Converting little endian to int

Question

First of all, I think this question is not C# independent. But can also be used in other languages like C.

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I'm now trying to parse a file format which stores integers in 4 bytes little-endian format. TBH, I don't know how the little-endian format nor big-endian format works.

But I need to convert them into an usable int variable.

For example, 02 00 00 00 = 2

So far, I have this code to convert the first 2 bytes: (I used FileStream.Read to store the raw datas into a buffer variable)

        int num = ((buffer[5] << 8) + buffer[4]);

But it will only convert the first two bytes. (02 00 in the example, not 02 00 00 00)

Any kind of help would be appreciated :)

Answer 1

int GetBigEndianIntegerFromByteArray(byte[] data, int startIndex) {
    return (data[startIndex] << 24)
         | (data[startIndex + 1] << 16)
         | (data[startIndex + 2] << 8)
         | data[startIndex + 3];
}

int GetLittleEndianIntegerFromByteArray(byte[] data, int startIndex) {
    return (data[startIndex + 3] << 24)
         | (data[startIndex + 2] << 16)
         | (data[startIndex + 1] << 8)
         | data[startIndex];
}

Answer 2

Using built-in methods:

byte[] data = { 2, 0, 0, 0 };
Array.Reverse(data);
int value = BitConverter.ToInt32(data, 0);

On the other side of the spectrum, using optimised unsafe code:

public static unsafe void SwapInts(int[] data) {
  int cnt = data.Length;
  fixed (int* d = data) {
    byte* p = (byte*)d;
    while (cnt-- > 0) {
      byte a = *p;
      p++;
      byte b = *p;
      *p = *(p + 1);
      p++;
      *p = b;
      p++;
      *(p - 3) = *p;
      *p = a;
      p++;
    }
  }
}

Note: You can use the BitConverter.IsLittleEndian to check if the computer is using little endian or big endian internally, so that you can make the code portable.