c# - Compare two arbitrary JToken-s of the same structure

Question

Thanks in advance. I appreciate any help.

I would like to compare two arbitrary JTokens of the same type and structure (Json.Net from NewtonSoft).

static int CompareTokens(JToken x, JToken y);  
// possible output: 0 / 1 / -1

The main goal is to be able use this method to sort two Json strings, so that even if in the begining they had the same data, but in the different order, in the end these are two exactly same strings. So the sort criterion doesn't really matter, it just matters that this criterion is always the same. And each small element of data should be taken into account.

JToken can be of one of next several types: Array, Boolean, Date, Float, Guid, Integer, Null, Object, Property, String, TimeSpan, Uri. I don't take into account comparing Bytes, Comment, Constructor, None, Undefined, Raw.

• It would be great to gain some idea about comparing JArrays and JObjects. That shoud be some recursive comparison, because JArrays may consist of other JArrays and JObjects and vice versa. Any idea would be appreciated.
• But knowing about comparing simpler types would also be very helpful. I wonder rather about knowing how to convert from JToken to actual type (than about knowing how to do it logically).
• JValue has IComparable implemented, but i didn't figure out how to convert simple typed JToken to JValue. Knowing about this would also be helpful.

This is quite a havy question. If i figure out how to do it, i'll put a +100 on it. And sorry for my English.

Answer 1

In Linq-to-JSON, JValue represents a primitive value (string, number, boolean, and so on). It implements IComparable<JValue>, so Json.NET takes care of sorting primitive values for you.

Building off of that, you're going to need to recursively descend the two JToken object hierarchies in parallel. When you encounter the first token with a different .Net type, or different properties (if not a JValue), or with a different value (if a JValue), you need to return back the comparison value.

Keep in mind the following:

• A comparison method should be reflexive, antisymmetric and transitive.
• Container tokens of different .Net type need to be ordered by type in some consistent manner.
• the child tokens of JArray and JConstructor are ordered.
• the child tokens of JObject are not, so they need to be compared in some stable, symmetric manner. Walking both in order of property name would seem to work.
• There is no obvious way to compare JRaw, so don't try, and let an exception get thrown.

The following is a prototype implementation:

public class JTokenComparer : IComparer<JToken>
{
    public static JTokenComparer Instance { get { return instance; } }

    static JTokenComparer instance;

    static JTokenComparer()
    {
        instance = new JTokenComparer();
    }

    readonly Dictionary<Type, KeyValuePair<int, IComparer<JToken>>> dict;

    JTokenComparer()
    {
        dict = new Dictionary<Type, KeyValuePair<int, IComparer<JToken>>>
        {
            // Order chosen semi-arbitrarily.  Putting values first seems reasonable though.
            {typeof(JValue), new KeyValuePair<int, IComparer<JToken>>(0, new JValueComparer()) },
            {typeof(JProperty), new KeyValuePair<int, IComparer<JToken>>(1, new JPropertyComparer()) },
            {typeof(JArray), new KeyValuePair<int, IComparer<JToken>>(2, new JArrayComparer()) },
            {typeof(JObject), new KeyValuePair<int, IComparer<JToken>>(3, new JObjectComparer()) },
            {typeof(JConstructor), new KeyValuePair<int, IComparer<JToken>>(4, new JConstructorComparer()) },
        };
    }

    #region IComparer<JToken> Members

    public int Compare(JToken x, JToken y)
    {
        if (x is JRaw || y is JRaw)
            throw new InvalidOperationException("Tokens of type JRaw cannot be sorted");
        if (object.ReferenceEquals(x, y))
            return 0;
        else if (x == null)
            return -1;
        else if (y == null)
            return 1;

        var typeData1 = dict[x.GetType()];
        var typeData2 = dict[y.GetType()];

        int comp;
        if ((comp = typeData1.Key.CompareTo(typeData2.Key)) != 0)
            return comp;
        if (typeData1.Value != typeData2.Value)
            throw new InvalidOperationException("inconsistent dictionary values"); // Internal error
        return typeData2.Value.Compare(x, y);
    }

    #endregion
}

abstract class JTokenComparerBase<TJToken> : IComparer<JToken> where TJToken : JToken
{
    protected TJToken CheckType(JToken item)
    {
        if (item != null && item.GetType() != typeof(TJToken))
            throw new ArgumentException(string.Format("Actual type {0} of token "{1}" does not match expected type {2}", item.GetType(), item, typeof(TJToken)));
        return (TJToken)item;
    }

    protected bool TryBaseCompare(TJToken x, TJToken y, out int comparison)
    {
        CheckType(x);
        CheckType(y);
        if (object.ReferenceEquals(x, y))
        {
            comparison = 0;
            return true;
        }
        else if (x == null)
        {
            comparison = -1;
            return true;
        }
        else if (y == null)
        {
            comparison = 1;
            return true;
        }
        comparison = 0;
        return false;
    }

    protected abstract int CompareDerived(TJToken x, TJToken y);

    protected int TokenCompare(JToken x, JToken y)
    {
        var tx = CheckType(x);
        var ty = CheckType(y);
        int comp;
        if (TryBaseCompare(tx, ty, out comp))
            return comp;
        return CompareDerived(tx, ty);
    }

    #region IComparer<JToken> Members

    int IComparer<JToken>.Compare(JToken x, JToken y)
    {
        return TokenCompare(x, y);
    }

    #endregion
}

abstract class JContainerOrderedComparerBase<TJToken> : JTokenComparerBase<TJToken> where TJToken : JContainer
{
    protected int CompareItemsInOrder(TJToken x, TJToken y)
    {
        int comp;
        // Dictionary order: sort on items before number of items.
        for (int i = 0, n = Math.Min(x.Count, y.Count); i < n; i++)
            if ((comp = JTokenComparer.Instance.Compare(x[i], y[i])) != 0)
                return comp;
        if ((comp = x.Count.CompareTo(y.Count)) != 0)
            return comp;
        return 0;
    }
}

class JPropertyComparer : JTokenComparerBase<JProperty>
{
    protected override int CompareDerived(JProperty x, JProperty y)
    {
        int comp;
        if ((comp = x.Name.CompareTo(y.Name)) != 0)
            return comp;
        return JTokenComparer.Instance.Compare(x.Value, y.Value);
    }
}

class JObjectComparer : JTokenComparerBase<JObject>
{
    protected override int CompareDerived(JObject x, JObject y)
    {
        int comp;
        // Dictionary order: sort on items before number of items.
        // Order both property sequences to preserve reflexivity.
        foreach (var propertyComp in x.Properties().OrderBy(p => p.Name).Zip(y.Properties().OrderBy(p => p.Name), (xp, yp) => JTokenComparer.Instance.Compare(xp, yp)))
            if (propertyComp != 0)
                return propertyComp;
        if ((comp = x.Count.CompareTo(y.Count)) != 0)
            return comp;
        return 0;
    }
}

class JArrayComparer : JContainerOrderedComparerBase<JArray>
{
    protected override int CompareDerived(JArray x, JArray y)
    {
        int comp;
        if ((comp = CompareItemsInOrder(x, y)) != 0)
            return comp;
        return 0;
    }
}

class JConstructorComparer : JContainerOrderedComparerBase<JConstructor>
{
    protected override int CompareDerived(JConstructor x, JConstructor y)
    {
        int comp;
        if ((comp = x.Name.CompareTo(y.Name)) != 0)
            return comp;
        if ((comp = CompareItemsInOrder(x, y)) != 0)
            return comp;
        return 0;
    }
}

class JValueComparer : JTokenComparerBase<JValue>
{
    protected override int CompareDerived(JValue x, JValue y)
    {
        return Comparer<JToken>.Default.Compare(x, y); // JValue implements IComparable<JValue>
    }
}

Lightly tested prototype fiddle.