Efficiently sort an IList without copying the source list

Question

Given the test case below how can I:

1. Sort the IList<TestObject> based on the index of a matching Id in the IList<int> list.
2. Unmatched values are moved to the end of the list and sorted by their original index. In this case, since 3 and 4 do not exist in the index list, we expect to see list[3] == 3 and list[4] == 4.
3. Whilst I know this can be achieved with linq, I need to resort the original list rather than creating a new one (due to how the list is stored).
4. The source list must be an IList (I can't use List<T>)

Here's the test:

    public class TestObject
    {
        public int Id { get; set; }
    }

    [Test]
    public void Can_reorder_using_index_list()
    {
        IList<TestObject> list = new List<TestObject>
        {
            new TestObject { Id = 1 },
            new TestObject { Id = 2 },
            new TestObject { Id = 3 },
            new TestObject { Id = 4 },
            new TestObject { Id = 5 }
        };

        IList<int> indexList = new[] { 10, 5, 1, 9, 2 };

        // TODO sort

        Assert.That(list[0].Id, Is.EqualTo(5));
        Assert.That(list[1].Id, Is.EqualTo(1));
        Assert.That(list[2].Id, Is.EqualTo(2));
        Assert.That(list[3].Id, Is.EqualTo(3));
        Assert.That(list[4].Id, Is.EqualTo(4));
    }

Update:

As requested, this is what I did try, but 1) it only works with List<T> and 2) I'm not sure it's the most efficient way:

       var clone = list.ToList();
        list.Sort((x, y) =>
        {
            var xIndex = indexList.IndexOf(x.Id);
            var yIndex = indexList.IndexOf(y.Id);

            if (xIndex == -1)
            {
                xIndex = list.Count + clone.IndexOf(x);
            }
            if (yIndex == -1)
            {
                yIndex = list.Count + clone.IndexOf(y);
            }

            return xIndex.CompareTo(yIndex);
        });

Update 2:

Thanks to @leppie, @jamiec, @mitch wheat - this is the working code:

    public class TestObjectComparer : Comparer<TestObject>
    {
        private readonly IList<int> indexList;
        private readonly Func<TestObject, int> currentIndexFunc;
        private readonly int listCount;

        public TestObjectComparer(IList<int> indexList, Func<TestObject, int> currentIndexFunc, int listCount)
        {
            this.indexList = indexList;
            this.currentIndexFunc = currentIndexFunc;
            this.listCount = listCount;
        }

        public override int Compare(TestObject x, TestObject y)
        {
            var xIndex = indexList.IndexOf(x.Id);
            var yIndex = indexList.IndexOf(y.Id);

            if (xIndex == -1)
            {
                xIndex = listCount + currentIndexFunc(x);
            }
            if (yIndex == -1)
            {
                yIndex = listCount + currentIndexFunc(y);
            }

            return xIndex.CompareTo(yIndex);
        }
    }

    [Test]
    public void Can_reorder_using_index_list()
    {
        IList<TestObject> list = new List<TestObject>
        {
            new TestObject { Id = 1 },
            new TestObject { Id = 2 },
            new TestObject { Id = 3 },
            new TestObject { Id = 4 },
            new TestObject { Id = 5 }
        };

        IList<int> indexList = new[] { 10, 5, 1, 9, 2, 4 };

        ArrayList.Adapter((IList)list).Sort(new TestObjectComparer(indexList, x => list.IndexOf(x), list.Count));

        Assert.That(list[0].Id, Is.EqualTo(5));
        Assert.That(list[1].Id, Is.EqualTo(1));
        Assert.That(list[2].Id, Is.EqualTo(2));
        Assert.That(list[3].Id, Is.EqualTo(3));
        Assert.That(list[4].Id, Is.EqualTo(4));
    }

Answer 1

You can try the following:

ArrayList.Adapter(yourilist).Sort();

Update:

A generic comparer:

class Comparer<T> : IComparer<T>, IComparer
{
  internal Func<T, T, int> pred;

  public int Compare(T x, T y)
  {
    return pred(x, y);  
  }

  public int Compare(object x, object y)
  {
    return Compare((T)x, (T)y);
  }
}

static class ComparerExtensions
{
  static IComparer Create<T>(Func<T, T, int> pred)
  {
    return new Comparer<T> { pred = pred };
  }

  public static void Sort<T>(this ArrayList l, Func<T, T, int> pred)
  {
    l.Sort(Create(pred));
  }
}

Usage:

ArrayList.Adapter(list).Sort<int>((x,y) => x - y);

Answer 2

Been looking at this for a bit, and indeed as previously said, your going to need ArrayList.Adapter, however you'll note it takes a non-generic IList so some casting will be required:

ArrayList.Adapter((IList)list)

You'll also need to write a comparer, of which the logic to do your sorting willl be contained. Excuse the name but:

public class WeirdComparer : IComparer,IComparer<TestObject>
{
    private IList<int> order;
    public WeirdComparer(IList<int> order)
    {
        this.order = order;
    }
    public int Compare(object x, object y)
    {
        return Compare((TestObject) x, (TestObject) y);
    }

    public int Compare(TestObject x, TestObject y)
    {
        if(order.Contains(x.Id))
        {
            if(order.Contains(y.Id))
            {
                return order.IndexOf(x.Id).CompareTo(order.IndexOf(y.Id));    
            }
            return -1;
        }
        else
        {
            if (order.Contains(y.Id))
            {
                return 1;
            }
            return x.Id.CompareTo(y.Id);
        }
    }
}

EDIT: Added implementation to above comparerr

Then the usage would be as follows:

IList<int> indexList = new[] { 10, 5, 1, 9, 2 };
ArrayList.Adapter((IList)list).Sort(new WeirdComparer(indexList));

By the way, this thread explains a nice way to turn this into an extension method which will make your code more reusable and easier to read IMO.

Answer 3

Here's a generic version of the comparer. IEntity<TId> is just a simple interface that has a property "Id" of type TId:

public class IndexComparer<T, TId> : Comparer<T> where T : IEntity<TId> where TId : IComparable
{
    private readonly IList<TId> order;
    private readonly int listCount;
    private readonly Func<T, int> currentIndexFunc;

    public IndexComparer(Func<T, int> currentIndexFunc, IList<TId> order, int listCount) {
        this.order = order;
        this.listCount = listCount;
        this.currentIndexFunc = currentIndexFunc;
    }

    public override int Compare(T x, T y)
    {
        var xIndex = order.IndexOf(x.Id);
        var yIndex = order.IndexOf(y.Id);

        if (xIndex == -1)
        {
            xIndex = listCount + currentIndexFunc(x);
        }
        if (yIndex == -1)
        {
            yIndex = listCount + currentIndexFunc(y);
        }

        return xIndex.CompareTo(yIndex);
    }
}

Working test:

[TestFixture]
public class OrderingTests
{
    public class TestObject : IEntity<int>
    {
        public int Id { get; set; }
    }

    [Test]
    public void Can_reorder_using_index_list()
    {
        IList<TestObject> list = new List<TestObject>
        {
            new TestObject { Id = 1 },
            new TestObject { Id = 2 },
            new TestObject { Id = 3 },
            new TestObject { Id = 4 },
            new TestObject { Id = 5 }
        };

        IList<int> indexList = new[] { 10, 5, 1, 9, 2, 4 };
        ArrayList.Adapter((IList)list)
            .Sort(new IndexComparer<TestObject, int>(x => list.IndexOf(x), indexList, list.Count));

        Assert.That(list[0].Id, Is.EqualTo(5));
        Assert.That(list[1].Id, Is.EqualTo(1));
        Assert.That(list[2].Id, Is.EqualTo(2));
        Assert.That(list[3].Id, Is.EqualTo(4));
        Assert.That(list[4].Id, Is.EqualTo(3));
    }
}

This meets the requirements as outlined in my original question. Unmatched elements are moved to the end of the list and then ordered by their original index.