AppsheetEpplus/FormulaParsing/Excel/Functions/DateTime/Workday.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace AppsheetEpplus;

public class Workday : ExcelFunction {
  public override CompileResult Execute(
      IEnumerable<FunctionArgument> arguments,
      ParsingContext context) {
    ValidateArguments(arguments, 2);
    var startDate = System.DateTime.FromOADate(ArgToInt(arguments, 0));
    var nWorkDays = ArgToInt(arguments, 1);
    var resultDate = System.DateTime.MinValue;
    var workdaysCounted = 0;
    var tmpDate = startDate;
    // first move forward to the first monday
    while (tmpDate.DayOfWeek != DayOfWeek.Monday && (nWorkDays - workdaysCounted) > 0) {
      if (!IsHoliday(tmpDate)) {
        workdaysCounted++;
      }
      tmpDate = tmpDate.AddDays(1);
    }
    // then calculate whole weeks
    var nWholeWeeks = (nWorkDays - workdaysCounted) / 5;
    tmpDate = tmpDate.AddDays(nWholeWeeks * 7);
    workdaysCounted += nWholeWeeks * 5;

    // calculate the rest
    while (workdaysCounted < nWorkDays) {
      tmpDate = tmpDate.AddDays(1);
      if (!IsHoliday(tmpDate)) {
        workdaysCounted++;
      }
    }
    resultDate = AdjustResultWithHolidays(tmpDate, arguments);
    return CreateResult(resultDate.ToOADate(), DataType.Date);
  }

  private System.DateTime AdjustResultWithHolidays(
      System.DateTime resultDate,
      IEnumerable<FunctionArgument> arguments) {
    if (arguments.Count() == 2) {
      return resultDate;
    }
    var holidays = arguments.ElementAt(2).Value as IEnumerable<FunctionArgument>;
    if (holidays != null) {
      foreach (var arg in holidays) {
        if (ConvertUtil.IsNumeric(arg.Value)) {
          var dateSerial = ConvertUtil.GetValueDouble(arg.Value);
          var holidayDate = System.DateTime.FromOADate(dateSerial);
          if (!IsHoliday(holidayDate)) {
            resultDate = resultDate.AddDays(1);
          }
        }
      }
    } else {
      var range = arguments.ElementAt(2).Value as ExcelDataProvider.IRangeInfo;
      if (range != null) {
        foreach (var cell in range) {
          if (ConvertUtil.IsNumeric(cell.Value)) {
            var dateSerial = ConvertUtil.GetValueDouble(cell.Value);
            var holidayDate = System.DateTime.FromOADate(dateSerial);
            if (!IsHoliday(holidayDate)) {
              resultDate = resultDate.AddDays(1);
            }
          }
        }
      }
    }
    return resultDate;
  }

  private bool IsHoliday(System.DateTime date) {
    return date.DayOfWeek == DayOfWeek.Saturday || date.DayOfWeek == DayOfWeek.Sunday;
  }
}