using System;
using System.Runtime.ExceptionServices;

namespace Umbraco.Core.Cache
{
    public static class SafeLazy
    {
        // an object that represent a value that has not been created yet
        internal static readonly object ValueNotCreated = new object();

        public static Lazy<object> GetSafeLazy(Func<object> getCacheItem)
        {
            // try to generate the value and if it fails,
            // wrap in an ExceptionHolder - would be much simpler
            // to just use lazy.IsValueFaulted alas that field is
            // internal
            return new Lazy<object>(() =>
            {
                try
                {
                    return getCacheItem();
                }
                catch (Exception e)
                {
                    return new ExceptionHolder(ExceptionDispatchInfo.Capture(e));
                }
            });
        }

        public static object GetSafeLazyValue(Lazy<object> lazy, bool onlyIfValueIsCreated = false)
        {
            // if onlyIfValueIsCreated, do not trigger value creation
            // must return something, though, to differentiate from null values
            if (onlyIfValueIsCreated && lazy.IsValueCreated == false) return ValueNotCreated;

            // if execution has thrown then lazy.IsValueCreated is false
            // and lazy.IsValueFaulted is true (but internal) so we use our
            // own exception holder (see Lazy<T> source code) to return null
            if (lazy.Value is ExceptionHolder) return null;

            // we have a value and execution has not thrown so returning
            // here does not throw - unless we're re-entering, take care of it
            try
            {
                return lazy.Value;
            }
            catch (InvalidOperationException e)
            {
                throw new InvalidOperationException("The method that computes a value for the cache has tried to read that value from the cache.", e);
            }
        }

        public class ExceptionHolder
        {
            public ExceptionHolder(ExceptionDispatchInfo e)
            {
                Exception = e;
            }

            public ExceptionDispatchInfo Exception { get; }
        }
    }
}