EPPlus/FormulaParsing/Excel/Functions/Math/Fact.cs

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

namespace OfficeOpenXml.FormulaParsing.Excel.Functions.Math;

public class Fact : ExcelFunction {
  public override CompileResult Execute(
      IEnumerable<FunctionArgument> arguments,
      ParsingContext context) {
    ValidateArguments(arguments, 1);
    var number = ArgToDecimal(arguments, 0);
    ThrowExcelErrorValueExceptionIf(() => number < 0, eErrorType.Na);
    var result = 1d;
    for (var x = 1; x < number; x++) {
      result *= x;
    }
    return CreateResult(result, DataType.Integer);
  }
}