using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
namespace Base64EncoderDemo
{
    public partial class FormBase64Encoder : Form
    {
        public FormBase64Encoder()
        {
            InitializeComponent();
            //參考Convert微軟源程序
            //https://referencesource.microsoft.com/#mscorlib/system/convert.cs,fc990bd1275d43d6
        }
        private void FormBase64Encoder_Load(object sender, EventArgs e)
        {
            rtxtMessage.ReadOnly = true;
            //編碼格式
            cboEncoding.Items.AddRange(new string[] { "ASCII", "Unicode", "UTF-8", "GBK" });
            cboEncoding.SelectedIndex = 0;
        }
        private void btnClear_Click(object sender, EventArgs e)
        {
            rtxtSourceString.Clear();
            rtxtBase64String.Clear();
            rtxtMessage.Clear();
        }
        /// <summary>
        /// 顯示提示消息
        /// </summary>
        /// <param name="content"></param>
        private void DisplayMessage(string content)
        {
            if (rtxtMessage.TextLength >= 20480)
            {
                rtxtMessage.Clear();
            }
            rtxtMessage.AppendText($"{DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss.fff")} -> {content}\n");
            rtxtMessage.ScrollToCaret();
        }
        private void btnConvertBase64_Click(object sender, EventArgs e)
        {
            rtxtBase64String.Clear();
            if (rtxtSourceString.Text.Trim().Length == 0)
            {
                rtxtSourceString.Focus();
                DisplayMessage("源字符串不能為空");
                return;
            }
            try
            {
                Encoding encoding = Encoding.GetEncoding(cboEncoding.Text);
                byte[] buffer = encoding.GetBytes(rtxtSourceString.Text.Trim());
                rtxtBase64String.Text = Convert.ToBase64String(buffer, Base64FormattingOptions.None);
                DisplayMessage($"轉(zhuǎn)換成功，Base64字符串【{rtxtBase64String.Text}】");
            }
            catch (Exception ex)
            {
                DisplayMessage($"轉(zhuǎn)換為Base64時(shí)出錯(cuò)：【{ex.Message}】");
            }
        }
        private void btnRestore_Click(object sender, EventArgs e)
        {
            rtxtSourceString.Clear();
            if (rtxtBase64String.Text.Trim().Length == 0)
            {
                rtxtBase64String.Focus();
                DisplayMessage("Base64字符串不能為空");
                return;
            }
            try
            {
                Encoding encoding = Encoding.GetEncoding(cboEncoding.Text);
                byte[] buffer = Convert.FromBase64String(rtxtBase64String.Text);
                rtxtSourceString.Text = encoding.GetString(buffer);
                DisplayMessage($"還原成功，源字符串【{rtxtSourceString.Text}】");
            }
            catch (Exception ex)
            {
                DisplayMessage($"還原字符串時(shí)出錯(cuò)：【{ex.Message}】");
            }
        }
    }
}

public static unsafe String ToBase64String(byte[] inArray, int offset, int length, Base64FormattingOptions options) {
            //Do data verfication
            if (inArray==null) 
                throw new ArgumentNullException("inArray");
            if (length<0)
                throw new ArgumentOutOfRangeException("length", Environment.GetResourceString("ArgumentOutOfRange_Index"));
            if (offset<0)
                throw new ArgumentOutOfRangeException("offset", Environment.GetResourceString("ArgumentOutOfRange_GenericPositive"));
            if (options < Base64FormattingOptions.None || options > Base64FormattingOptions.InsertLineBreaks)
                throw new ArgumentException(Environment.GetResourceString("Arg_EnumIllegalVal", (int)options));
            Contract.Ensures(Contract.Result<string>() != null);
            Contract.EndContractBlock();
            int inArrayLength;
            int stringLength;
            inArrayLength = inArray.Length;
            if (offset > (inArrayLength - length))
                throw new ArgumentOutOfRangeException("offset", Environment.GetResourceString("ArgumentOutOfRange_OffsetLength"));
            if (inArrayLength == 0)
                return String.Empty;
            bool insertLineBreaks = (options == Base64FormattingOptions.InsertLineBreaks);
            //Create the new string.  This is the maximally required length.
            stringLength = ToBase64_CalculateAndValidateOutputLength(length, insertLineBreaks);
            string returnString = string.FastAllocateString(stringLength);
            fixed (char* outChars = returnString){
                fixed (byte* inData = inArray) {
                    int j = ConvertToBase64Array(outChars,inData,offset,length, insertLineBreaks);
                    BCLDebug.Assert(returnString.Length == j, "returnString.Length == j");
                    return returnString;
                }
            }
        }

internal static readonly char[] base64Table = {'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O',
                                                       'P','Q','R','S','T','U','V','W','X','Y','Z','a','b','c','d',
                                                       'e','f','g','h','i','j','k','l','m','n','o','p','q','r','s',
                                                       't','u','v','w','x','y','z','0','1','2','3','4','5','6','7',
                                                       '8','9','+','/','=' };        
        private const Int32 base64LineBreakPosition = 76;     

[System.Security.SecurityCritical]  // auto-generated
        private static unsafe int ConvertToBase64Array(char* outChars, byte* inData, int offset, int length, bool insertLineBreaks) {
            int lengthmod3 = length%3;
            int calcLength = offset + (length - lengthmod3);
            int j=0;
            int charcount = 0;
            //Convert three bytes at a time to base64 notation.  This will consume 4 chars.
            int i;
            // get a pointer to the base64Table to avoid unnecessary range checking
            fixed(char* base64 = base64Table) {
                for (i=offset; i<calcLength; i+=3)
                {
                    if (insertLineBreaks) {
                        if (charcount == base64LineBreakPosition) {
                            outChars[j++] = '\r';
                            outChars[j++] = '\n';
                            charcount = 0;
                        }
                        charcount += 4;
                    }
                    outChars[j] = base64[(inData[i]&0xfc)>>2];
                    outChars[j+1] = base64[((inData[i]&0x03)<<4) | ((inData[i+1]&0xf0)>>4)];
                    outChars[j+2] = base64[((inData[i+1]&0x0f)<<2) | ((inData[i+2]&0xc0)>>6)];
                    outChars[j+3] = base64[(inData[i+2]&0x3f)];
                    j += 4;
                }
                //Where we left off before
                i =  calcLength;
                if (insertLineBreaks && (lengthmod3 !=0) && (charcount == base64LineBreakPosition)) {
                    outChars[j++] = '\r';
                    outChars[j++] = '\n';
                }
                switch(lengthmod3)
                {
                case 2: //One character padding needed
                    outChars[j] = base64[(inData[i]&0xfc)>>2];
                    outChars[j+1] = base64[((inData[i]&0x03)<<4)|((inData[i+1]&0xf0)>>4)];
                    outChars[j+2] = base64[(inData[i+1]&0x0f)<<2];
                    outChars[j+3] = base64[64]; //Pad
                    j+=4;
                    break;
                case 1: // Two character padding needed
                    outChars[j] = base64[(inData[i]&0xfc)>>2];
                    outChars[j+1] = base64[(inData[i]&0x03)<<4];
                    outChars[j+2] = base64[64]; //Pad
                    outChars[j+3] = base64[64]; //Pad
                    j+=4;
                    break;
                }
            }
            return j;
        }
        private static int ToBase64_CalculateAndValidateOutputLength(int inputLength, bool insertLineBreaks) {
            long outlen = ((long)inputLength) / 3 * 4;          // the base length - we want integer division here. 
            outlen += ((inputLength % 3) != 0) ? 4 : 0;         // at most 4 more chars for the remainder
            if (outlen == 0)
                return 0;
            if (insertLineBreaks) {
                long newLines = outlen / base64LineBreakPosition;
                if ((outlen % base64LineBreakPosition) == 0) {
                    --newLines;    
                }
                outlen += newLines * 2;              // the number of line break chars we'll add, "\r\n"
            }
            // If we overflow an int then we cannot allocate enough
            // memory to output the value so throw
            if (outlen > int.MaxValue)
                throw new OutOfMemoryException();
            return (int)outlen;
        }