Add AddBusinessDays function and corresponding tests

src/Albatross.Expression.Test/Operations/DateOperationTests.cs

@@ -1,4 +1,5 @@
 using NUnit.Framework;
+using System;
 
 namespace Albatross.Expression.Test.Operations
 {
@@ -52,6 +53,21 @@ public class DateOperationTests
         [TestCase("format(Date(\"2015-2-10\"), \"yyyy-MM-dd\")", ExpectedResult = "2015-02-10")]
         [TestCase("GetWorkingDays(Date(\"2023-08-29\"), Date(\"2023-09-03\"),1,1)", ExpectedResult = 4d)]
 
+        // Add Business Days
+        [TestCase("AddBusinessDays(CreateDate(2025, 4, 4), 5)", ExpectedResult = "2025-04-10")] 
+        [TestCase("AddBusinessDays(CreateDate(2025, 4, 5), 2)", ExpectedResult = "2025-04-08")] 
+        [TestCase("AddBusinessDays(CreateDate(2025, 4, 12), 1)", ExpectedResult = "2025-04-14")]
+        [TestCase("AddBusinessDays(CreateDate(2025, 4, 11), 3)", ExpectedResult = "2025-04-15")] 
+        [TestCase("AddBusinessDays(CreateDate(2025, 4, 10), -1)", ExpectedResult = "2025-04-10")] 
+        [TestCase("AddBusinessDays(CreateDate(2025, 4, 11), -5)", ExpectedResult = "2025-04-07")] 
+        [TestCase("AddBusinessDays(CreateDate(2025, 4, 14), -1)", ExpectedResult = "2025-04-14")]
+        [TestCase("AddBusinessDays(CreateDate(2025, 4, 12), 10)", ExpectedResult = "2025-04-25")]
+        [TestCase("AddBusinessDays(CreateDate(2025, 4, 8), 3, 1, 1)", ExpectedResult = "2025-04-10")] 
+        [TestCase("AddBusinessDays(CreateDate(2025, 4, 10), 3, 1, 1)", ExpectedResult = "2025-04-13")] 
+        [TestCase("AddBusinessDays(CreateDate(2025, 4, 12), 1, 1, 1)", ExpectedResult = "2025-04-13")] 
+        [TestCase("AddBusinessDays(CreateDate(2025, 4, 11), 3, 5, 2)", ExpectedResult = "2025-04-13")] 
+        [TestCase("AddBusinessDays(CreateDate(2025, 4, 10), 8, 5, 3)", ExpectedResult = "2025-04-20")] 
+
         public object OperationsTesting(string expression)
         {
             return Factory.Instance.Create().Compile(expression).EvalValue(null);

src/Albatross.Expression/Functions/Date/AddBusinessDays.cs

@@ -0,0 +1,82 @@
+using Albatross.Expression.Documentation;
+using Albatross.Expression.Documentation.Attributes;
+using Albatross.Expression.Tokens;
+using System;
+using System.Collections.Generic;
+
+namespace Albatross.Expression.Functions.Date
+{
+    [FunctionDoc(Group.Date, "{token}( , )",
+@"### Add or Subtract Business Days
+#### Inputs:
+- `date`: Date (mandatory) - The starting date.
+- `count`: Number (mandatory) - Number of business days to add (positive for future, negative for past).
+- `startOfWeek`: Start of the week as a value from 1 (Monday) to 7 (Sunday) (Optional, default: 1).
+- `weekendLength`: Integer (1 or 2) indicating the length of the weekend (Optional, default: 2).
+#### Outputs:
+- A calculated `DateTime`, adjusted by the number of business days."
+    )]
+    [ParserOperation]
+    public class AddBusinessDays : PrefixOperationToken
+    {
+        public override string Name => "AddBusinessDays";
+
+        public override bool Symbolic => false;
+
+        public override int MinOperandCount => 2;
+
+        public override int MaxOperandCount => 4;
+
+        public override object EvalValue(Func<string, object> context)
+        {
+            var date = (DateTime)Convert.ChangeType(Operands[0].EvalValue(context), typeof(DateTime));
+            var count = (int)Convert.ChangeType(Operands[1].EvalValue(context), typeof(int));
+
+            var startOfWeek = DayOfWeek.Monday;
+            if (Operands.Count > 2)
+                startOfWeek = (DayOfWeek)(((int)Convert.ChangeType(Operands[2].EvalValue(context), typeof(int)) - 1) % 7);
+
+            var weekendLength = 2;
+            if (Operands.Count > 3)
+                weekendLength = (int)Convert.ChangeType(Operands[3].EvalValue(context), typeof(int));
+
+            var adjustedDate = AdjustBusinessDays(date, count, startOfWeek, weekendLength);
+            return adjustedDate.ToString("yyyy-MM-dd");
+        }
+
+        private DateTime AdjustBusinessDays(DateTime date, int count, DayOfWeek startOfWeek, int weekendLength)
+        {
+            var weekendDays = CalculateWeekendDays(startOfWeek, weekendLength);
+
+            var step = count > 0 ? 1 : -1;
+            var daysMoved = 0;
+
+            if (!weekendDays.Contains(date.DayOfWeek))
+                daysMoved++;
+
+            while (daysMoved < Math.Abs(count))
+            {
+                date = date.AddDays(step);
+
+                if (!weekendDays.Contains(date.DayOfWeek))
+                    daysMoved++;
+            }
+
+            return date;
+        }
+
+        private HashSet<DayOfWeek> CalculateWeekendDays(DayOfWeek startOfWeek, int weekendLength)
+        {
+            var weekendDays = new HashSet<DayOfWeek>();
+
+            var endOfWeek = ((int)startOfWeek + 7 - 1) % 7; 
+            for (var i = 0; i < weekendLength; i++)
+            {
+                var weekendDayIndex = (endOfWeek - i + 7) % 7; 
+                weekendDays.Add((DayOfWeek)weekendDayIndex);
+            }
+
+            return weekendDays;
+        }
+    }
+}