C#的Hash码实现源码

桃扇骨 2021-07-04 13:31 448阅读 0赞

public int GetHashCode(string s)
        { 
            int hash = 5381;
            int len = s.Length;
    
            for (int i = 0; i < len; i++)
            { 
                int c = Convert.ToInt32(s[i]);
                hash = ((hash << 5) + hash) ^ c;
            }
    
            return hash;
        }

## `System\String.cs` ##

// Gets a hash code for this string. If strings A and B are such that A.Equals(B), then
            // they will return the same hash code.
            [System.Security.SecuritySafeCritical]  // auto-generated
            [ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail)]
            public override int GetHashCode() { 
     
    #if FEATURE_RANDOMIZED_STRING_HASHING
                if(HashHelpers.s_UseRandomizedStringHashing)
                { 
                    return InternalMarvin32HashString(this, this.Length, 0);
                }
    #endif // FEATURE_RANDOMIZED_STRING_HASHING
     
                unsafe { 
                    fixed (char *src = this) { 
                        Contract.Assert(src[this.Length] == '\0', "src[this.Length] == '\\0'");
                        Contract.Assert( ((int)src)%4 == 0, "Managed string should start at 4 bytes boundary");
     
    #if WIN32
                        int hash1 = (5381<<16) + 5381;
    #else
                        int hash1 = 5381;
    #endif
                        int hash2 = hash1;
     
    #if WIN32
                        // 32 bit machines.
                        int* pint = (int *)src;
                        int len = this.Length;
                        while (len > 2)
                        { 
                            hash1 = ((hash1 << 5) + hash1 + (hash1 >> 27)) ^ pint[0];
                            hash2 = ((hash2 << 5) + hash2 + (hash2 >> 27)) ^ pint[1];
                            pint += 2;
                            len  -= 4;
                        }
     
                        if (len > 0)
                        { 
                            hash1 = ((hash1 << 5) + hash1 + (hash1 >> 27)) ^ pint[0];
                        }
    #else
                        int     c;
                        char *s = src;
                        while ((c = s[0]) != 0) { 
                            hash1 = ((hash1 << 5) + hash1) ^ c;
                            c = s[1];
                            if (c == 0)
                                break;
                            hash2 = ((hash2 << 5) + hash2) ^ c;
                            s += 2;
                        }
    #endif
    #if DEBUG
                        // We want to ensure we can change our hash function daily.
                        // This is perfectly fine as long as you don't persist the
                        // value from GetHashCode to disk or count on String A 
                        // hashing before string B. Those are bugs in your code.
                        hash1 ^= ThisAssembly.DailyBuildNumber;
    #endif
                        return hash1 + (hash2 * 1566083941);
                    }
                }
            }