iOS 关于大小端以及一些数据补位

   <h3><strong>关于大小端</strong></h3>    <p>所谓大端数据就是数据的高字节保存在内存的低地址中，而数据的低字节保存在内存的高地址中，这样的存储模式有点儿类似于把数据当作字符串顺序处理：地址由小向大增加，而数据从高位往低位放；这和我们的阅读习惯一致。</p>    <p>所谓的小端数据就是数据的高字节保存在内存的高地址中，而数据的低字节保存在内存的低地址中，这种存储模式将地址的高低和数据位权有效地结合起来，高地址部分权值高，低地址部分权值低。（简单的来说就是反过来存放数据了）</p>    <h3><strong>关于补位</strong></h3>    <p>数据补位，是指在将数据按照8个字节一段进行加密或解密得到一段8个字节的密文或者明文，最后一段不足8个字节，按照需求补足8个字节（通常补00或者FF，根据实际要求不同）进行计算，之后按照顺序将计算所得的数据连在一起即可。</p>    <h3><strong>举个栗子:chestnut:！</strong></h3>    <p>假设要发送一段这样的数据（16进制数据）:</p>    <p>| head | cmdlen(2字节) | cmd | playloadLen (4 字节) | playload</p>    <p>这段数据的内容大概是包含了：我有一个头部标记，然后用头部之后的两个字节作为描述何种命令的长度，然后接上的是何种命令，然后接上了附带信息的长度（4个字节），然后是附带信息。</p>    <p>需要发送的信息为： | https(头部) | len | openStream | len2 | {@"速度与激情.h264",@"fileName"}</p>    <pre>  <code class="language-objectivec">//  //  CtpTools.m  //  NetworkTest  //  //  Created by 飞天江郎 on 10/11/2016.  //  Copyright © 2016 EzioChen. All rights reserved.  //    //普通字符串转换为十六进制的。    + (NSString *)hexStringFromString:(NSString *)string{      NSData *myD = [string dataUsingEncoding:NSUTF8StringEncoding];      Byte *bytes = (Byte *)[myD bytes];      //下面是Byte 转换为16进制。      NSString *hexStr=@"";      for(int i=0;i<[myD length];i++)        {          NSString *newHexStr = [NSString stringWithFormat:@"%x",bytes[i]&0xff];///16进制数            if([newHexStr length]==1)                hexStr = [NSString stringWithFormat:@"%@0%@",hexStr,newHexStr];            else                hexStr = [NSString stringWithFormat:@"%@%@",hexStr,newHexStr];      }         return hexStr;     }    //编写一个NSData类型数据  +(NSMutableData*)HexStringToData:(NSString*)str{        NSString *command = str;      command = [command stringByReplacingOccurrencesOfString:@" " withString:@""];      NSMutableData *commandToSend= [[NSMutableData alloc] init];      unsigned char whole_byte;      char byte_chars[3] = {'\0','\0','\0'};      int i;      for (i=0; i < [command length]/2; i++) {          byte_chars[0] = [command characterAtIndex:i*2];          byte_chars[1] = [command characterAtIndex:i*2+1];          whole_byte = strtol(byte_chars, NULL, 16);          [commandToSend appendBytes:&whole_byte length:1];      }      return commandToSend;  }    //补位的方法  +(NSString*)addString:(NSString*)string Length:(NSInteger)length OnString:(NSString*)str{        NSMutableString * nullStr = [[NSMutableString alloc] initWithString:@""];      if ((length-str.length)> 0) {          for (int i = 0; i< (length-str.length); i++) {              [nullStr appendString:string];          }      }      return [NSString stringWithFormat:@"%@%@",nullStr,str];  }    //把整型转化为16进制的，然后在补位，转换成大端数据  +(NSString *)intToHexString:(NSInteger)number length:(NSInteger)len{        NSString *result = [self addString:@"0" Length:len OnString:[self ToHex:(unsigned int)number]];      NSData *data = [self HexStringToData:result];      NSData *lastData = [self dataTransfromBigOrSmall:data];        result = [self dataChangeToString:lastData];        return result;    }    //把int 变成 16进制的字符串  +(NSString *)ToHex:(unsigned int)number  {      NSString * result = [NSString stringWithFormat:@"%x",number];      if (result.length<2) {          result = [NSString stringWithFormat:@"0%x",number];      }      return [result uppercaseString];  }    //把data（十六进制）转化为为十进制整型  +(NSInteger)dataToInt:(NSData *)data{          NSInteger temp;      double sum = 0.0;      NSString * string = [self dataChangeToString:data];      NSInteger length = string.length;          for (int i = 0 ; i < length; i++) {          NSString * tempStr = [string substringWithRange:NSMakeRange(i, 1)];          int tempInt = [self StringToInt:tempStr];          double tempDouble = tempInt * pow(16.0, (double)(length-1-i));          sum = sum + tempDouble;      }        temp = (NSInteger)sum;      return temp;    }    //大小端数据转换（其实还有更简便的方法，不过看起来这个方法是最直观的）  +(NSData *)dataTransfromBigOrSmall:(NSData *)data{        NSString *tmpStr = [self dataChangeToString:data];      NSMutableArray *tmpArra = [NSMutableArray array];      for (int i = 0 ;i<data.length*2 ;i+=2) {          NSString *str = [tmpStr substringWithRange:NSMakeRange(i, 2)];          [tmpArra addObject:str];      }        NSArray *lastArray = [[tmpArra reverseObjectEnumerator] allObjects];        NSMutableString *lastStr = [NSMutableString string];        for (NSString *str in lastArray) {            [lastStr appendString:str];        }        NSData *lastData = [self HexStringToData:lastStr];        return lastData;    }    +(NSString*)dataChangeToString:(NSData*)data{        NSString * string = [NSString stringWithFormat:@"%@",data];      string = [string stringByReplacingOccurrencesOfString:@"<" withString:@""];      string = [string stringByReplacingOccurrencesOfString:@">" withString:@""];      string = [string stringByReplacingOccurrencesOfString:@" " withString:@""];      return string;    }    +(int)StringToInt:(NSString*)string{        int temp;        temp = [string intValue];        if ([string isEqual:@"a"]||[string isEqual:@"A"]) {          temp = 10;      }else if ([string isEqual:@"b"]||[string isEqual:@"B"])      {          temp = 11;      }else if ([string isEqual:@"c"]||[string isEqual:@"C"])      {          temp = 12;      }else if ([string isEqual:@"d"]||[string isEqual:@"D"])      {          temp = 13;      }else if ([string isEqual:@"e"]||[string isEqual:@"E"])      {          temp = 14;      }else if ([string isEqual:@"f"]||[string isEqual:@"F"])      {          temp = 15;      }      return temp;  }    //hexString 转换成 String  + (NSString *)stringFromHexString:(NSString *)hexString {        char *myBuffer = (char *)malloc((int)[hexString length] / 2 + 1);      bzero(myBuffer, [hexString length] / 2 + 1);      for (int i = 0; i < [hexString length] - 1; i += 2) {          unsigned int anInt;          NSString * hexCharStr = [hexString substringWithRange:NSMakeRange(i, 2)];          NSScanner * scanner = [[NSScanner alloc] initWithString:hexCharStr];          [scanner scanHexInt:&anInt];          myBuffer[i / 2] = (char)anInt;      }      NSString *unicodeString = [NSString stringWithCString:myBuffer encoding:4];        return unicodeString;     }</code></pre>    <p><strong>调用方法</strong></p>    <pre>  <code class="language-objectivec">NSString *headStr = @"https";      NSString *playload = @"{\"fileName\":\"速度与激情.h264\"}";      NSString *cmd = @"openStream";        NSString *headHex = [self hexStringFromString:headStr];        NSString *cmdHex = [self hexStringFromString:cmd];        NSString *cmdLen = [self intToHexString:cmdHex.length/2 length:4];        NSString *playloadHex = [self hexStringFromString:playload];        NSString *playloadHexLen = [self intToHexString:playloadHex.length/2 length:8];        NSString *allHexStr = [self stringWithFormat:@"%@%@%@%@%@",headHex,cmdLen,cmdHex,playloadHexLen,playloadHex];         NSData *lastData = [self HexStringToData:allHexStr];       NSLog(@"lastData=>%@",lastData);</code></pre>    <p>最后打印出的Data，就是对应数据的16进制标准格式了。</p>    <p>其实还有一些可优化的地方，仔细的观察下每次的Data组装，然后你就会发现NSData本来就已经是16进制的，而大小端的转换其实还有跟简单的转换比方说是这种：</p>    <pre>  <code class="language-objectivec">#include <sys/cdefs.h>    #define ntohs(x) __DARWIN_OSSwapInt16(x)  #define htons(x) __DARWIN_OSSwapInt16(x)    #define ntohl(x) __DARWIN_OSSwapInt32(x)  #define htonl(x) __DARWIN_OSSwapInt32(x)</code></pre>    <p>这里写成这样完全是方便理解、</p>    <p> </p>    <p>来自：http://www.jianshu.com/p/79f349409cbf</p>    <p> </p>