面试准备

Hashtable是很经常在面试中遇到的数据结构，因为他的O(1)操作时间和O(n)空间

之所以自己写一份是因为：

• 加深对于hashtable的理解
• 某些公司面试的时候需要coding.......

开放地址法  Xn=(Xn-1 +b ) % size

理论上b要和size是要精心选择的，不过我这边没有做特别的处理，101的默认size是从c#源代码中抄袭的。。。。

代码尽量简单一点是为了理解方便

hashtable快满的时候扩展一倍空间，数据和标志位还有key 这三个数组都要扩展

删除的时候不能直接删除元素，只能打一个标志（因为用了开放地方方法）

目前只支持string和int类型的key（按位131进制）

非线程安全- 因为这是范例代码

支持泛型

  
    public class Hashtable<T>
        
    {
        public Hashtable()
        {
            this.dataArray = new T[this.m];
            this.avaiableCapacity = this.m;
            this.keyArray = new int[this.m];
            for (int i = 0; i < this.keyArray.Length; i++)
            {
                this.keyArray[i] = -1;
            }
            this.flagArray = new bool[this.m];
        }

        private int m = 101;

        private int l = 1;

        private int avaiableCapacity;

        private double factor = 0.35;

        private T[] dataArray;

        private int[] keyArray;

        private bool[] flagArray;

        public void Add(string s, T item)
        {
            if (string.IsNullOrEmpty(s))
            {
                throw new ArgumentNullException("s");
            }

            if ((double)this.avaiableCapacity / this.m < this.factor)
            {
                this.ExtendCapacity();
            }

            var code = HashtableHelper.GetStringHash(s);
            this.AddItem(code, item, this.dataArray, code, this.keyArray, this.flagArray);
        }

        public T Get(string s)
        {
            if (string.IsNullOrEmpty(s))
            {
                throw new ArgumentNullException("s");
            }

            var code = HashtableHelper.GetStringHash(s);
            return this.GetItem(code, this.dataArray, code, this.keyArray, this.flagArray);
        }

        private void ExtendCapacity()
        {
            this.m *= 2;
            this.avaiableCapacity += this.m;
            T[] newItems = new T[this.m];
            int[] newKeys = new int[this.m];
            bool[] newFlags = new bool[this.m];

            for (int i = 0; i < newKeys.Length; i++)
            {
                newKeys[i] = -1;
            }

            for (int i = 0; i < this.dataArray.Length; i++)
            {
                if (this.keyArray[i] >= 0 && !this.flagArray[i])
                {
                    //var code = HashtableHelper.GetStringHash(s);
                    this.AddItem(
                        this.keyArray[i],
                        this.dataArray[i],
                        newItems,
                        this.keyArray[i],
                        newKeys,
                        this.flagArray);
                }
            }
            this.dataArray = newItems;
            this.keyArray = newKeys;
            this.flagArray = newFlags;
            // throw new NotImplementedException();
        }

        private int AddItem(int code, T item, T[] data, int hashCode, int[] keys, bool[] flags)
        {
            int address = code % this.m;
            if (keys[address] < 0)
            {
                data[address] = item;
                keys[address] = hashCode;
                this.avaiableCapacity--;
                return address;
            }
            else if (keys[address] == hashCode)
            {
                if (flags[address])
                {
                    flags[address] = false;
                    data[address] = item;
                    return address;
                }
                throw new ArgumentException("duplicated key");
            }
            else
            {
                int nextAddress = address + this.l; //open addressing Xn=Xn-1 + b
                return this.AddItem(nextAddress, item, data, hashCode, keys, flags);
            }
        }

        private T GetItem(int code, T[] data, int hashCode, int[] keys, bool[] flags)
        {
            int address = code % this.m;
            if (keys[address] < 0)
            {
                return default(T);
            }
            else if (keys[address] == hashCode)
            {
                if (flags[address])
                {
                    return default(T);
                }
                return data[address];
            }
            else
            {
                int nextAddress = address + this.l; //open addressing Xn=Xn-1 + b
                return this.GetItem(nextAddress, data, hashCode, keys, flags);
            }
        }

        public void Delete(string s)
        {
            if (string.IsNullOrEmpty(s))
            {
                throw new ArgumentNullException("s");
            }

            var code = HashtableHelper.GetStringHash(s);
            this.DeleteItem(code, this.dataArray, code, this.keyArray, this.flagArray);
        }

        private void DeleteItem(int code, T[] data, int hashCode, int[] keys, bool[] flags)
        {
            int address = code % this.m;
            if (keys[address] < 0)
            {
                return;
                //not exist
            }
            else if (keys[address] == hashCode)
            {
                if (!this.flagArray[address])
                {
                    flags[address] = true;
                    this.avaiableCapacity++;
                }
            }
            else
            {
                int nextAddress = address + this.l; //open addressing Xn=Xn-1 + b
                this.DeleteItem(nextAddress, data, hashCode, keys, flags);
            }
        }
    }


    public class HashtableHelper
    {
        public static int GetStringHash(string s)
        {
            if (string.IsNullOrEmpty(s))
            {
                throw new ArgumentNullException("s");
            }

            var bytes = Encoding.ASCII.GetBytes(s);
            int checksum = GetBytesHash(bytes, 0, bytes.Length);
            return checksum;
        }

        public static int GetBytesHash(byte[] array, int ibStart, int cbSize)
        {
            if (array == null || array.Length == 0)
            {
                throw new ArgumentNullException("array");
            }

            int checksum = 0;
            for (int i = ibStart; i < (ibStart + cbSize); i++)
            {
                checksum = (checksum * 131) + array[i];
            }
            return checksum;
        }

        public static int GetBytesHash(char[] array, int ibStart, int cbSize)
        {
            if (array == null || array.Length == 0)
            {
                throw new ArgumentNullException("array");
            }

            int checksum = 0;
            for (int i = ibStart; i < (ibStart + cbSize); i++)
            {
                checksum = (checksum * 131) + array[i];
            }
            return checksum;
        }
    }