What is the fastest way to convert a float[] to a byte[]?

Question

I would like to get a byte[] from a float[] as quickly as possible, without looping through the whole array (via a cast, probably). Unsafe code is fine. Thanks!

I am looking for a byte array 4 time longer than the float array (the dimension of the byte array will be 4 times that of the float array, since each float is composed of 4 bytes). I'll pass this to a BinaryWriter.

EDIT: To those critics screaming "premature optimization": I have benchmarked this using ANTS profiler before I optimized. There was a significant speed increase because the file has a write-through cache and the float array is exactly sized to match the sector size on the disk. The binary writer wraps a file handle created with pinvoke'd win32 API. The optimization occurs since this lessens the number of function calls.

And, with regard to memory, this application creates massive caches which use plenty of memory. I can allocate the byte buffer once and re-use it many times--the double memory usage in this particular instance amounts to a roundoff error in the overall memory consumption of the app.

So I guess the lesson here is not to make premature assumptions ;)

Answer 1

There is a dirty fast (not unsafe code) way of doing this:

[StructLayout(LayoutKind.Explicit)]
struct BytetoDoubleConverter
{
    [FieldOffset(0)]
    public Byte[] Bytes;

    [FieldOffset(0)]
    public Double[] Doubles;
}
//...
static Double Sum(byte[] data)
{
    BytetoDoubleConverter convert = new BytetoDoubleConverter { Bytes = data };
    Double result = 0;
    for (int i = 0; i < convert.Doubles.Length / sizeof(Double); i++)
    {
        result += convert.Doubles[i];
    }
    return result;
}

This will work, but I'm not sure of the support on Mono or newer versions of the CLR. The only strange thing is that the array.Length is the bytes length. This can be explained because it looks at the array length stored with the array, and because this array was a byte array that length will still be in byte length. The indexer does think about the Double being eight bytes large so no calculation is necessary there.

I've looked for it some more, and it's actually described on MSDN, How to: Create a C/C++ Union by Using Attributes (C# and Visual Basic), so chances are this will be supported in future versions. I am not sure about Mono though.

Answer 2

Premature optimization is the root of all evil! @Vlad's suggestion to iterate over each float is a much more reasonable answer than switching to a byte[]. Take the following table of runtimes for increasing numbers of elements (average of 50 runs):

Elements      BinaryWriter(float)      BinaryWriter(byte[])
-----------------------------------------------------------
10               8.72ms                    8.76ms
100              8.94ms                    8.82ms
1000            10.32ms                    9.06ms
10000           32.56ms                   10.34ms
100000         213.28ms                  739.90ms
1000000       1955.92ms                10668.56ms

There is little difference between the two for small numbers of elements. Once you get into the huge number of elements range, the time spent copying from the float[] to the byte[] far outweighs the benefits.

So go with what is simple:

float[] data = new float[...];
foreach(float value in data)
{
    writer.Write(value);
}

Answer 3

There is a way which avoids memory copying and iteration.

You can use a really ugly hack to temporary change your array to another type using (unsafe) memory manipulation.

I tested this hack in both 32 & 64 bit OS, so it should be portable.

The source + sample usage is maintained at https://gist.github.com/1050703 , but for your convenience I'll paste it here as well:

public static unsafe class FastArraySerializer
{
    [StructLayout(LayoutKind.Explicit)]
    private struct Union
    {
        [FieldOffset(0)] public byte[] bytes;
        [FieldOffset(0)] public float[] floats;
    }

    [StructLayout(LayoutKind.Sequential, Pack = 1)]
    private struct ArrayHeader
    {
        public UIntPtr type;
        public UIntPtr length;
    }

    private static readonly UIntPtr BYTE_ARRAY_TYPE;
    private static readonly UIntPtr FLOAT_ARRAY_TYPE;

    static FastArraySerializer()
    {
        fixed (void* pBytes = new byte[1])
        fixed (void* pFloats = new float[1])
        {
            BYTE_ARRAY_TYPE = getHeader(pBytes)->type;
            FLOAT_ARRAY_TYPE = getHeader(pFloats)->type;
        }
    }

    public static void AsByteArray(this float[] floats, Action<byte[]> action)
    {
        if (floats.handleNullOrEmptyArray(action)) 
            return;

        var union = new Union {floats = floats};
        union.floats.toByteArray();
        try
        {
            action(union.bytes);
        }
        finally
        {
            union.bytes.toFloatArray();
        }
    }

    public static void AsFloatArray(this byte[] bytes, Action<float[]> action)
    {
        if (bytes.handleNullOrEmptyArray(action)) 
            return;

        var union = new Union {bytes = bytes};
        union.bytes.toFloatArray();
        try
        {
            action(union.floats);
        }
        finally
        {
            union.floats.toByteArray();
        }
    }

    public static bool handleNullOrEmptyArray<TSrc,TDst>(this TSrc[] array, Action<TDst[]> action)
    {
        if (array == null)
        {
            action(null);
            return true;
        }

        if (array.Length == 0)
        {
            action(new TDst[0]);
            return true;
        }

        return false;
    }

    private static ArrayHeader* getHeader(void* pBytes)
    {
        return (ArrayHeader*)pBytes - 1;
    }

    private static void toFloatArray(this byte[] bytes)
    {
        fixed (void* pArray = bytes)
        {
            var pHeader = getHeader(pArray);

            pHeader->type = FLOAT_ARRAY_TYPE;
            pHeader->length = (UIntPtr)(bytes.Length / sizeof(float));
        }
    }

    private static void toByteArray(this float[] floats)
    {
        fixed(void* pArray = floats)
        {
            var pHeader = getHeader(pArray);

            pHeader->type = BYTE_ARRAY_TYPE;
            pHeader->length = (UIntPtr)(floats.Length * sizeof(float));
        }
    }
}

And the usage is:

var floats = new float[] {0, 1, 0, 1};
floats.AsByteArray(bytes =>
{
    foreach (var b in bytes)
    {
        Console.WriteLine(b);
    }
});

Answer 4

If you do not want any conversion to happen, I would suggest Buffer.BlockCopy().

public static void BlockCopy(
    Array src,
    int srcOffset,
    Array dst,
    int dstOffset,
    int count
)

For example:

float[] floatArray = new float[1000];
byte[] byteArray = new byte[floatArray.Length * 4];

Buffer.BlockCopy(floatArray, 0, byteArray, 0, byteArray.Length);

Answer 5

You're better-off letting the BinaryWriter do this for you. There's going to be iteration over your entire set of data regardless of which method you use, so there's no point in playing with bytes.

Answer 6

Although you can obtain a byte* pointer using unsafe and fixed, you cannot convert the byte* to byte[] in order for the writer to accept it as a parameter without performing data copy. Which you do not want to do as it will double your memory footprint and add an extra iteration over the inevitable iteration that needs to be performed in order to output the data to disk.

Instead, you are still better off iterating over the array of floats and writing each float to the writer individually, using the Write(double) method. It will still be fast because of buffering inside the writer. See sixlettervariables's numbers.

Answer 7

Using the new Span<> in .Net Core 2.1 or later...

byte[] byteArray2 = MemoryMarshal.Cast<float, byte>(floatArray).ToArray();

Or, if Span can be used instead, then a direct reinterpret cast can be done: (very fast - zero copying)

Span<byte> byteArray3 = MemoryMarshal.Cast<float, byte>(floatArray);
// with span we can get a byte, set a byte, iterate, and more.
byte someByte = byteSpan[2]; 
byteSpan[2] = 33;

I did some crude benchmarks. The time taken for each is in the comments. [release/no debugger/x64]

float[] floatArray = new float[100];
for (int i = 0; i < 100; i++) floatArray[i] = i *  7.7777f;
Stopwatch start = Stopwatch.StartNew();
for (int j = 0; j < 100; j++)
{
    start.Restart();
    for (int k = 0; k < 1000; k++)
    {
        Span<byte> byteSpan = MemoryMarshal.Cast<float, byte>(floatArray);
    }
    long timeTaken1 = start.ElapsedTicks; ////// 0 ticks  //////

    start.Restart();
    for (int k = 0; k < 1000; k++)
    {
        byte[] byteArray2 = MemoryMarshal.Cast<float, byte>(floatArray).ToArray();
    }
    long timeTaken2 = start.ElapsedTicks; //////  26 ticks  //////

    start.Restart();
    for (int k = 0; k < 1000; k++)
    {
        byte[] byteArray = new byte[sizeof(float) * floatArray.Length];
        for (int i = 0; i < floatArray.Length; i++)
            BitConverter.GetBytes(floatArray[i]).CopyTo(byteArray, i * sizeof(float));
    }
    long timeTaken3 = start.ElapsedTicks;  //////  1310  ticks //////

    start.Restart();
    for (int k = 0; k < 1000; k++)
    {
        byte[] byteArray = new byte[sizeof(float) * floatArray.Length];
        Buffer.BlockCopy(floatArray, 0, byteArray, 0, byteArray.Length);
    }
    long timeTaken4 = start.ElapsedTicks;  ////// 33 ticks  //////

    start.Restart();
    for (int k = 0; k < 1000; k++)
    {
        byte[] byteArray = new byte[sizeof(float) * floatArray.Length];
        MemoryStream memStream = new MemoryStream();
        BinaryWriter writer = new BinaryWriter(memStream);
        foreach (float value in floatArray)
            writer.Write(value);
        writer.Close();
    }
    long timeTaken5 = start.ElapsedTicks;   ////// 1080 ticks   //////

    Console.WriteLine($"{timeTaken1/10,6} {timeTaken2 / 10,6} {timeTaken3 / 10,6} {timeTaken4 / 10,6} {timeTaken5 / 10,6} ");
}

Answer 8

We have a class called LudicrousSpeedSerialization and it contains the following unsafe method:

    static public byte[] ConvertFloatsToBytes(float[] data)
    {
        int n = data.Length;
        byte[] ret = new byte[n * sizeof(float)];
        if (n == 0) return ret;

        unsafe
        {
            fixed (byte* pByteArray = &ret[0])
            {
                float* pFloatArray = (float*)pByteArray;
                for (int i = 0; i < n; i++)
                {
                    pFloatArray[i] = data[i];
                }
            }
        }

        return ret;
    }

Answer 9

Although it basically does do a for loop behind the scenes, it does do the job in one line

byte[] byteArray = floatArray.Select(
                    f=>System.BitConverter.GetBytes(f)).Aggregate(
                    (bytes, f) => {List<byte> temp = bytes.ToList(); temp.AddRange(f); return temp.ToArray(); });