NSData的转换为基本变量与IEEE-754或二进制补码？ [英] Convert NSData to primitive variable with ieee-754 or twos-complement?

问题描述

我在的OBJ-C 和可可新的程序员。我是一个试图写将用于读取二进制文件的框架（灵活的图像传输系统或 FITS 二进制文件，通常是由天文学家使用）。二进制数据，我感兴趣提取，可以有不同的格式和我通过阅读的头FITS 文件中获取其属性。

I am new programmer in Obj-C and cocoa. Im a trying to write a framework which will be used to read a binary files (Flexible Image Transport System or FITS binary files, usually used by astronomers). The binary data, that I am interested to extract, can have various formats and I get its properties by reading the header of the FITS file.

到现在为止，我设法创建一个类来存储在的内容符合文件和头隔离到一个的NSString 对象和二进制数据转换成的NSData 对象。我也设法写方法，它让我从中提取标题是国米preT二进制数据非常有价值的核心价值观。

Up to now, I manage to create a class to store the content of the FITS file and to isolate the header into a NSString object and the binary data into a NSData object. I also manage to write method which allow me to extract the key values from the header that are very valuable to interpret the binary data.

现在我想在的NSData 对象转换为基本类型数组的（数组双， INT ，短 ...）。但是，在这里，我被卡住，并会AP preciate任何帮助。

I am now trying to convert the NSData object into a primitive array (array of double, int, short ...). But, here, I get stuck and would appreciate any help.

据我对 FITS 文件，我有PT取决于价值的二进制数据5的可能性，除$ P $的 BITPIX 键：

According to the documentation I have about the FITS file, I have 5 possibilities to interpret the binary data depending on the value of the BITPIX key:

BITPIX value | Data represented
  8          | Char or unsigned binary int
 16          | 16-bit two's complement binary integer
 32          | 32-bit two's complement binary integer
 64          | 64-bit two's complement binary integer
-32          | IEEE single precision floating-point
-64          | IEEE double precision floating-point

我已经写code的和平，表明波纹管，要尽量在的NSData 转换为原始数组。

// self reefer to my FITS class which contain a NSString object  
// with the content of the header and a NSData object with the binary data. 

-(void*) GetArray
{
switch (BITPIX)
{
    case 8:
        return [self GetArrayOfUInt];
        break;
    case 16:
        return [self GetArrayOfInt];
        break;
    case 32:
        return [self GetArrayOfLongInt];
        break;
    case 64:
        return [self GetArrayOfLongLong];
        break;
    case -32:
        return [self GetArrayOfFloat];
        break;
    case -64:
        return [self GetArrayOfDouble];
        break;
    default:
        return NULL;
}
}

// then I show you the method to convert the NSData into a primitive array.
// I restrict my example to the case of 'double'. Code is similar for other methods
// just change double by 'unsigned int' (BITPIX 8), 'short' (BITPIX 16)
// 'int' (BITPIX 32) 'long lon' (BITPIX 64), 'float' (BITPIX -32). 

-(double*) GetArrayOfDouble
{
int Nelements=[self NPIXEL]; // Metod to extract, from the header 
                             // the number of element into the array
NSLog(@"TOTAL NUMBER OF ELEMENTS [%i]\n",Nelements);

//CREATE THE ARRAY
double (*array)[Nelements];

// Get the total number of bits in the binary data
int Nbit = abs(BITPIX)*GCOUNT*(PCOUNT + Nelements); // GCOUNT and PCOUNT are defined
                                                        // into the header
NSLog(@"TOTAL NUMBER OF BIT [%i]\n",Nbit);
int i=0;

    //FILL THE ARRAY
double Value;

for(int bit=0; bit < Nbit; bit+=sizeof(double))
{
    [Img getBytes:&Value range:NSMakeRange(bit,sizeof(double))];
    NSLog(@"[%i]:(%u)%.8G\n",i,bit,Value);
        (*array)[i]=Value;
    i++;

}

return (*array);

}

不过，我在打印循环的价值从预期值非常不同的（用比较正式的FITS软件）。因此，我认为在的OBJ-C 双击不使用 IEEE-754 约定还有< STRONG>的OBJ-C INT 不是的补码的。我真的不熟悉这个两公约（ IEEE 和补码的），并想知道我该怎么做这种转换是的OBJ-C

However, the value I print in the loop are very different from the expected values (compared using official FITS software). Therefore, I think that the Obj-C double does not use the IEEE-754 convention as well as the Obj-C int are not twos-complement. I am really not familiar with this two convention (IEEE and twos-complement) and would like to know how I can do this conversion with Obj-C.

在提前许多感谢任何帮助或信息。

In advance many thanks for any help or information.

推荐答案

非常感谢。这个问题真的涉及到endian和我没有意识到这一点。说实话，我从来没有真正意识到尾数类型的，因为到现在为止，我从来没有遇到这个问题。

Thanks a lot. The problem was really related to the endian and I was not aware of this. Honestly, I was never really aware of the endian type because, up to now, I was never confronted to the problem.

顺便说一句，我找到解决方案，感谢所有为你的话。因为它可以为其他开发人员有用，我会建议code的和平成为可能，这样的NSData转换为原始的，什么都尾数类型。这是我觉得（与基金会和可可框架在OS X 10.5和10.6的工作）的解决方案，它可能不是最好的，所以，需要您自担风险使用;）

By the way, I find the solution, thanks all for your remarks. Because it can be useful for other developer, I will propose a peace of code which allow to convert NSData to primitive, what ever the endian type. This is the solution I find (work with the Foundation and cocoa framework on Os X 10.5 and 10.6) and it may not be the best, so, use at your own risk ;)

// 1- CREATE AN ARRAY OF SIZE Nelements;
int Nelements = XXX;
double (*array)[Nelements];

// 2- Create the swapped double -> a double with the wrong bit order
NSSwappedDouble swappedValue;

// 3- The correct value
double Value;

// 4- Fill your array
int i=0, bit=0;
int BitStep=sizeof(double);

while(i<Nelements)
{
    [MyNSData getBytes:&swappedValue range:NSMakeRange(bit,step)];
    Value = NSSwapBigDoubleToHost(swappedValue);   // or NSSwapLittleDoubleToHost depending of your endian type.
    (*array)[i]=Value;
    i++; bit+=BitStep;

}

希望这是有用的。

Hope this can be useful.

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