数据结构1：Hashtable原理

面试场景1：

面试官：你了解过数据结构吗？

我：平时会用到的数组，List，Hashtable，Dictionary等

面试官：你知道他们的原理吗？

我：心里默默的说一句会用不就行了吗...怎么问题那么多，嘴上却说我不太清楚，但是我知道List，Hashtable，Dictionary线程不安全，一顿瞎吹...

面试官：给你台机器，实现一个线程安全的MyList

我：OMG...

以上面试场景，是我曾经真实遇到过的，非常遗憾和尴尬，我那次面试也基本就到此为止了，结果不出意外的就没有下文了。

下面是我总结的数据结构方面的一点内容，欢迎大家point out mistake

一．Hashtable

Hashtable h = new Hashtable();

h.Add("a", "a");
h.Add("b", "a");
var a=h["b"];
h["b"] = "c";

  h.Contains("a");
  h.Remove("a");
  h.ContainsKey("b");

构造函数:

// Constructs a new hashtable. The hashtable is created with an initial
// capacity of zero and a load factor of 1.0.
public Hashtable() : this(0, 1.0f) {
}
 
 
// Constructs a new hashtable with the given initial capacity and load
// factor. The capacity argument serves as an indication of the
// number of entries the hashtable will contain. When this number (or an
// approximation) is known, specifying it in the constructor can eliminate
// a number of resizing operations that would otherwise be performed when
// elements are added to the hashtable. The loadFactor argument
// indicates the maximum ratio of hashtable entries to hashtable buckets.
// Smaller load factors cause faster average lookup times at the cost of
// increased memory consumption. A load factor of 1.0 generally provides
// the best balance between speed and size.
// 
public Hashtable(int capacity, float loadFactor) {
    if (capacity < 0)
        throw new ArgumentOutOfRangeException("capacity", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
    if (!(loadFactor >= 0.1f && loadFactor <= 1.0f))
        throw new ArgumentOutOfRangeException("loadFactor", Environment.GetResourceString("ArgumentOutOfRange_HashtableLoadFactor", .1, 1.0));
    Contract.EndContractBlock();

    // Based on perf work, .72 is the optimal load factor for this table.  
    this.loadFactor = 0.72f * loadFactor;

    double rawsize = capacity / this.loadFactor;
    if (rawsize > Int32.MaxValue)
        throw new ArgumentException(Environment.GetResourceString("Arg_HTCapacityOverflow"));

    // Avoid awfully small sizes
    int hashsize = (rawsize > InitialSize) ? HashHelpers.GetPrime((int)rawsize) : InitialSize;
    buckets = new bucket[hashsize];  //buckets是放数据的桶

    loadsize = (int)(this.loadFactor * hashsize);
    isWriterInProgress = false;
    // Based on the current algorithm, loadsize must be less than hashsize.
    Contract.Assert( loadsize < hashsize, "Invalid hashtable loadsize!");
}

// The hash table data.
// This cannot be serialised
private struct bucket {  //结构体，存储hashtable中的kv,hash_coll存k的hashcode
    public Object key;
    public Object val;
    public int hash_coll;   // Store hash code; sign bit means there was a collision.
}

private bucket[] buckets;  

// Adds an entry with the given key and value to this hashtable. An
// ArgumentException is thrown if the key is null or if the key is already
// present in the hashtable.
// 
public virtual void Add(Object key, Object value) {  //字典的添加方法
    Insert(key, value, true);
}

// Inserts an entry into this hashtable. This method is called from the Set
// and Add methods. If the add parameter is true and the given key already
// exists in the hashtable, an exception is thrown.
[ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail)]
private void Insert (Object key, Object nvalue, bool add) {

    if (key == null) {
        throw new ArgumentNullException("key", Environment.GetResourceString("ArgumentNull_Key"));
    }
    Contract.EndContractBlock();
    if (count >= loadsize) {  //扩容
        expand();
    }
    else if(occupancy > loadsize && count > 100) {
        rehash();
    }
    
    uint seed;  //seed为k的hashcode
    uint incr;  //不同k的hashcode不同，但是取余buckets.Lenght后，索引会冲突，冲突后需要将k所在索引+incr
    // Assume we only have one thread writing concurrently.  Modify
    // buckets to contain new data, as long as we insert in the right order.
    uint hashcode = InitHash(key, buckets.Length, out seed, out incr);
    int  ntry = 0;
    int emptySlotNumber = -1; // We use the empty slot number to cache the first empty slot. We chose to reuse slots
    // create by remove that have the collision bit set over using up new slots.
    int bucketNumber = (int) (seed % (uint)buckets.Length);  //数据存放在buckets数组中的索引
    do {

        // Set emptySlot number to current bucket if it is the first available bucket that we have seen
        // that once contained an entry and also has had a collision.
        // We need to search this entire collision chain because we have to ensure that there are no 
        // duplicate entries in the table.
        if (emptySlotNumber == -1 && (buckets[bucketNumber].key == buckets) && (buckets[bucketNumber].hash_coll < 0))//(((buckets[bucketNumber].hash_coll & unchecked(0x80000000))!=0)))
            emptySlotNumber = bucketNumber;

        // Insert the key/value pair into this bucket if this bucket is empty and has never contained an entry
        // OR
        // This bucket once contained an entry but there has never been a collision
        if ((buckets[bucketNumber].key == null) ||  如果key==null说明此索引位置为空，可以存放当前值，否则说明当前值不为空，要么是更新操作，要么是需要将索引+incr继续遍历
            (buckets[bucketNumber].key == buckets && ((buckets[bucketNumber].hash_coll & unchecked(0x80000000))==0))) {

            // If we have found an available bucket that has never had a collision, but we've seen an available
            // bucket in the past that has the collision bit set, use the previous bucket instead
            if (emptySlotNumber != -1) // Reuse slot
                bucketNumber = emptySlotNumber;

            isWriterInProgress = true;                    
            buckets[bucketNumber].val = nvalue;   //将值存放到该索引位置
            buckets[bucketNumber].key  = key;
            buckets[bucketNumber].hash_coll |= (int) hashcode;  //k的hashcode值，大于0的无符号整形
            count++;
            UpdateVersion();
            isWriterInProgress = false;   

            return;
        }

        // The current bucket is in use
        // OR
        // it is available, but has had the collision bit set and we have already found an available bucket
        if (((buckets[bucketNumber].hash_coll & 0x7FFFFFFF) == hashcode) &&   //hashcode冲突时候hash_coll值为负的，需要转成正值，当前bucket中存放的k为参数中的key
            KeyEquals (buckets[bucketNumber].key, key)) {
            if (add) {
                throw new ArgumentException(Environment.GetResourceString("Argument_AddingDuplicate__", buckets[bucketNumber].key, key)); //如果是添加操作，key已存在，否则更新key对应的值
            }
      
            isWriterInProgress = true;                    
            buckets[bucketNumber].val = nvalue;
            UpdateVersion();                    
            isWriterInProgress = false; 
            
            return;
        }

        // The current bucket is full, and we have therefore collided.  We need to set the collision bit
        // UNLESS
        // we have remembered an available slot previously.
        if (emptySlotNumber == -1) {// We don't need to set the collision bit here since we already have an empty slot
            if( buckets[bucketNumber].hash_coll >= 0 ) {
                buckets[bucketNumber].hash_coll |= unchecked((int)0x80000000);
                occupancy++;
            }
        }

        bucketNumber = (int) (((long)bucketNumber + incr)% (uint)buckets.Length);  //当前索引位置存放了别的key的值，key对应的bucket的索引位置+incr后取余bucket.Length为当前key的索引              
    } while (++ntry < buckets.Length);

    // This code is here if and only if there were no buckets without a collision bit set in the entire table
    if (emptySlotNumber != -1)
    {
        // We pretty much have to insert in this order.  Don't set hash
        // code until the value & key are set appropriately.
       
        isWriterInProgress = true;                    
        buckets[emptySlotNumber].val = nvalue;
        buckets[emptySlotNumber].key  = key;
        buckets[emptySlotNumber].hash_coll |= (int) hashcode;
        count++;
        UpdateVersion();                
        isWriterInProgress = false;     

        return;
    }

    // If you see this assert, make sure load factor & count are reasonable.
    // Then verify that our double hash function (h2, described at top of file)
    // meets the requirements described above. You should never see this assert.
    Contract.Assert(false, "hash table insert failed!  Load factor too high, or our double hashing function is incorrect.");
    throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_HashInsertFailed"));
}

// ‘InitHash’ is basically an implementation of classic DoubleHashing (see http://en.wikipedia.org/wiki/Double_hashing)  
//
// 1) The only ‘correctness’ requirement is that the ‘increment’ used to probe 
//    a. Be non-zero
//    b. Be relatively prime to the table size ‘hashSize’. (This is needed to insure you probe all entries in the table before you ‘wrap’ and visit entries already probed)
// 2) Because we choose table sizes to be primes, we just need to insure that the increment is 0 < incr < hashSize
//
// Thus this function would work: Incr = 1 + (seed % (hashSize-1))
// 
// While this works well for ‘uniformly distributed’ keys, in practice, non-uniformity is common. 
// In particular in practice we can see ‘mostly sequential’ where you get long clusters of keys that ‘pack’. 
// To avoid bad behavior you want it to be the case that the increment is ‘large’ even for ‘small’ values (because small 
// values tend to happen more in practice). Thus we multiply ‘seed’ by a number that will make these small values
// bigger (and not hurt large values). We picked HashPrime (101) because it was prime, and if ‘hashSize-1’ is not a multiple of HashPrime
// (enforced in GetPrime), then incr has the potential of being every value from 1 to hashSize-1. The choice was largely arbitrary.
// 
// Computes the hash function:  H(key, i) = h1(key) + i*h2(key, hashSize).
// The out parameter seed is h1(key), while the out parameter 
// incr is h2(key, hashSize).  Callers of this function should 
// add incr each time through a loop.
private uint InitHash(Object key, int hashsize, out uint seed, out uint incr) {
    // Hashcode must be positive.  Also, we must not use the sign bit, since
    // that is used for the collision bit.
    uint hashcode = (uint) GetHash(key) & 0x7FFFFFFF;
    seed = (uint) hashcode; //当前key的hashcode
    // Restriction: incr MUST be between 1 and hashsize - 1, inclusive for
    // the modular arithmetic to work correctly.  This guarantees you'll
    // visit every bucket in the table exactly once within hashsize 
    // iterations.  Violate this and it'll cause obscure bugs forever.
    // If you change this calculation for h2(key), update putEntry too!
    incr = (uint)(1 + ((seed * HashPrime) % ((uint)hashsize - 1))); //hashcode扩大固定素数倍数后取余 buckets.Length
    return hashcode; //当前key的hashcode
}

// Increases the bucket count of this hashtable. This method is called from
// the Insert method when the actual load factor of the hashtable reaches
// the upper limit specified when the hashtable was constructed. The number
// of buckets in the hashtable is increased to the smallest prime number
// that is larger than twice the current number of buckets, and the entries
// in the hashtable are redistributed into the new buckets using the cached
// hashcodes.
private void expand()  {
    int rawsize = HashHelpers.ExpandPrime(buckets.Length);//buckets数据扩容时，计算新数组Length，如何扩容详见 HashHelpers.ExpandPrime方法，下面粘贴了部分源码
    rehash(rawsize, false);
}

private void rehash( int newsize, bool forceNewHashCode ) 
{
 
    // reset occupancy
    occupancy=0;

    // Don't replace any internal state until we've finished adding to the 
    // new bucket[].  This serves two purposes: 
    //   1) Allow concurrent readers to see valid hashtable contents 
    //      at all times
    //   2) Protect against an OutOfMemoryException while allocating this 
    //      new bucket[].
    bucket[] newBuckets = new bucket[newsize]; //扩容后的新的buckets

    // rehash table into new buckets
    int nb;
    for (nb = 0; nb < buckets.Length; nb++){
        bucket oldb = buckets[nb];
        if ((oldb.key != null) && (oldb.key != buckets)) {
            int hashcode = ((forceNewHashCode ? GetHash(oldb.key) : oldb.hash_coll) & 0x7FFFFFFF);                              
            putEntry(newBuckets, oldb.key, oldb.val, hashcode);//旧buckets中的数据存放到新buckets中
        }
    }

    // New bucket[] is good to go - replace buckets and other internal state.
    Thread.BeginCriticalRegion();     
    isWriterInProgress = true;
    buckets = newBuckets;
    loadsize = (int)(loadFactor * newsize);
    UpdateVersion();
    isWriterInProgress = false;
          
    Thread.EndCriticalRegion();           
    // minimun size of hashtable is 3 now and maximum loadFactor is 0.72 now.
    Contract.Assert(loadsize < newsize, "Our current implementaion means this is not possible.");
    return;
    }
    
private void putEntry (bucket[] newBuckets, Object key, Object nvalue, int hashcode)
{
    Contract.Assert(hashcode >= 0, "hashcode >= 0");  // make sure collision bit (sign bit) wasn't set.

    uint seed = (uint) hashcode;
    uint incr = (uint)(1 + ((seed * HashPrime) % ((uint)newBuckets.Length - 1)));
    int bucketNumber = (int) (seed % (uint)newBuckets.Length);     //不需要重新计算hashcode,但是需要重新获取索引值       
    do {

        if ((newBuckets[bucketNumber].key == null) || (newBuckets[bucketNumber].key == buckets)) { //当前索引未使用，将key存入当前索引
            newBuckets[bucketNumber].val = nvalue;
            newBuckets[bucketNumber].key = key;
            newBuckets[bucketNumber].hash_coll |= hashcode;
            return;
        }
        
        if( newBuckets[bucketNumber].hash_coll >= 0 ) {
        newBuckets[bucketNumber].hash_coll |= unchecked((int)0x80000000);  //当前索引冲突，标识为负的hashcode
            occupancy++; //冲突占用+1
        }
        bucketNumber = (int) (((long)bucketNumber + incr)% (uint)newBuckets.Length);  //索引+incr后取余，继续寻找当前key的索引              
    } while (true);
}

HashHelpers中的ExpandPrime方法：

HashHelpers中的ExpandPrime方法：
// Returns size of hashtable to grow to.
public static int ExpandPrime(int oldSize)
{
    int newSize = 2 * oldSize;  //扩大为原来两倍

    // 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)  //新容量大于数组最大长度，默认最大数组长度
    {
        Contract.Assert( MaxPrimeArrayLength == GetPrime(MaxPrimeArrayLength), "Invalid MaxPrimeArrayLength");
        return MaxPrimeArrayLength;
    }

    return GetPrime(newSize);
}

public static int GetPrime(int min) 
{
    if (min < 0)
        throw new ArgumentException(Environment.GetResourceString("Arg_HTCapacityOverflow"));
    Contract.EndContractBlock();

    for (int i = 0; i < primes.Length; i++)   //从素数数组中寻找一个大于当前值的最小素数
    {
        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) //素数数组中没有找到，继续+2的找，感觉这儿有点小问题，这么找是有可能比MaxPrimeArrayLength大的，不知道微软大牛们是如何想的，可能是我们用的时候不会放那么多数据吧,-_-||
    {
        if (IsPrime(i) && ((i - 1) % Hashtable.HashPrime != 0))
            return i;
    }
    return min;
}

为啥可以这么写：

var a=h["b"];
h["b"] = "c";

看源码：

// Returns the value associated with the given key. If an entry with the
// given key is not found, the returned value is null.
// 
public virtual Object this[Object key] {
    get {
        if (key == null) {
            throw new ArgumentNullException("key", Environment.GetResourceString("ArgumentNull_Key"));
        }
        Contract.EndContractBlock();

        uint seed; //key的hashcode
        uint incr; //如果key的hashcode取余buckets.Length 即 key所在的bucket索引冲突，新索引需要挪的长度

        
        // Take a snapshot of buckets, in case another thread does a resize
        bucket[] lbuckets = buckets;
        uint hashcode = InitHash(key, lbuckets.Length, out seed, out incr);  //上面已经分析过了，计算key的hashcode
        int  ntry = 0;

        bucket b;
        int bucketNumber = (int) (seed % (uint)lbuckets.Length);                
        do
        {
            int currentversion;

            //     A read operation on hashtable has three steps:
            //        (1) calculate the hash and find the slot number.
            //        (2) compare the hashcode, if equal, go to step 3. Otherwise end.
            //        (3) compare the key, if equal, go to step 4. Otherwise end.
            //        (4) return the value contained in the bucket.
            //     After step 3 and before step 4. A writer can kick in a remove the old item and add a new one 
            //     in the same bukcet. So in the reader we need to check if the hash table is modified during above steps.
            //
            // Writers (Insert, Remove, Clear) will set 'isWriterInProgress' flag before it starts modifying 
            // the hashtable and will ckear the flag when it is done.  When the flag is cleared, the 'version'
            // will be increased.  We will repeat the reading if a writer is in progress or done with the modification 
            // during the read.
            //
            // Our memory model guarantee if we pick up the change in bucket from another processor, 
            // we will see the 'isWriterProgress' flag to be true or 'version' is changed in the reader.
            //                    
            int spinCount = 0;
            do {
                // this is violate read, following memory accesses can not be moved ahead of it.
                currentversion = version;
                b = lbuckets[bucketNumber];                        

                // The contention between reader and writer shouldn't happen frequently.
                // But just in case this will burn CPU, yield the control of CPU if we spinned a few times.
                // 8 is just a random number I pick. 
                if( (++spinCount) % 8 == 0 ) {   
                    Thread.Sleep(1);   // 1 means we are yeilding control to all threads, including low-priority ones.
                }
            } while ( isWriterInProgress || (currentversion != version) ); //hashtable在更新扩容写的时候，或者版本不同时，读的时候要个锁哦，

            if (b.key == null) { //如果key==null,说明没有当前key，返回null
                return null;
            }
            if (((b.hash_coll & 0x7FFFFFFF) == hashcode) && //如果索引冲突，hash_coll值为hashcode负值，需要转成正的比较，相等就返回值
                KeyEquals (b.key, key))
                return b.val;  
            bucketNumber = (int) (((long)bucketNumber + incr)% (uint)lbuckets.Length);     //如果索引冲突且循环次数小于buckets.Lenght，继续循环寻找当前key所在的索引值                             
        } while (b.hash_coll < 0 && ++ntry < lbuckets.Length);
        return null;
    }

    set {
        Insert(key, value, false);
    }
}

好了，到此为止吧，hashcode应该大致了解了吧，基本上就是用一个结构体的数组，key二次hash来实现的，

个人想法：buckets数组扩容时，需要重新计算所有key的索引，旧数组的数据需要挪到新数组，旧数组就是gc需要管理的内存垃圾，这个数组较大时，会存储到大对象堆吗？这个不太了解，暂时留个问题吧，待以后再了解后再做记录吧

以上是framework4.8的部分源码，跟.net core 6.1的有微小的区别

参考：

https://referencesource.microsoft.com/#mscorlib/system/collections/hashtable.cs,10fefb6e0ae510dd