RESOLVED: Conversion from 2-part 64bit Floating Point to Fixed Point

[pgm575]

So I have already figured out how to convert 32-bit (4 byte) Floating Point numbers to Fixed Point using VB's data types from a previous forum thread, in the form shown below-

<StructLayout(LayoutKind.Explicit)> _
    Public Structure DataUnion
        <FieldOffset(0)> Public FloatValue As Single
        <FieldOffset(0)> Public LongValue As Long
        <FieldOffset(0)> Public ByteValue As Byte
    End Structure

I simply send my value into the Long field and retrieve it from the Single field.

Now I have a slightly more interesting problem, though- I need to combine (not add, but concatenate) two 4 byte (Single) values into one 8 byte (Double) value, convert to Fixed Point, and round down to Single.

Here is an example of what I mean:
I am working with a piece of hardware that outputs values in 8 byte (64-bit, or Double precision) Floating Point numbers. A typical Hex value would be 40 40 DD E4 2D 3B A2 6D. Unfortunately, the hardware that I use for data capture can only receive 4 byte (32-bit, or Single precision) values, so I capture two of them at an offset (i.e. Part 1 is 40 40 DD E4 and Part 2 is 2D 3B A2 6D). There is an added level of complexity, in that the software that is doing the conversion exposes these values (in its API) as decimal values (1077992932 and 758882925 would be my inputs in this example), so I have to convert them to another form first to even combine them.

When I convert the 8-byte number by hand, I get the proper decimal fixed point value (roughly 33.733 or so) but I'm not really sure how to convert to the proper data type and combine these values in VB. Has anyone done this before?

[pgm575]

So I've gotten closer but I'm not quite there yet. What I have attempted to do is convert my two singles to hexadecimal representations, combine them as strings, and I will then convert them back. This code is shown below, in its entirety-

Imports Microsoft.Win32
Imports System.Runtime.InteropServices
 
Module Module1
 
    <StructLayout(LayoutKind.Explicit)> _
    Public Structure DataUnion
        <FieldOffset(0)> Public FloatValue As Single
        <FieldOffset(0)> Public FloatLongValue As Double
        <FieldOffset(0)> Public LongValue As Long
        <FieldOffset(0)> Public ByteValue As Byte
    End Structure
 
    Sub Main()
        Dim p1 As Single, p2 As Single
        Dim ba1() As Byte, ba2() As Byte, wholeByte(7) As Byte
        Dim hex1 As String, hex2 As String, whole As String
        Dim suLong As Int64
        Dim su As DataUnion
 
        p1 = 1077992931
        ba1 = BitConverter.GetBytes(p1)
        hex1 = BitConverter.ToString(ba1)
 
        p2 = 831725571
        ba2 = BitConverter.GetBytes(p2)
        hex2 = BitConverter.ToString(ba2)
 
        whole = hex1 + "-" + hex2
        ba1.CopyTo(wholeByte, 0)
        ba2.CopyTo(wholeByte, 4)
 
        Console.WriteLine("Part 1 is {0}d and part 2 is {1}d" & vbCrLf, p1, p2)
 
        Console.WriteLine("Part1 byte array is {0}", hex1)
        Console.WriteLine("Part2 byte array is {0}", hex2)
        Console.WriteLine("Whole byte array is {0}", whole)
 
        Console.WriteLine(vbCrLf)
 
        suLong = BitConverter.ToInt64(wholeByte, 0)
        su.LongValue = suLong
 
        Console.WriteLine("suLong is {0} and su.LongValue is {1}", suLong, su.LongValue)
        Console.WriteLine("su.FloatValue is {0}", su.FloatValue)
    End Sub
 
End Module

Unfortunately, when I run this Console application I get incorrect Hex values. For example, "Part1" is correctly converted by Windows Calculator to 40 40 DD E2, whereas this console application converts it to BC 81 80 4E. The MS documentation states that GetBytes method returns a byte array of the given value, so does anyone see what I'm missing here?

[pgm575]

Alright, more progress but still not quite there. If the above p1 = 1077992931d is used and I implement the code

Dim samp1 As String = Hex(p1)
Console.WriteLine("Part1 converted to Hex directly is {0}", samp1)

at the bottom of my Main Sub, then I get an output of 40 40 DE 00h. This converts back to 1077992960d, but I would've expected to see 40 40 DD E3h as the output.

Given the decimal values I'm seeing, this looks less like rounding and more like a different conversion method. It needs to be exactly accurate, though, since I'm combining these representations to make that one 64-bit number.

[pgm575]

So now I've gotten the pieces to convert correctly but the whole Hex value does not convert back to decimal properly. See code below.

Imports Microsoft.Win32
Imports System.Runtime.InteropServices

Module Module1

    <StructLayout(LayoutKind.Explicit)> _
    Public Structure DataUnion
        <FieldOffset(0)> Public FixedPointValue As Single
        <FieldOffset(0)> Public LongValue As Long
        <FieldOffset(0)> Public ByteValue As Byte
    End Structure

    Sub Main()
        Dim p1 As Int32, p2 As Int32
        Dim wholeByte2(7) As Byte
        Dim su As DataUnion

        p1 = 1077992931 'properly converts to 40 40 DD E3h
        p2 = 831725571 'properly converts to 31 93 20 03h

        Dim samp1 As String = Hex(p1), samp2 As String = Hex(p2), wholeHexStr As String = samp1 & samp2

        Console.WriteLine("Part1 converted to Hex directly is {0}. Part 2 is {1}.", samp1, samp2)
        Console.WriteLine("Whole is {0}.", wholeHexStr)

        Dim j As Integer = 0
        For i = 0 To wholeHexStr.Length - 2
            wholeByte2(j) = Byte.Parse(wholeHexStr.Substring(i, 2), Globalization.NumberStyles.HexNumber)
            i += 1
            j += 1
        Next
        Console.WriteLine("Whole with splits as usable value is {0}.", BitConverter.ToString(wholeByte2))
        Console.WriteLine(vbCrLf)

        Dim ConLong As Long = BitConverter.ToInt64(wholeByte2, 0) 'should convert to 4629944384795910146d
        'actually converts to 225341823854133312d
        su.LongValue = ConLong
        Console.WriteLine("Complete Long in structure is {0}", su.LongValue)
        Console.WriteLine("Converted value is {0}", su.FixedPointValue)
    End Sub

End Module

Anyone know why my Byte-to-Long conversion isn't working as expected?

[pgm575]

Ended up resolving this myself. Looks like the Hex() method was converting in Big Endian format, whereas the number structure was converting in Little Endian format. The proper conversion is shown below.

Imports Microsoft.Win32
Imports System.Runtime.InteropServices

Module Module1

    <StructLayout(LayoutKind.Explicit)> _
    Public Structure DataUnion
        <FieldOffset(0)> Public FixedPointValue As Double
        <FieldOffset(0)> Public LongValue As Long
        <FieldOffset(0)> Public ByteValue As Byte
    End Structure

    Sub Main()
        Dim p1 As Int32, p2 As Int32
        Dim wholeByte(7) As Byte
        Dim su As DataUnion
        Dim TempValue As Double, FinalValue As Single

        p1 = 1077992931 'properly converts to 40 40 DD E3h
        p2 = 831725571 'properly converts to 31 93 20 03h

        Dim samp1 As String = Hex(p1), samp2 As String = Hex(p2), wholeHexStr As String = samp1 & samp2

        Console.WriteLine("Part1 converted to Hex directly is {0}. Part 2 is {1}.", samp1, samp2)
        Console.WriteLine("Whole is {0}.", wholeHexStr)

        Dim j As Integer = 0
        For i = 0 To wholeHexStr.Length - 2
            wholeByte(7 - j) = Byte.Parse(wholeHexStr.Substring(i, 2), Globalization.NumberStyles.HexNumber)
            i += 1
            j += 1
        Next
        Console.WriteLine("Whole with splits as system-usable value is {0}.", BitConverter.ToString(wholeByte))
        Console.WriteLine(vbCrLf)

        Dim ConLong As Long = (BitConverter.ToInt64(wholeByte, 0)) 'properly converts to 4629944384795910147d
        su.LongValue = ConLong
        Console.WriteLine("Complete Long in structure is {0}", su.LongValue) '4629944384795910147d
        Console.WriteLine("Which happens to be {0} in Hex", Hex(su.LongValue)) '40 40 DD E3 31 93 20 03h

        TempValue = su.FixedPointValue
        Console.WriteLine("Converted value is {0}", TempValue)
        FinalValue = Convert.ToSingle(TempValue)
        Console.WriteLine("Final value is {0}", FinalValue)
    End Sub

End Module