Int32Set.cs

using System;
using System.Collections.Generic;

namespace UrbanScience
{
    public struct Int32Set
    {
        private readonly int[] buckets;

        private readonly Slot[] slots;

        public Int32Set(IReadOnlyList<int> collection)
        {
            ////if (collection == null)
            ////{
            ////    throw new ArgumentNullException("collection");
            ////}

            int count = collection.Count;
            int size = GetPrime(count);

            this.buckets = new int[size];
            this.slots = new Slot[size];

            for (int i = 0; i < count; i++)
            {
                int value = collection[i];
                int bucket = value % this.buckets.Length;
                this.slots[i].value = value;
                this.slots[i].next = this.buckets[bucket] - 1;
                this.buckets[bucket] = i + 1;
            }
        }

        public bool Contains(int item)
        {
            ////if (this.buckets == null)
            ////{
            ////    return false;
            ////}

            for (int i = this.buckets[item % this.buckets.Length] - 1; i >= 0; i = this.slots[i].next)
            {
                if (this.slots[i].value == item)
                {
                    return true;
                }
            }

            return false;
        }

        private struct Slot
        {
            internal int value;
            internal int next;
        }

        // https://github.com/Microsoft/referencesource/blob/9da503f9ef21e8d1f2905c78d4e3e5cbb3d6f85a/mscorlib/system/collections/hashtable.cs#L1691-L1878
        #region HashHelpers

        private static readonly int[] Primes = {
            3, 7, 11, 17, 23, 29, 37, 47, 59, 71, 89, 107, 131, 163, 197, 239, 293, 353, 431, 521, 631, 761, 919,
            1103, 1327, 1597, 1931, 2333, 2801, 3371, 4049, 4861, 5839, 7013, 8419, 10103, 12143, 14591,
            17519, 21023, 25229, 30293, 36353, 43627, 52361, 62851, 75431, 90523, 108631, 130363, 156437,
            187751, 225307, 270371, 324449, 389357, 467237, 560689, 672827, 807403, 968897, 1162687, 1395263,
            1674319, 2009191, 2411033, 2893249, 3471899, 4166287, 4999559, 5999471, 7199369};

        private static int GetPrime(int min)
        {
            for (int i = 0; i < Primes.Length; i++)
            {
                int prime = Primes[i];
                if (prime >= min)
                {
                    return prime;
                }
            }

            for (int i = (min | 1); i < Int32.MaxValue; i += 2)
            {
                // The % 101 in the base version strictly benefits Hashtable.
                ////if (IsPrime(i) && ((i - 1) % 101 != 0))
                if (IsPrime(i))
                {
                    return i;
                }
            }

            return min;
        }

        private static bool IsPrime(int candidate)
        {
            if ((candidate & 1) == 0)
            {
                return candidate == 2;
            }

            int limit = (int)Math.Sqrt(candidate);
            for (int divisor = 3; divisor <= limit; divisor += 2)
            {
                if ((candidate % divisor) == 0)
                {
                    return false;
                }
            }

            return true;
        }

        #endregion
    }
}