using Connected.Entities.Query;
using System.Linq.Expressions;

namespace Connected.Expressions.Query;

public abstract class QueryProvider : IAsyncQueryProvider
{
    public IQueryable CreateQuery(Expression expression)
    {
        var type = expression.Type.GetElementType();
        var generic = typeof(EntityQuery<>).MakeGenericType(new Type[] { type });

        if (generic is null)
            throw new NullReferenceException(nameof(type));

        var instance = Activator.CreateInstance(generic, new object[] { this, expression }) as IQueryable;

        if (instance is null)
            throw new NullReferenceException(nameof(type));

        return instance;
    }

    public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
    {
        return new EntityQuery<TElement>(this, expression);
    }

    public object? Execute(Expression expression)
    {
        return OnExecute(expression);
    }

    public object? Execute(Expression expression, CancellationToken cancellationToken = default)
    {
        return OnExecute(expression);
    }

    public TResult Execute<TResult>(Expression expression)
    {
        return (TResult)OnExecute(expression);
    }

    protected virtual object? OnExecute(Expression expression)
    {
        return default;
    }
}