汎用按分器 in .NET

Prorate.cs

/// Usage
void Main() {
    var weights = new decimal[] { 4, 6, 7.3m };
    
    Prorate(total: 1014, weights: weights, ProrationRounding.SpecifiedOrder).Dump(nameof(ProrationRounding.SpecifiedOrder));
    Prorate(total: 1014, weights: weights, ProrationRounding.DecimalPartDescending).Dump(nameof(ProrationRounding.DecimalPartDescending));
    ProrateExact(total: 1014, weights: weights).Dump(nameof(ProrateExact));
}

/// <summary>
/// 整数按分で生じた端数の処理方法を表します。
/// </summary>
public enum ProrationRounding {

    /// <summary>
    /// 指定された重み配列の順に端数を割り振ります。
    /// </summary>
    /// <remarks>低コスト</remarks>
    SpecifiedOrder,

    /// <summary>
    /// 小数部の大きい順に端数を割り振ります。
    /// </summary>
    /// <remarks>高コスト</remarks>
    DecimalPartDescending,
}

public static IEnumerable<int> Prorate(
      int total
    , IEnumerable<decimal> weights
    , ProrationRounding rounding
) {
    if (weights is null) { throw new ArgumentNullException(nameof(weights)); }

    List<(int integerPart, decimal decimalPart)> proratedList = new();
    int remaineds = total;
    foreach (decimal prorated in ProrateExact(total, weights)) {
        int integerPart = decimal.ToInt32(prorated);  // NOTE: https://docs.microsoft.com/en-us/dotnet/api/system.decimal.toint32?view=net-5.0#remarks
        remaineds -= integerPart;

        proratedList.Add((
            integerPart,
            decimalPart: prorated % 1
        ));
    }
    if (remaineds is 0) { return proratedList.Select(t => t.integerPart); }

    switch (rounding) {
        case ProrationRounding.SpecifiedOrder:
            return proratedList.Select(t => {
                if (remaineds is 0) { return t.integerPart; }
                if (t.decimalPart is 0) { return t.integerPart; }

                remaineds--;
                return t.integerPart + 1;
            });

        case ProrationRounding.DecimalPartDescending:
            return proratedList
                .Select((parts, i) => (parts, i))
                .OrderByDescending(t => t.parts.decimalPart)
                .Select(t => {
                    if (remaineds is 0) { return (prorated: t.parts.integerPart, t.i); }

                    remaineds--;
                    return (prorated: t.parts.integerPart + 1, t.i);
                })
                .OrderBy(t => t.i)
                .Select(t => t.prorated)
                ;

        default:
            throw new ArgumentException(nameof(rounding));
    }
}

public static IEnumerable<decimal> ProrateExact(
      decimal total
    , IEnumerable<decimal> weights
) {
    if (weights is null) { throw new ArgumentNullException(nameof(weights)); }

    switch (weights) {
        case IReadOnlyCollection<decimal>:
        case ICollection<decimal>:
            break;

        default:
            weights = weights.ToArray();
            break;
    }
    decimal totalWeight = weights.Sum();

    return weights.Select(weight => total * weight / totalWeight);
}