ds4/src/module/rolls/check-evaluation.ts

export function assignSubChecksToDice(
    dice: number[],
    checkTargetNumber: number,
    {
        maximumCoupResult = 1,
    }: {
        maximumCoupResult?: number;
    } = {},
): { result: number; checkTargetNumber: number }[] {
    const requiredNumberOfDice = Math.ceil(checkTargetNumber / 20);
    if (dice.length !== requiredNumberOfDice || requiredNumberOfDice < 1) {
        throw new Error(game.i18n.localize("DS4.ErrorInvalidNumberOfDice")); // TODO: add i18n
    }

    const checkTargetNumberForLastSubCheck = checkTargetNumber - 20 * (requiredNumberOfDice - 1);

    const indexOfSmallestNonCoup = findIndexOfSmallestNonCoup(dice, maximumCoupResult);
    const indexOfFirstCoup = dice.findIndex((die) => die <= maximumCoupResult);
    const indexForLastSubCheck = shouldUseCoupForLastSubCheck(
        indexOfSmallestNonCoup,
        indexOfFirstCoup,
        dice,
        checkTargetNumberForLastSubCheck,
    )
        ? indexOfFirstCoup
        : indexOfSmallestNonCoup;

    return dice.map((die, index) => ({
        result: die,
        checkTargetNumber: index === indexForLastSubCheck ? checkTargetNumberForLastSubCheck : 20,
    }));
}

function findIndexOfSmallestNonCoup(dice: number[], maximumCoupResult: number): number {
    return dice
        .map((die, index) => [die, index])
        .filter((indexedDie) => indexedDie[0] > maximumCoupResult)
        .reduce(
            (smallestIndexedDie, indexedDie) =>
                indexedDie[0] < smallestIndexedDie[0] ? indexedDie : smallestIndexedDie,
            [Infinity, -1],
        )[1];
}

function shouldUseCoupForLastSubCheck(
    indexOfSmallestNonCoup: number,
    indexOfFirstCoup: number,
    dice: number[],
    checkTargetNumberForLastSubCheck: number,
) {
    return (
        indexOfFirstCoup !== -1 &&
        (indexOfSmallestNonCoup === -1 ||
            (dice[indexOfSmallestNonCoup] > checkTargetNumberForLastSubCheck &&
                dice[indexOfSmallestNonCoup] + checkTargetNumberForLastSubCheck > 20))
    );
}