Reading bytes from a binary file - c++

Question

I have got a binary file and i need to store from ( x, y ) position z amount of bytes. For example I have got this sequence of bytes :
00000000 49 49 49 49 05 00 00 00 08 00 00 00 1a 00 00 00 | y0
00000010 39 a6 82 f8 47 8b b8 10 78 97 f1 73 56 d9 6f 00 | y1
00000020 58 99 d5 3b ac 7b 7b 40 b6 2e 9f 0a 69 b2 ac a0 | y2
________________________________________________________________
x0 x1 x2 x3 x4 x5 x6 x7 x8 x9 x10 x11 x12 x13 x14 x15
Every 2 merged numbers represents 1 byte ( it's taken from hexdump -C - coding is a little endian ). 49 = 1 byte, f8 is 1 byte etc ...
( x , y ) means position. For example if i put for x = 2,y = 2 i get position ( 2, 2 ) that means i start reading bytes from position y2, x2. In this case i start at byte d5 If i put z = 3 that means i want to store 3 bytes in this case those bytes are d5, 3b, ac.
I can calculate the position by a simple formula :
position = 16 * y + x
position = 16 * 2 + 2 // i put x = 2, y = 2 to formula
position = 34 // get number 34, that means i will start storing at 35th byte in this case it's d5
binaryFile . seekg ( position ) // jump to position in a file ( itc it's 99 )
binaryFile . read ( ( char * )&dest, z )) // save z bytes
If i put z = 3 i will store 3 bytes : d5, 3b, ac.
But somtimes coefficient z , x , y are not integers:
If i put y = 2, x = 1,5 and z = 3 // ( 1,5, 2 ) That means i have to jump at byte not 99 but d5 then store 2 bytes in this case d5 , 3b and add to them half byte from byte 99 and half byte from byte ac because the starting position was x = 1,5. How could I do that ??

Answer 1

You have to extend to byte boundaries on both ends, and first read the area that you want to write. So, if you want to write two bytes, you will have to read three bytes.
Then you will have to do appropriate bit shifting and masking to get your bits in the right places.
For example, if you are going to write two bytes shifted ½ byte, you would start with something like this:
unsigned char *mydata = MyDataToWrite();
unsigned char temp[bigEnough];
binaryFile.input(temp, 3);
temp[0] = (temp[0] & 0xf0) | (mydata[0] >> 4);
// more code here to put bits in temp
binaryFile.output(temp, 3);
Once you have your data in temp, write the 3 bytes back to the same location from which the were read.
I'm not going to write the whole thing here, but I hope this gives you an idea you can work from