DBCHM/MJTop.Data/BWofter.Converters/EqualityComparers/StringCollectionComparer.cs

/*
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Copyright (c) 2017, B. Wofter
All rights reserved.

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* Redistributions of source code must retain the above copyright notice, this
  list of conditions and the following disclaimer.

* Redistributions in binary form must reproduce the above copyright notice,
  this list of conditions and the following disclaimer in the documentation
  and/or other materials provided with the distribution.

* Neither the name of the copyright holder nor the names of its
  contributors may be used to endorse or promote products derived from
  this software without specific prior written permission.

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 */
namespace BWofter.Converters.EqualityComparers
{
    using System;
    using System.Collections.Generic;
    /// <summary><para>A utility class that is used to compare string collections within the cache object.</para></summary>
    public class StringCollectionComparer : IEqualityComparer<ICollection<string>>
    {
        private static readonly Lazy<StringCollectionComparer> instance = new Lazy<StringCollectionComparer>(true);
        /// <summary><para>Returns the singleton of <see cref="StringCollectionComparer"/>.</para></summary>
        /// <returns>The singleton of <see cref="StringCollectionComparer"/>.</returns>
        public static IEqualityComparer<ICollection<string>> GetInstance() => instance.Value;
        /// <summary><para>Determines whether the specified objects are equal.</para></summary>
        /// <param name="x"><para>The first object of type <see cref="ICollection{T}"/> of <see cref="string"/> values to compare.</para></param>
        /// <param name="x"><para>The second object of type <see cref="ICollection{T}"/> of <see cref="string"/> values to compare.</para></param>
        /// <returns><para><see langword="true"/> if the specified objects are equal; otherwise, <see langword="false"/>.</para></returns>
        public bool Equals(ICollection<string> x, ICollection<string> y)
        {
            //Determines if x and y are the same instance and returns true.
            if (x == y)
            {
                return true;
            }
            //Determines if either x or y is null and returns false.
            else if (x == null || y == null)
            {
                return false;
            }
            //Determines if x is a different length from y and returns false.
            else if (x.Count != y.Count)
            {
                return false;
            }
            //Determines if x is a hash set and uses it as the contains target.
            else if (x is HashSet<string>)
            {
                //Iterate over the values in y and determine if x contains them. If any are not contained in x, return false.
                foreach (string s in y)
                {
                    if (!x.Contains(s))
                    {
                        return false;
                    }
                }
            }
            //The default option.
            else
            {
                //Iterate over the values in x and determine if y contains them. If any are not contained in y, return false.
                foreach (string s in x)
                {
                    if (!y.Contains(s))
                    {
                        return false;
                    }
                }
            }
            return true;
        }
        /// <summary><para>Returns a hash code for the specified object.</para></summary>
        /// <param name="obj"><para>The <see cref="Object"/> for which a hash code is to be returned.</para></param>
        /// <returns><para>A hash code for the specified object.</para></returns>
        public int GetHashCode(ICollection<string> obj)
        {
            //Determine if obj is null and return 0.
            if (obj == null)
            {
                return 0;
            }
            int hashCode = 2;
            //Enter an overflow unchecked areaa, iterate over the strings in the collection, and create a compound hash code using the hash code of each string.
            unchecked
            {
                foreach (string s in obj)
                {
                    hashCode <<= (obj?.GetHashCode() ?? 10) * 230182;
                }
            }
            return hashCode;
        }
    }
}