Next Rating

nextRating.ts

type NextRatingFn = (rating: number, actualPoints: number, expectedPoints: number) => [nextRating: number, ratingChange: number]
type KFactorFunction = (rating: number) => number

function createNextRatingFn(kFactor: number | KFactorFunction): NextRatingFn {
    const kFactorFn: KFactorFunction = typeof kFactor === 'number'
        ? () => kFactor
        : kFactor

    return (rating: number, actualPoints: number, expectedPoints: number): [number, number] => {
        const change = Math.round(kFactorFn(rating) * (actualPoints - expectedPoints))
        const nextRating = rating + change

        return [nextRating, change]
    }
}