array.slice(),
        temp = array.slice(1).map(v => Math.max(v - 1, 0)),
        offset;
    
    while (temp.length) {
        temp.forEach((v, i) => result[i] += v);
        temp = temp.slice(1).map(dec);
    }

    temp = array.slice(0, -1).map(dec);
    while (temp.length) {
        offset = result.length - temp.length;
        temp.forEach((v, i) => result[i + offset] += v);
        temp = temp.slice(0, -1).map(dec);
    }
    return result;
}

console.log(...spread([0, 0, 0, 4, 0, 0, 0])); // [1, 2, 3, 4, 3, 2, 1]
console.log(...spread([0, 0, 0, 3, 0, 2, 0])); // [0, 1, 2, 3, 3, 3, 1]
console.log(...spread([3, 0, 0, 0]));          // [3, 2, 1, 0]