1  using System;
  2  using System.Numerics;
  3  using System.Runtime.CompilerServices;
  4  
  5  namespace Spv.Generator
  6  {
  7      /// <summary>
  8      /// Similar to System.HashCode, but without introducing random values.
  9      /// The same primes and shifts are used.
 10      /// </summary>
 11      internal static class DeterministicHashCode
 12      {
 13          private const uint Prime1 = 2654435761U;
 14          private const uint Prime2 = 2246822519U;
 15          private const uint Prime3 = 3266489917U;
 16          private const uint Prime4 = 668265263U;
 17  
 18          public static int GetHashCode(string value)
 19          {
 20              uint hash = (uint)value.Length + Prime1;
 21  
 22              for (int i = 0; i < value.Length; i++)
 23              {
 24                  hash += (hash << 7) ^ value[i];
 25              }
 26  
 27              return (int)MixFinal(hash);
 28          }
 29  
 30          public static int Combine<T>(ReadOnlySpan<T> values)
 31          {
 32              uint hashCode = Prime2;
 33              hashCode += 4 * (uint)values.Length;
 34  
 35              foreach (T value in values)
 36              {
 37                  uint hc = (uint)(value?.GetHashCode() ?? 0);
 38                  hashCode = MixStep(hashCode, hc);
 39              }
 40  
 41              return (int)MixFinal(hashCode);
 42          }
 43  
 44          public static int Combine<T1, T2>(T1 value1, T2 value2)
 45          {
 46              uint hc1 = (uint)(value1?.GetHashCode() ?? 0);
 47              uint hc2 = (uint)(value2?.GetHashCode() ?? 0);
 48  
 49              uint hash = Prime2;
 50              hash += 8;
 51  
 52              hash = MixStep(hash, hc1);
 53              hash = MixStep(hash, hc2);
 54  
 55              return (int)MixFinal(hash);
 56          }
 57  
 58          public static int Combine<T1, T2, T3>(T1 value1, T2 value2, T3 value3)
 59          {
 60              uint hc1 = (uint)(value1?.GetHashCode() ?? 0);
 61              uint hc2 = (uint)(value2?.GetHashCode() ?? 0);
 62              uint hc3 = (uint)(value3?.GetHashCode() ?? 0);
 63  
 64              uint hash = Prime2;
 65              hash += 12;
 66  
 67              hash = MixStep(hash, hc1);
 68              hash = MixStep(hash, hc2);
 69              hash = MixStep(hash, hc3);
 70  
 71              return (int)MixFinal(hash);
 72          }
 73  
 74          public static int Combine<T1, T2, T3, T4>(T1 value1, T2 value2, T3 value3, T4 value4)
 75          {
 76              uint hc1 = (uint)(value1?.GetHashCode() ?? 0);
 77              uint hc2 = (uint)(value2?.GetHashCode() ?? 0);
 78              uint hc3 = (uint)(value3?.GetHashCode() ?? 0);
 79              uint hc4 = (uint)(value4?.GetHashCode() ?? 0);
 80  
 81              uint hash = Prime2;
 82              hash += 16;
 83  
 84              hash = MixStep(hash, hc1);
 85              hash = MixStep(hash, hc2);
 86              hash = MixStep(hash, hc3);
 87              hash = MixStep(hash, hc4);
 88  
 89              return (int)MixFinal(hash);
 90          }
 91  
 92          [MethodImpl(MethodImplOptions.AggressiveInlining)]
 93          private static uint MixStep(uint hashCode, uint mixValue)
 94          {
 95              return BitOperations.RotateLeft(hashCode + mixValue * Prime3, 17) * Prime4;
 96          }
 97  
 98          [MethodImpl(MethodImplOptions.AggressiveInlining)]
 99          private static uint MixFinal(uint hash)
100          {
101              hash ^= hash >> 15;
102              hash *= Prime2;
103              hash ^= hash >> 13;
104              hash *= Prime3;
105              hash ^= hash >> 16;
106              return hash;
107          }
108      }
109  }