#region License and Terms
// MoreLINQ - Extensions to LINQ to Objects
// Copyright (c) 2010 Leopold Bushkin. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#endregion

namespace MoreLinq
{
    using System;
    using System.Collections.Generic;

    public static partial class MoreEnumerable
    {
        /// <summary>
        /// Produces a projection of a sequence by evaluating pairs of elements separated by a negative offset.
        /// </summary>
        /// <remarks>
        /// This operator evaluates in a deferred and streaming manner.<br/>
        /// For elements prior to the lag offset, <c>default(T)</c> is used as the lagged value.<br/>
        /// </remarks>
        /// <typeparam name="TSource">The type of the elements of the source sequence</typeparam>
        /// <typeparam name="TResult">The type of the elements of the result sequence</typeparam>
        /// <param name="source">The sequence over which to evaluate lag</param>
        /// <param name="offset">The offset (expressed as a positive number) by which to lag each value of the sequence</param>
        /// <param name="resultSelector">A projection function which accepts the current and lagged items (in that order) and returns a result</param>
        /// <returns>A sequence produced by projecting each element of the sequence with its lagged pairing</returns>

        public static IEnumerable<TResult> Lag<TSource, TResult>(this IEnumerable<TSource> source, int offset, Func<TSource, TSource, TResult> resultSelector)
        {
            return Lag(source, offset, default, resultSelector);
        }

        /// <summary>
        /// Produces a projection of a sequence by evaluating pairs of elements separated by a negative offset.
        /// </summary>
        /// <remarks>
        /// This operator evaluates in a deferred and streaming manner.<br/>
        /// </remarks>
        /// <typeparam name="TSource">The type of the elements of the source sequence</typeparam>
        /// <typeparam name="TResult">The type of the elements of the result sequence</typeparam>
        /// <param name="source">The sequence over which to evaluate lag</param>
        /// <param name="offset">The offset (expressed as a positive number) by which to lag each value of the sequence</param>
        /// <param name="defaultLagValue">A default value supplied for the lagged value prior to the lag offset</param>
        /// <param name="resultSelector">A projection function which accepts the current and lagged items (in that order) and returns a result</param>
        /// <returns>A sequence produced by projecting each element of the sequence with its lagged pairing</returns>

        public static IEnumerable<TResult> Lag<TSource, TResult>(this IEnumerable<TSource> source, int offset, TSource defaultLagValue, Func<TSource, TSource, TResult> resultSelector)
        {
            if (source == null) throw new ArgumentNullException(nameof(source));
            if (resultSelector == null) throw new ArgumentNullException(nameof(resultSelector));
            // NOTE: Theoretically, we could assume that negative (or zero-offset) lags could be
            //       re-written as: sequence.Lead( -lagBy, resultSelector ). However, I'm not sure
            //       that it's an intuitive - or even desirable - behavior. So it's being omitted.
            if (offset <= 0) throw new ArgumentOutOfRangeException(nameof(offset));

            return _(); IEnumerable<TResult> _()
            {
                using (var iter = source.GetEnumerator())
                {
                    var i = offset;
                    var lagQueue = new Queue<TSource>(offset);
                    // until we progress far enough, the lagged value is defaultLagValue
                    var hasMore = true;
                    // NOTE: The if statement below takes advantage of short-circuit evaluation
                    //       to ensure we don't advance the iterator when we reach the lag offset.
                    //       Do not reorder the terms in the condition!
                    while (i-- > 0 && (hasMore = iter.MoveNext()))
                    {
                        lagQueue.Enqueue(iter.Current);
                        // until we reach the lag offset, the lagged value is the defaultLagValue
                        yield return resultSelector(iter.Current, defaultLagValue);
                    }

                    if (hasMore) // check that we didn't consume the sequence yet
                    {
                        // now the lagged value is derived from the sequence
                        while (iter.MoveNext())
                        {
                            var lagValue = lagQueue.Dequeue();
                            yield return resultSelector(iter.Current, lagValue);
                            lagQueue.Enqueue(iter.Current);
                        }
                    }
                }
            }
        }
    }
}
