關(guān)于大小端

所謂大端數(shù)據(jù)就是數(shù)據(jù)的高字節(jié)保存在內(nèi)存的低地址中，而數(shù)據(jù)的低字節(jié)保存在內(nèi)存的高地址中，這樣的存儲(chǔ)模式有點(diǎn)兒類似于把數(shù)據(jù)當(dāng)作字符串順序處理：地址由小向大增加，而數(shù)據(jù)從高位往低位放；這和我們的閱讀習(xí)慣一致。

所謂的小端數(shù)據(jù)就是數(shù)據(jù)的高字節(jié)保存在內(nèi)存的高地址中，而數(shù)據(jù)的低字節(jié)保存在內(nèi)存的低地址中，這種存儲(chǔ)模式將地址的高低和數(shù)據(jù)位權(quán)有效地結(jié)合起來(lái)，高地址部分權(quán)值高，低地址部分權(quán)值低。（簡(jiǎn)單的來(lái)說(shuō)就是反過(guò)來(lái)存放數(shù)據(jù)了）

關(guān)于補(bǔ)位

數(shù)據(jù)補(bǔ)位，是指在將數(shù)據(jù)按照8個(gè)字節(jié)一段進(jìn)行加密或解密得到一段8個(gè)字節(jié)的密文或者明文，最后一段不足8個(gè)字節(jié)，按照需求補(bǔ)足8個(gè)字節(jié)（通常補(bǔ)00或者FF，根據(jù)實(shí)際要求不同）進(jìn)行計(jì)算，之后按照順序?qū)⒂?jì)算所得的數(shù)據(jù)連在一起即可。

舉個(gè)栗子??！

假設(shè)要發(fā)送一段這樣的數(shù)據(jù)（16進(jìn)制數(shù)據(jù)）:

| head | cmdlen(2字節(jié)) | cmd | playloadLen (4 字節(jié)) | playload

這段數(shù)據(jù)的內(nèi)容大概是包含了：我有一個(gè)頭部標(biāo)記，然后用頭部之后的兩個(gè)字節(jié)作為描述何種命令的長(zhǎng)度，然后接上的是何種命令，然后接上了附帶信息的長(zhǎng)度（4個(gè)字節(jié)），然后是附帶信息。

需要發(fā)送的信息為：**| https(頭部) | len | openStream | len2 | {@"速度與激情.h264",@"fileName"} **

//
//  CtpTools.m
//  NetworkTest
//
//  Created by 飛天江郎 on 10/11/2016.
//  Copyright ? 2016 EzioChen. All rights reserved.
//

//普通字符串轉(zhuǎn)換為十六進(jìn)制的。

+ (NSString *)hexStringFromString:(NSString *)string{
    NSData *myD = [string dataUsingEncoding:NSUTF8StringEncoding];
    Byte *bytes = (Byte *)[myD bytes];
    //下面是Byte 轉(zhuǎn)換為16進(jìn)制。
    NSString *hexStr=@"";
    for(int i=0;i<[myD length];i++)
        
    {
        NSString *newHexStr = [NSString stringWithFormat:@"%x",bytes[i]&0xff];///16進(jìn)制數(shù)
        
        if([newHexStr length]==1)
            
            hexStr = [NSString stringWithFormat:@"%@0%@",hexStr,newHexStr];
        
        else
            
            hexStr = [NSString stringWithFormat:@"%@%@",hexStr,newHexStr];
    }   
    return hexStr;   
}

//編寫一個(gè)NSData類型數(shù)據(jù)
+(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;
}

//補(bǔ)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];
}

//把整型轉(zhuǎn)化為16進(jìn)制的，然后在補(bǔ)位，轉(zhuǎn)換成大端數(shù)據(jù)
+(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 變成 16進(jìn)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（十六進(jìn)制）轉(zhuǎn)化為為十進(jìn)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;

}

//大小端數(shù)據(jù)轉(zhuǎn)換（其實(shí)還有更簡(jiǎn)便的方法，不過(guò)看起來(lái)這個(gè)方法是最直觀的）
+(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 轉(zhuǎn)換成 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; 
    
}

調(diào)用方法

    NSString *headStr = @"https";
    NSString *playload = @"{\"fileName\":\"速度與激情.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，就是對(duì)應(yīng)數(shù)據(jù)的16進(jìn)制標(biāo)準(zhǔn)格式了。

其實(shí)還有一些可優(yōu)化的地方，仔細(xì)的觀察下每次的Data組裝，然后你就會(huì)發(fā)現(xiàn)NSData本來(lái)就已經(jīng)是16進(jìn)制的，而大小端的轉(zhuǎn)換其實(shí)還有跟簡(jiǎn)單的轉(zhuǎn)換比方說(shuō)是這種：

#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)

這里寫成這樣完全是方便理解、