using System;
using System.Diagnostics.Contracts;
using System.Runtime.CompilerServices;
using System.Runtime.ConstrainedExecution;
using System.Runtime.Serialization;
using System.Threading;

namespace Cryville.Common.Collections {
	internal static class HashHelpers {
#if FEATURE_RANDOMIZED_STRING_HASHING
        public const int HashCollisionThreshold = 100;
        public static bool s_UseRandomizedStringHashing = String.UseRandomizedHashing();
#endif

		// Table of prime numbers to use as hash table sizes. 
		// A typical resize algorithm would pick the smallest prime number in this array
		// that is larger than twice the previous capacity. 
		// Suppose our Hashtable currently has capacity x and enough elements are added 
		// such that a resize needs to occur. Resizing first computes 2x then finds the 
		// first prime in the table greater than 2x, i.e. if primes are ordered 
		// p_1, p_2, ..., p_i, ..., it finds p_n such that p_n-1 < 2x < p_n. 
		// Doubling is important for preserving the asymptotic complexity of the 
		// hashtable operations such as add.  Having a prime guarantees that double 
		// hashing does not lead to infinite loops.  IE, your hash function will be 
		// h1(key) + i*h2(key), 0 <= i < size.  h2 and the size must be relatively prime.
		public 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};

		// Used by Hashtable and Dictionary's SeralizationInfo .ctor's to store the SeralizationInfo
		// object until OnDeserialization is called.
		private static ConditionalWeakTable<object, SerializationInfo> s_SerializationInfoTable;

		internal static ConditionalWeakTable<object, SerializationInfo> SerializationInfoTable {
			get {
				if (s_SerializationInfoTable == null) {
					ConditionalWeakTable<object, SerializationInfo> newTable = new ConditionalWeakTable<object, SerializationInfo>();
					Interlocked.CompareExchange(ref s_SerializationInfoTable, newTable, null);
				}

				return s_SerializationInfoTable;
			}

		}

		[ReliabilityContract(Consistency.WillNotCorruptState, Cer.Success)]
		public static bool IsPrime(int candidate) {
			if ((candidate & 1) != 0) {
				int limit = (int)System.Math.Sqrt (candidate);
				for (int divisor = 3; divisor <= limit; divisor += 2) {
					if ((candidate % divisor) == 0)
						return false;
				}
				return true;
			}
			return (candidate == 2);
		}

		internal const Int32 HashPrime = 101;

		[ReliabilityContract(Consistency.WillNotCorruptState, Cer.Success)]
		public static int GetPrime(int min) {
			if (min < 0)
				throw new ArgumentException("Hashtable's capacity overflowed and went negative. Check load factor, capacity and the current size of the table.");
			Contract.EndContractBlock();

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

			//outside of our predefined table. 
			//compute the hard way. 
			for (int i = (min | 1); i < Int32.MaxValue; i += 2) {
				if (IsPrime(i) && ((i - 1) % HashPrime != 0))
					return i;
			}
			return min;
		}

		public static int GetMinPrime() {
			return primes[0];
		}

		// Returns size of hashtable to grow to.
		public static int ExpandPrime(int oldSize) {
			int newSize = 2 * oldSize;

			// Allow the hashtables to grow to maximum possible size (~2G elements) before encoutering capacity overflow.
			// Note that this check works even when _items.Length overflowed thanks to the (uint) cast
			if ((uint)newSize > MaxPrimeArrayLength && MaxPrimeArrayLength > oldSize) {
				Contract.Assert(MaxPrimeArrayLength == GetPrime(MaxPrimeArrayLength), "Invalid MaxPrimeArrayLength");
				return MaxPrimeArrayLength;
			}

			return GetPrime(newSize);
		}


		// This is the maximum prime smaller than Array.MaxArrayLength
		public const int MaxPrimeArrayLength = 0x7FEFFFFD;
	}
}