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# How to calculate fast CRC16-CCITT checksum in VB

## How to calculate fast CRC16-CCITT checksum in VB

(OP)
Here is the code to calculate the CRC16-CCITT checksum
which uses the x^16,x^12,x^5,x^1 polynomial.
It uses a lookup table for speed.
DataByte() is the byte array which the checksum is calculated for.
In the example the array is loaded from a hex string in
Text1.Text.
The initial value is loaded from Text2.Text.
The CRC is displayed in Text3.Text.
The lookup table is pasted in at the end of the code.
It should be copied into a text file called CRCTAB.txt
and saved in the same folder as the Project.

Option Explicit
Dim CRCTab(255) As Long '               Array for single byte CRCs loaded from table
Dim DataByte() As Byte '                Array for byte data

Private Sub CmdStart_Click()
Dim TempStr As String
Dim DataSize As Long
Dim X As Long
Dim Index As Long
Dim TempLng As Integer
Dim CRC As Long

TempStr = Text1.Text '                              Load text box hex bytes
DataSize = (Len(TempStr) / 2) - 1 '                 Calc nunber of pairs of chrs
ReDim DataByte(DataSize) '                          Resize the array
Index = 1
For X = 0 To DataSize '                             Load array into memory
DataByte(X) = "&h" & Mid(TempStr, Index, 2)
Index = Index + 2
Next

CRC = CLng("&h" & Text2.Text) '                     Load initial value (normally 0xFFFF)

For X = 0 To DataSize '                             Loop through data bytes
TempLng = ((CRC \ 256) Xor DataByte(X)) '           Shift left (>>8) XOR with data
CRC = ((CRC * 256) And 65535) Xor CRCTab(TempLng) ' Shift right (<<8) prevent overflow, XOR with table
Next
Text3.Text = Right("0000" & Hex(CRC), 4)

End Sub

Dim X As Long
Dim TempStr As String

Open App.Path & "\CRCTAB.txt" For Input As #1
For X = 0 To 255
Input #1, TempStr
CRCTab(X) = CLng(TempStr)
Next
Close #1

End Sub

&h0000, &h1021, &h2042, &h3063, &h4084, &h50A5, &h60C6, &h70E7,&h8108, &h9129, &hA14A, &hB16B, &hC18C, &hD1AD, &hE1CE, &hF1EF,&h1231, &h0210, &h3273, &h2252, &h52B5, &h4294, &h72F7, &h62D6,&h9339, &h8318, &hB37B, &hA35A, &hD3BD, &hC39C, &hF3FF, &hE3DE,&h2462, &h3443, &h0420, &h1401, &h64E6, &h74C7, &h44A4, &h5485,&hA56A, &hB54B, &h8528, &h9509, &hE5EE, &hF5CF, &hC5AC, &hD58D,&h3653, &h2672, &h1611, &h0630, &h76D7, &h66F6, &h5695, &h46B4,&hB75B, &hA77A, &h9719, &h8738, &hF7DF, &hE7FE, &hD79D, &hC7BC,&h48C4, &h58E5, &h6886, &h78A7, &h0840, &h1861, &h2802, &h3823,&hC9CC, &hD9ED, &hE98E, &hF9AF, &h8948, &h9969, &hA90A, &hB92B,&h5AF5, &h4AD4, &h7AB7, &h6A96, &h1A71, &h0A50, &h3A33, &h2A12,&hDBFD, &hCBDC, &hFBBF, &hEB9E, &h9B79, &h8B58, &hBB3B, &hAB1A,&h6CA6, &h7C87, &h4CE4, &h5CC5, &h2C22, &h3C03, &h0C60, &h1C41,&hEDAE, &hFD8F, &hCDEC, &hDDCD, &hAD2A, &hBD0B, &h8D68, &h9D49,&h7E97, &h6EB6, &h5ED5, &h4EF4, &h3E13, &h2E32, &h1E51, &h0E70,&hFF9F, &hEFBE, &hDFDD, &hCFFC, &hBF1B, &hAF3A, &h9F59, &h8F78,&h9188, &h81A9, &hB1CA, &hA1EB, &hD10C, &hC12D, &hF14E, &hE16F,&h1080, &h00A1, &h30C2, &h20E3, &h5004, &h4025, &h7046, &h6067,&h83B9, &h9398, &hA3FB, &hB3DA, &hC33D, &hD31C, &hE37F, &hF35E,&h02B1, &h1290, &h22F3, &h32D2, &h4235, &h5214, &h6277, &h7256,&hB5EA, &hA5CB, &h95A8, &h8589, &hF56E, &hE54F, &hD52C, &hC50D,&h34E2, &h24C3, &h14A0, &h0481, &h7466, &h6447, &h5424, &h4405,&hA7DB, &hB7FA, &h8799, &h97B8, &hE75F, &hF77E, &hC71D, &hD73C,&h26D3, &h36F2, &h0691, &h16B0, &h6657, &h7676, &h4615, &h5634,&hD94C, &hC96D, &hF90E, &hE92F, &h99C8, &h89E9, &hB98A, &hA9AB,&h5844, &h4865, &h7806, &h6827, &h18C0, &h08E1, &h3882, &h28A3,&hCB7D, &hDB5C, &hEB3F, &hFB1E, &h8BF9, &h9BD8, &hABBB, &hBB9A,&h4A75, &h5A54, &h6A37, &h7A16, &h0AF1, &h1AD0, &h2AB3, &h3A92,&hFD2E, &hED0F, &hDD6C, &hCD4D, &hBDAA, &hAD8B, &h9DE8, &h8DC9,&h7C26, &h6C07, &h5C64, &h4C45, &h3CA2, &h2C83, &h1CE0, &h0CC1,&hEF1F, &hFF3E, &hCF5D, &hDF7C, &hAF9B, &hBFBA, &h8FD9, &h9FF8,&h6E17, &h7E36, &h4E55, &h5E74, &h2E93, &h3EB2, &h0ED1, &h1EF0

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