Solution for new GetPreviousOccurence

NCrontab.Signed/NCrontab.Signed.csproj

@@ -31,6 +31,8 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <Compile Include="..\NCrontab\Utils\DateTimeComponents.cs" Link="Utils\DateTimeComponents.cs" />
+    <Compile Include="..\NCrontab\Utils\Incrementor.cs" Link="Utils\Incrementor.cs" />
     <Compile Include="..\SolutionInfo.cs" />
     <Compile Include="..\NCrontab\*.cs" />
     <None Include="..\COPYING.txt" Pack="true" PackagePath="$(PackageLicenseFile)" />

NCrontab/CrontabField.Iterator.cs

@@ -0,0 +1,52 @@
+using NCrontab.Utils;
+
+namespace NCrontab
+{
+    public sealed partial class CrontabField
+    {
+        internal sealed class Iterator
+        {
+            readonly CrontabField _crontabField;
+            readonly bool _forwardMoving;
+
+            public Iterator(CrontabField crontabField, bool forwardMoving)
+            {
+                _crontabField = crontabField;
+                _forwardMoving = forwardMoving;
+            }
+
+            /// <summary>
+            /// Returns the first valid value (according to filter) beginning at <paramref name="start"/>.
+            /// </summary>
+            /// <param name="start"></param>
+            /// <returns></returns>
+            public int Next(int start)
+            {
+                if (BeyondLowerBoundValueSet(start))
+                    return LowerBoundValueSet;
+
+                var startIndex = _crontabField.ValueToIndex(start);
+                var lastIndex = _crontabField.ValueToIndex(UpperBoundValueSet);
+
+                var incrementor = new Incrementor(_forwardMoving);
+                for (var i = startIndex; incrementor.BeforeOrEqual(i, lastIndex); i = incrementor.Increment(i))
+                {
+                    if (_crontabField._bits[i])
+                        return _crontabField.IndexToValue(i);
+                }
+
+                return nil;
+            }
+
+            public int LowerBound => _forwardMoving ? _crontabField.GetFirst() : _crontabField.GetLast();
+
+            public int LowerBoundValueSet => _forwardMoving ? _crontabField._minValueSet : _crontabField._maxValueSet;
+            public int UpperBoundValueSet => _forwardMoving ? _crontabField._maxValueSet : _crontabField._minValueSet;
+
+            public bool BeyondLowerBoundValueSet(int start) => _forwardMoving ?
+                                                                start < _crontabField._minValueSet :
+                                                                start > _crontabField._maxValueSet;
+
+        }
+    }
+}

NCrontab/CrontabField.cs

@@ -20,6 +20,7 @@
 
 namespace NCrontab
 {
+    using NCrontab.Utils;
     #region Imports
 
     using System;
@@ -37,6 +38,8 @@ namespace NCrontab
 
     public sealed partial class CrontabField : ICrontabField
     {
+        internal const int nil = -1;
+
         readonly BitArray _bits;
         /* readonly */ int _minValueSet;
         /* readonly */ int _maxValueSet;
@@ -111,36 +114,32 @@ public static CrontabField DaysOfWeek(string expression) =>
             _impl = impl ?? throw new ArgumentNullException(nameof(impl));
             _bits = new BitArray(impl.ValueCount);
             _minValueSet = int.MaxValue;
-            _maxValueSet = -1;
+            _maxValueSet = nil;
         }
 
         /// <summary>
         /// Gets the first value of the field or -1.
         /// </summary>
 
-        public int GetFirst() => _minValueSet < int.MaxValue ? _minValueSet : -1;
+        public int GetFirst() => _minValueSet < int.MaxValue ? _minValueSet : nil;
+
+        /// <summary>
+        /// Gets the last value of the field or -1.
+        /// </summary>
+        public int GetLast() => _maxValueSet >= 0 ? _maxValueSet : nil;
 
         /// <summary>
         /// Gets the next value of the field that occurs after the given
         /// start value or -1 if there is no next value available.
         /// </summary>
 
-        public int Next(int start)
-        {
-            if (start < _minValueSet)
-                return _minValueSet;
-
-            var startIndex = ValueToIndex(start);
-            var lastIndex = ValueToIndex(_maxValueSet);
+        public int Next(int start) => new Iterator(this, true).Next(start);
 
-            for (var i = startIndex; i <= lastIndex; i++)
-            {
-                if (_bits[i])
-                    return IndexToValue(i);
-            }
-
-            return -1;
-        }
+        /// <summary>
+        /// Gets the previous value of the field that occurs before the given
+        /// start value or -1 if there is no previous value available.
+        /// </summary>
+        public int Prev(int start) => new Iterator(this, false).Next(start);
 
         int IndexToValue(int index) => index + _impl.MinValue;
         int ValueToIndex(int value) => value - _impl.MinValue;

NCrontab/CrontabFieldImpl.cs

@@ -90,7 +90,7 @@ public void Format(ICrontabField field, TextWriter writer, bool noNames)
             var next = field.GetFirst();
             var count = 0;
 
-            while (next != -1)
+            while (next != CrontabField.nil)
             {
                 var first = next;
                 int last;
@@ -219,7 +219,7 @@ T InternalParse<T>(string str, CrontabFieldAccumulator<T> acc, T success, Func<E
 
             if (str.Length == 1 && str[0] == '*')
             {
-                return acc(-1, -1, every ?? 1, success, errorSelector);
+                return acc(CrontabField.nil, CrontabField.nil, every ?? 1, success, errorSelector);
             }
 
             //