(Go) Dropbox Content Hash

Demonstrates how to compute the Dropbox content_hash of a file. (This is the "content_hash" found in the Dropbox FileMetadata object.

Note: This example requires Chilkat v9.5.0.79 or greater because it uses the fac.ReadBlockBd method.

For more information, see https://www.dropbox.com/developers/reference/content-hash

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    // This example requires the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    crypt := chilkat.NewCrypt2()

    // To calculate the content_hash of a file:
    // Split the file into blocks of 4 MB (4,194,304 or 4 * 1024 * 1024 bytes). The last block (if any) may be smaller than 4 MB.
    // Compute the hash of each block using SHA-256.
    // Concatenate the hash of all blocks in the binary format to form a single binary string.
    // Compute the hash of the concatenated string using SHA-256. Output the resulting hash in hexadecimal format.

    crypt.SetHashAlgorithm("sha256")
    crypt.SetEncodingMode("hex_lower")

    // We're going to calculate the content_hash for the Milky Way JPG image linked here:  https://www.dropbox.com/developers/reference/content-hash

    fac := chilkat.NewFileAccess()
    bd := chilkat.NewBinData()
    bdHashes := chilkat.NewBinData()

    success := fac.OpenForRead("qa_data/jpg/milky-way-nasa.jpg")
    if success == false {
        fmt.Println(fac.LastErrorText())
        crypt.DisposeCrypt2()
        fac.DisposeFileAccess()
        bd.DisposeBinData()
        bdHashes.DisposeBinData()
        return
    }

    blockSize := 4194304
    numBlocks := fac.GetNumBlocks(blockSize)

    i := 0
    for (i < numBlocks) {
        // Read the next 4MB block into bd.
        bd.Clear()
        success = fac.ReadBlockBd(i,blockSize,bd)

        hashHex := crypt.HashBdENC(bd)
        fmt.Println(i, ": ", *hashHex)
        bdHashes.AppendEncoded(*hashHex,"hex_lower")

        i = i + 1
    }

    fac.FileClose()

    // Hash the concatenated SHA-256 hashes.
    content_hash := crypt.HashBdENC(bdHashes)
    fmt.Println("content_hash = ", *content_hash)

    crypt.DisposeCrypt2()
    fac.DisposeFileAccess()
    bd.DisposeBinData()
    bdHashes.DisposeBinData()