EPPlus/FormulaParsing/Excel/Functions/DateTime/Weeknum.cs

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using OfficeOpenXml.FormulaParsing.ExpressionGraph;

namespace OfficeOpenXml.FormulaParsing.Excel.Functions.DateTime;

public class Weeknum : ExcelFunction {
  public override CompileResult Execute(
      IEnumerable<FunctionArgument> arguments,
      ParsingContext context) {
    ValidateArguments(arguments, 1, eErrorType.Value);
    var dateSerial = ArgToDecimal(arguments, 0);
    var date = System.DateTime.FromOADate(dateSerial);
    var startDay = DayOfWeek.Sunday;
    if (arguments.Count() > 1) {
      var argStartDay = ArgToInt(arguments, 1);
      switch (argStartDay) {
        case 1:
          startDay = DayOfWeek.Sunday;
          break;
        case 2:
        case 11:
          startDay = DayOfWeek.Monday;
          break;
        case 12:
          startDay = DayOfWeek.Tuesday;
          break;
        case 13:
          startDay = DayOfWeek.Wednesday;
          break;
        case 14:
          startDay = DayOfWeek.Thursday;
          break;
        case 15:
          startDay = DayOfWeek.Friday;
          break;
        case 16:
          startDay = DayOfWeek.Saturday;
          break;
        default:
          // Not supported
          ThrowExcelErrorValueException(eErrorType.Num);
          break;
      }
    }
    if (DateTimeFormatInfo.CurrentInfo == null) {
      throw new InvalidOperationException(
          "Could not execute Weeknum function because DateTimeFormatInfo.CurrentInfo was null");
    }
    var week = DateTimeFormatInfo.CurrentInfo.Calendar.GetWeekOfYear(
        date,
        CalendarWeekRule.FirstDay,
        startDay);
    return CreateResult(week, DataType.Integer);
  }
}