using System;
using System.Collections;
using System.Collections.Generic;
using System.Runtime.CompilerServices;

namespace Foodsoft.Alpm
{
    internal class ReadOnlyCollectionWrapper<TImpl, TElement> : IReadOnlyCollection<TElement> where TImpl : struct,
        IReadOnlyItemsAccessor<TElement>
    {
        private TImpl _impl;
        private SafeAlpmHandle SafeAlpmHandle => _impl.SafeAlpmHandle;

        public ReadOnlyCollectionWrapper(TImpl impl)
        {
            _impl = impl;
        }

        private static unsafe IntPtr ListNext(IntPtr list) => ((alpm_list_t*) list)->next;
        private static unsafe IntPtr ListData(IntPtr list) => ((alpm_list_t*) list)->data;
        public IEnumerator<TElement> GetEnumerator()
        {
            var safeAlpmHandle = _impl.SafeAlpmHandle;
            var release = false;

            RuntimeHelpers.PrepareConstrainedRegions();
            try
            {
                safeAlpmHandle.DangerousAddRef(ref release);
                if (!release) throw new ObjectDisposedException(_impl.GetType().FullName);
                for (var list = _impl.GetItems(); list != IntPtr.Zero; list = ListNext(list))
                {
                    yield return _impl.PtrToItem(ListData(list));
                }
            }
            finally
            {
                if (release)
                    safeAlpmHandle.DangerousRelease();
            }
        }

        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();

        public int Count => (int) alpm.alpm_list_count(_impl.GetItems());
    }
}