MergeSorted.cs

using System;
using System.Collections.Generic;

namespace PASaveEditor {
    // MergedSorted implementation by Brian Rasmussen @ http://stackoverflow.com/a/9809450
    internal static class MergeSortedExtension {
        public static IEnumerable<T> MergeSorted<T>(this IEnumerable<T> first, IEnumerable<T> second,
                                                    Func<T, T, int> comparer) {
            using (var firstEnumerator = first.GetEnumerator()) {
                using (var secondEnumerator = second.GetEnumerator()) {
                    var elementsLeftInFirst = firstEnumerator.MoveNext();
                    var elementsLeftInSecond = secondEnumerator.MoveNext();
                    while (elementsLeftInFirst || elementsLeftInSecond) {
                        if (!elementsLeftInFirst) {
                            do {
                                yield return secondEnumerator.Current;
                            } while (secondEnumerator.MoveNext());
                            yield break;
                        }

                        if (!elementsLeftInSecond) {
                            do {
                                yield return firstEnumerator.Current;
                            } while (firstEnumerator.MoveNext());
                            yield break;
                        }

                        if (comparer(firstEnumerator.Current, secondEnumerator.Current) < 0) {
                            yield return firstEnumerator.Current;
                            elementsLeftInFirst = firstEnumerator.MoveNext();
                        } else {
                            yield return secondEnumerator.Current;
                            elementsLeftInSecond = secondEnumerator.MoveNext();
                        }
                    }
                }
            }
        }
    }
}