(Swift 2) 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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func chilkatTest() {
    // This example requires the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    let crypt = CkoCrypt2()

    // 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.HashAlgorithm = "sha256"
    crypt.EncodingMode = "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

    let fac = CkoFileAccess()
    let bd = CkoBinData()
    let bdHashes = CkoBinData()

    var success: Bool = fac.OpenForRead("qa_data/jpg/milky-way-nasa.jpg")
    if success == false {
        print("\(fac.LastErrorText)")
        return
    }

    var blockSize: Int = 4194304
    var numBlocks: Int = fac.GetNumBlocks(blockSize).intValue

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

        var hashHex: String? = crypt.HashBdENC(bd)
        print("\(i): \(hashHex!)")
        bdHashes.AppendEncoded(hashHex, encoding: "hex_lower")

        i = i + 1
    }

    fac.FileClose()

    // Hash the concatenated SHA-256 hashes.
    var content_hash: String? = crypt.HashBdENC(bdHashes)
    print("content_hash = \(content_hash!)")

}