LehrerApp/LehrerApp.Core/Services/GradingService.cs

using LehrerApp.Core.Models;

namespace LehrerApp.Core.Services;

public class GradingService
{
    public string CalculateGrade(double achieved, double maximum, List<GradingKeyEntry> key)
    {
        if (maximum <= 0) return "-";
        var percent = achieved / maximum * 100.0;
        return key.OrderByDescending(k => k.MinPercent)
                  .FirstOrDefault(k => percent >= k.MinPercent)
                  ?.Grade ?? key.OrderBy(k => k.MinPercent).First().Grade;
    }
    public static List<GradingKeyEntry> DefaultKey1To6() =>
    [
        new() { Grade = "1", MinPercent = 87.5 },
        new() { Grade = "2", MinPercent = 75.0 },
        new() { Grade = "3", MinPercent = 62.5 },
        new() { Grade = "4", MinPercent = 50.0 },
        new() { Grade = "5", MinPercent = 25.0 },
        new() { Grade = "6", MinPercent = 0.0  },
    ];
    public static List<GradingKeyEntry> DefaultKey0To15() =>
    [
        new() { Grade = "15", MinPercent = 95.0 },
        new() { Grade = "14", MinPercent = 90.0 },
        new() { Grade = "13", MinPercent = 85.0 },
        new() { Grade = "12", MinPercent = 80.0 },
        new() { Grade = "11", MinPercent = 75.0 },
        new() { Grade = "10", MinPercent = 70.0 },
        new() { Grade = "9",  MinPercent = 65.0 },
        new() { Grade = "8",  MinPercent = 60.0 },
        new() { Grade = "7",  MinPercent = 55.0 },
        new() { Grade = "6",  MinPercent = 50.0 },
        new() { Grade = "5",  MinPercent = 45.0 },
        new() { Grade = "4",  MinPercent = 40.0 },
        new() { Grade = "3",  MinPercent = 33.0 },
        new() { Grade = "2",  MinPercent = 27.0 },
        new() { Grade = "1",  MinPercent = 20.0 },
        new() { Grade = "0",  MinPercent = 0.0  },
    ];
    public string? ValidateGradingKey(List<GradingKeyEntry> entries)
    {
        if (entries.Count < 2) return "Der Notenschlüssel braucht mindestens zwei Stufen.";
        if (entries.Any(e => string.IsNullOrWhiteSpace(e.Grade)))
            return "Jede Stufe braucht eine Bezeichnung.";
        if (entries.Any(e => e.MinPercent < 0 || e.MinPercent > 100))
            return "Prozentgrenzen müssen zwischen 0 und 100 liegen.";
        var sorted = entries.OrderByDescending(e => e.MinPercent).ToList();
        if (sorted.Select(e => e.MinPercent).Distinct().Count() != sorted.Count)
            return "Prozentgrenzen dürfen sich nicht doppeln.";
        if (sorted.Last().MinPercent != 0)
            return "Die unterste Stufe muss bei 0 % liegen (lückenlose Abdeckung).";
        return null;
    }

    public double WeightedAverage(List<(string Grade, double Weight)> grades)
    {
        var numeric = grades
            .Select(g => (Value: int.TryParse(g.Grade, out var n) ? (int?)n : null, g.Weight))
            .Where(g => g.Value.HasValue).ToList();
        if (numeric.Count == 0) return 0;
        var total = numeric.Sum(g => g.Weight);
        return total == 0 ? 0 : numeric.Sum(g => g.Value!.Value * g.Weight) / total;
    }
}