What is idiomatic C # for unpacking an integer from a byte array?
I am parsing a binary file format. It encodes an integer using four bytes in a way that will naturally conform to a C # uint type.
What is the most C # / idiomatic way to implement this feature:
uint ReadUint(byte[] buffer);
Suppose the buffer contains 4 elements. The complete answer can look at some of the common byte orders caused by small / large arguments in the file and document the ones it chooses to parse.
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The simplest (but a bit dangerous re endianness):
return BitConverter.ToUInt32(buffer, 0);
Also, bit shift is ok (as per your own answer) - or you can use Jon EndianBitConverter in MiscUtil which handles translations.
(edit)
The little bit shift version I use in protobuf-net is very similar to your version - I just read them in ascending order and use bitwise (not numeric) complement:
return ((uint)buffer[0])
| (((uint)buffer[1]) << 8)
| (((uint)buffer[2]) << 16)
| (((uint)buffer[3]) << 24);
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I would normally use the BitConverter class for this. In your case, the BitConverter.ToUInt32 () method .
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This answer is actually an extended comment (hence the wiki) comparing BitConverter and bitrate performance using + vs |; it only applies to micro-optimization !!
Results first:
BitConverter: 972ms, chk=1855032704
Bitwise: 740ms, chk=1855032704
ReadLength: 1316ms, chk=1855032704
Or the results, if configured to allow non-zero base offsets:
BitConverter: 905ms, chk=1855032704
Bitwise: 1058ms, chk=1855032704
ReadLength: 1244ms, chk=1855032704
And the code:
using System;
using System.Diagnostics;
static class Program
{
static void Main()
{
byte[] buffer = BitConverter.GetBytes((uint)123);
const int LOOP = 50000000;
uint chk = 0;
var watch = Stopwatch.StartNew();
for (int i = 0; i < LOOP; i++)
{
chk += BitConverter.ToUInt32(buffer, 0);
}
watch.Stop();
Console.WriteLine("BitConverter: " + watch.ElapsedMilliseconds
+ "ms, chk=" + chk);
chk = 0;
watch = Stopwatch.StartNew();
for (int i = 0; i < LOOP; i++)
{
chk += Bitwise(buffer);
}
watch.Stop();
Console.WriteLine("Bitwise: " + watch.ElapsedMilliseconds
+ "ms, chk=" + chk);
chk = 0;
watch = Stopwatch.StartNew();
for (int i = 0; i < LOOP; i++)
{
chk += ReadLength(buffer);
}
watch.Stop();
Console.WriteLine("ReadLength: " + watch.ElapsedMilliseconds
+ "ms, chk=" + chk);
Console.ReadKey();
}
static uint Bitwise(byte[] buffer)
{
return ((uint)buffer[0])
| (((uint)buffer[1]) << 8)
| (((uint)buffer[2]) << 16)
| (((uint)buffer[3]) << 24);
}
static uint ReadLength(byte[] buffer)
{
uint result = ((uint)buffer[3]) << 24;
result += ((uint)buffer[2]) << 16;
result += ((uint)buffer[1]) << 8;
result += buffer[0];
return result;
}
}
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As someone from C, this is how I implement this function:
static uint ReadLength(byte[] buffer)
{
uint result = ((uint) buffer[3]) << 24;
result |= ((uint) buffer[2]) << 16;
result |= ((uint) buffer[1]) << 8;
result |= buffer[offset];
return result;
}
This parses the Wikipedia asserted format, in little-endian style, in the .NET version running on i386 / Vista p>
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byte[] ba = new byte[]{ 0x10, 0xFF, 0x11, 0x01 } ;
var ui = BitConverter.ToUInt32(ba, 0);
Use the BitConverter class .
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