using System;
using System.Collections.Generic;

namespace Umbraco.Core
{
    public class TopoGraph
    {
        internal const string CycleDependencyError = "Cyclic dependency.";
        internal const string MissingDependencyError = "Missing dependency.";
    }

    /// <summary>
    /// Represents a generic DAG that can be topologically sorted.
    /// </summary>
    /// <typeparam name="TKey">The type of the keys.</typeparam>
    /// <typeparam name="TItem">The type of the items.</typeparam>
    public class TopoGraph<TKey, TItem> : TopoGraph
    {
        private readonly Func<TItem, TKey> _getKey;
        private readonly Func<TItem, IEnumerable<TKey>> _getDependencies;
        private readonly Dictionary<TKey, TItem> _items = new Dictionary<TKey, TItem>();

        /// <summary>
        /// Initializes a new instance of the <see cref="TopoGraph{TKey, TItem}"/> class.
        /// </summary>
        /// <param name="getKey">A method that returns the key of an item.</param>
        /// <param name="getDependencies">A method that returns the dependency keys of an item.</param>
        public TopoGraph(Func<TItem, TKey> getKey, Func<TItem, IEnumerable<TKey>> getDependencies)
        {
            _getKey = getKey;
            _getDependencies = getDependencies;
        }

        /// <summary>
        /// Adds an item to the graph.
        /// </summary>
        /// <param name="item">The item.</param>
        public void AddItem(TItem item)
        {
            var key = _getKey(item);
            _items[key] = item;
        }

        /// <summary>
        /// Adds items to the graph.
        /// </summary>
        /// <param name="items">The items.</param>
        public void AddItems(IEnumerable<TItem> items)
        {
            foreach (var item in items)
                AddItem(item);
        }

        /// <summary>
        /// Gets the sorted items.
        /// </summary>
        /// <param name="throwOnCycle">A value indicating whether to throw on cycles, or just ignore the branch.</param>
        /// <param name="throwOnMissing">A value indicating whether to throw on missing dependency, or just ignore the dependency.</param>
        /// <param name="reverse">A value indicating whether to reverse the order.</param>
        /// <returns>The (topologically) sorted items.</returns>
        public IEnumerable<TItem> GetSortedItems(bool throwOnCycle = true, bool throwOnMissing = true, bool reverse = false)
        {
            var sorted = new TItem[_items.Count];
            var visited = new HashSet<TItem>();
            var index = reverse ? _items.Count - 1 : 0;
            var incr = reverse ? -1 : +1;

            foreach (var item in _items.Values)
                Visit(item, visited, sorted, ref index, incr, throwOnCycle, throwOnMissing);

            return sorted;
        }

        private static bool Contains(TItem[] items, TItem item, int start, int count)
        {
            return Array.IndexOf(items, item, start, count) >= 0;
        }

        private void Visit(TItem item, ISet<TItem> visited, TItem[] sorted, ref int index, int incr, bool throwOnCycle, bool throwOnMissing)
        {
            if (visited.Contains(item))
            {
                // visited but not sorted yet = cycle
                var start = incr > 0 ? 0 : index;
                var count = incr > 0 ? index : sorted.Length - index;
                if (throwOnCycle && Contains(sorted, item, start, count) == false)
                    throw new Exception(CycleDependencyError);
                return;
            }

            visited.Add(item);

            var keys = _getDependencies(item);
            var dependencies = keys == null ? null : FindDependencies(keys, throwOnMissing);

            if (dependencies != null)
                foreach (var dep in dependencies)
                    Visit(dep, visited, sorted, ref index, incr, throwOnCycle, throwOnMissing);

            sorted[index] = item;
            index += incr;
        }

        private IEnumerable<TItem> FindDependencies(IEnumerable<TKey> keys, bool throwOnMissing)
        {
            foreach (var key in keys)
            {
                TItem value;
                if (_items.TryGetValue(key, out value))
                    yield return value;
                else if (throwOnMissing)
                    throw new Exception(MissingDependencyError);
            }
        }
    }
}