关于大小端
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。
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）
关于补位
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。
举个栗子🌰！
假设要发送一段这样的数据（16进制数据）:
| head | cmdlen(2字节) | cmd | playloadLen (4 字节) | playload
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。
需要发送的信息为：**| https(头部) | len | openStream | len2 | {@"速度与激情.h264",@"fileName"} **
//
// 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;
}
调用方法
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);
最后打印出的Data，就是对应数据的16进制标准格式了。
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：
#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)
这里写成这样完全是方便理解、
