/**
 * Choose a pair at random among a list of weighted pairs.
 *
 * @param pairs List of weighted pairs, with the weight coming first.
 * @return Selected pair, or null if there is nothing to choose from.
 */
export const choose = pairs =>
{
    const total = pairs.reduce((prev, [weight, _]) => prev + weight, 0);

    if (total === 0)
    {
        return null;
    }

    const value = Math.random() * total;
    let sum = 0;
    let index = 0;

    while (value >= sum)
    {
        sum += pairs[index][0];
        index += 1;
    }

    return pairs[index - 1];
};