[读书笔记]数据字典

这是一个关于数据字典使用的例子

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace DictionaryExample
{
    public struct EmployeeId : IEquatable<EmployeeId>
    {
        private readonly char prefix;
        private readonly int number;

        public EmployeeId(string id)
        {
            if (id == null)
                throw new ArgumentNullException("id");
            
            prefix = (id.ToUpper())[0];
            int numLength = id.Length - 1;
            number = int.Parse(id.Substring(1, numLength > 6 ? 6 : numLength));
        }

        public override string ToString()
        {
            return prefix.ToString() + string.Format("{0,6:000000}", number);
        }

        public override int GetHashCode()
        {
            return (number ^ number << 16) * 0x15051505;
        }

        public bool Equals(EmployeeId other)
        {
           return (prefix == other.prefix && number == other.number);
        }
    }

    public class Employee
    {
        private string name;
        private decimal salary;
        private readonly EmployeeId id;

        public Employee(EmployeeId id, string name, decimal salary)
        {
            this.id = id;
            this.name = name;
            this.salary = salary;
        }

        public override string ToString()
        {
            return String.Format("{0}: {1, -20} {2:C}", id.ToString(), name, salary);
        }

    }

    class Program
    {
        static void Main(string[] args)
        {
            Dictionary<EmployeeId, Employee> employees = new Dictionary<EmployeeId, Employee>(31);

            EmployeeId idJeff = new EmployeeId("C7102");
            Employee jeff = new Employee(idJeff, "Jeff Gordon", 5164580.00m);
            employees.Add(idJeff, jeff);
            Console.WriteLine(jeff);

            EmployeeId idTony = new EmployeeId("C7105");
            Employee Tony = new Employee(idTony, "Tony Stewart", 4814200.00m);
            employees.Add(idTony, Tony);
            Console.WriteLine(Tony);

            EmployeeId idDenny = new EmployeeId("C8011");
            Employee Denny = new Employee(idDenny, "Denny Hamlin", 3718710.00m);
            employees.Add(idDenny, Denny);
            Console.WriteLine(Denny);

            EmployeeId idCarl = new EmployeeId("F7908");
            Employee Carl = new Employee(idCarl, "Carl Edwards", 3285710.00m);
            employees[idCarl] = Carl;
            Console.WriteLine(Carl);

            EmployeeId idMatt = new EmployeeId("F7203");
            Employee Matt = new Employee(idMatt, "Matt Kensenth", 4520330.00m);
            employees.Add(idMatt, Matt);
            Console.WriteLine(Matt);

            while (true)
            {
                Console.Write("Enter employee id (X to exit)>");

                string userInput = Console.ReadLine();
                userInput = userInput.ToUpper();

                if (userInput == "X") break;

                EmployeeId id = new EmployeeId(userInput);
                Employee employee;
                if (!employees.TryGetValue(id, out employee))
                {
                    Console.WriteLine("Employee with id " + "{0} does not exist", id);
                }
                else
                {
                    Console.WriteLine(employee);
                }
            }

        }
    }
}

主要关注这里面的对于GetHashCode的重载的算法。这里GetHashCode使用的算法将数字向左移动16位，再与原来的数字进行异或操作，最后将结果乘以十六进制数15051505。散列码在整数取值区域上的分布相当均匀。