(Go) MIME Content-Transfer-Encoding Header Field

Explains the Content-Transfer-Encoding header field and how it affects how data is stored in the MIME.

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    // The Content-Transfer-Encoding header field is typically set to one of these values:
    // 
    //   base64
    //   quoted-printable
    //   binary
    //   8bit
    //   7bit

    // If the encoding is base64 or quoted-printable, then the bytes of the content are encoded as such.
    // The values "8bit", "7bit", and "binary" all imply that NO encoding has been performed.

    // Here are some guidelines and hints:
    // 
    // 1) "8bit", "7bit", and "binary" encodings (i.e. NO encoding) results in the smallest MIME.

    // 2) A "binary" encoding implies non-text data.  Binary MIME CANNOT be retrieved as a string.  It can only be retrieved as a byte array.

    // 3) "8bit" and "7bit" encodings imply text.  
    //    7bit implies that the text is us-ascii (all byte values less than or equal to 0x7F).
    //    8bit implies that the text contains non-us-ascii chars.

    // 4) Base64 is the best choice for encoding non-text data, such as PDF's, images, etc.

    // 5) quoted-printable is the best choice for encoding text data where most chars are going to be us-ascii. This would typically 
    //    include all Western European languages.  For Asian, Arabic, Hebrew, etc. where most chars in the text are non-us-ascii,
    //    the most efficient encoding would be base64.  

    // Let's demonstrate with this small JPG image.
    var success bool
    sbJpgBase64 := chilkat.NewStringBuilder()
    sbJpgBase64.Append("/9j/4AAQSkZJRgABAQEASABIAAD//gAmRmlsZSB3cml0dGVuIGJ5IEFkb2JlIFBob3Rvc2hvcD8g\r\n")
    sbJpgBase64.Append("NC4w/9sAQwAQCwwODAoQDg0OEhEQExgoGhgWFhgxIyUdKDozPTw5Mzg3QEhcTkBEV0U3OFBtUVdf\r\n")
    sbJpgBase64.Append("YmdoZz5NcXlwZHhcZWdj/9sAQwEREhIYFRgvGhovY0I4QmNjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj\r\n")
    sbJpgBase64.Append("Y2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj/8IAEQgAFAAUAwERAAIRAQMRAf/EABcAAAMBAAAA\r\n")
    sbJpgBase64.Append("AAAAAAAAAAAAAAIDBAX/xAAYAQADAQEAAAAAAAAAAAAAAAABAgMEAP/aAAwDAQACEAMQAAAB2kZY\r\n")
    sbJpgBase64.Append("NNEijWKddfTmLgALWH//xAAbEAACAgMBAAAAAAAAAAAAAAABAgMRAAQSE//aAAgBAQABBQL0XqN+\r\n")
    sbJpgBase64.Append("pM2aqJGMiqFFCyg7z//EABwRAAICAgMAAAAAAAAAAAAAAAERAAIQIQMSUf/aAAgBAwEBPwHqU5aq\r\n")
    sbJpgBase64.Append("Axx+y1tMQl4elj//xAAcEQEAAQUBAQAAAAAAAAAAAAABEQACEBIhA1H/2gAIAQIBAT8B3Bhqy7Zc\r\n")
    sbJpgBase64.Append("enyiwmGgDhiOzj//xAAdEAABAwUBAAAAAAAAAAAAAAABAAIREBIhIkFR/9oACAEBAAY/ArZyn+Cg\r\n")
    sbJpgBase64.Append("xtxWuJaoCnqDuin/xAAcEAABBAMBAAAAAAAAAAAAAAABABEhYRAxQVH/2gAIAQEAAT8hkEwPUUR9\r\n")
    sbJpgBase64.Append("DYfE4nxtRpIkBTsayuALIiuY/9oADAMBAAIAAwAAABDWPTsf/8QAGhEAAwADAQAAAAAAAAAAAAAA\r\n")
    sbJpgBase64.Append("AAEREDFBIf/aAAgBAwEBPxC0DVPcWm+Ce4OesrkE6bjH/8QAGBEBAQEBAQAAAAAAAAAAAAAAAREA\r\n")
    sbJpgBase64.Append("QRD/2gAIAQIBAT8QahMiOc8YgSrnTY3ELclHXn//xAAcEAEBAAIDAQEAAAAAAAAAAAABEQAhMUFx\r\n")
    sbJpgBase64.Append("EFH/2gAIAQEAAT8Qn3igmSZSj+c4N4zapMy9IjFV98wncN2iuLFsCEbDGxQkI6RO/n//2Q==\r\n")

    mime := chilkat.NewMime()
    mime.SetContentType("image/jpeg")
    mime.SetBodyFromEncoded("base64",*sbJpgBase64.GetAsString())

    fmt.Println(*mime.GetMime())
    fmt.Println("-")

    // The result:
    // 
    // 	Content-Type: image/jpeg
    // 	Content-Transfer-Encoding: base64
    // 
    // 	/9j/4AAQSkZJRgABAQEASABIAAD//gAmRmlsZSB3cml0dGVuIGJ5IEFkb2JlIFBob3Rvc2hvcD8g
    // 	NC4w/9sAQwAQCwwODAoQDg0OEhEQExgoGhgWFhgxIyUdKDozPTw5Mzg3QEhcTkBEV0U3OFBtUVdf
    // 	YmdoZz5NcXlwZHhcZWdj/9sAQwEREhIYFRgvGhovY0I4QmNjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj
    // 	Y2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj/8IAEQgAFAAUAwERAAIRAQMRAf/EABcAAAMBAAAA
    // 	AAAAAAAAAAAAAAIDBAX/xAAYAQADAQEAAAAAAAAAAAAAAAABAgMEAP/aAAwDAQACEAMQAAAB2kZY
    // 	NNEijWKddfTmLgALWH//xAAbEAACAgMBAAAAAAAAAAAAAAABAgMRAAQSE//aAAgBAQABBQL0XqN+
    // 	pM2aqJGMiqFFCyg7z//EABwRAAICAgMAAAAAAAAAAAAAAAERAAIQIQMSUf/aAAgBAwEBPwHqU5aq
    // 	Axx+y1tMQl4elj//xAAcEQEAAQUBAQAAAAAAAAAAAAABEQACEBIhA1H/2gAIAQIBAT8B3Bhqy7Zc
    // 	enyiwmGgDhiOzj//xAAdEAABAwUBAAAAAAAAAAAAAAABAAIREBIhIkFR/9oACAEBAAY/ArZyn+Cg
    // 	xtxWuJaoCnqDuin/xAAcEAABBAMBAAAAAAAAAAAAAAABABEhYRAxQVH/2gAIAQEAAT8hkEwPUUR9
    // 	DYfE4nxtRpIkBTsayuALIiuY/9oADAMBAAIAAwAAABDWPTsf/8QAGhEAAwADAQAAAAAAAAAAAAAA
    // 	AAEREDFBIf/aAAgBAwEBPxC0DVPcWm+Ce4OesrkE6bjH/8QAGBEBAQEBAQAAAAAAAAAAAAAAAREA
    // 	QRD/2gAIAQIBAT8QahMiOc8YgSrnTY3ELclHXn//xAAcEAEBAAIDAQEAAAAAAAAAAAABEQAhMUFx
    // 	EFH/2gAIAQEAAT8Qn3igmSZSj+c4N4zapMy9IjFV98wncN2iuLFsCEbDGxQkI6RO/n//2Q==
    // 

    // Set the Content-Transfer-Encoding to "binary" by setting the Encoding property.
    mime.SetEncoding("binary")

    // Try to get the MIME string.  This CANNOT be done.  The bytes of the JPG image do not
    // represent chars, and to return a string means that bytes must be interpreted according
    // to some character encoding (such as utf-8).  Non-text binary bytes can only be 
    // contained in a string IF encoded in some way. Encodings such as Base64, quoted-printable,
    // URL, etc. exist to make it possible to represent binary data in string format.   
    fmt.Println(*mime.GetMime())
    fmt.Println("-")

    // We CAN get the binary MIME as bytes..
    var mimeBytes []byte
    mimeBytes = mime.GetMimeBytes()

    // Regardless of the Content-Transfer-Encoding, the 
    // body content can always be retrieved and the body bytes
    // decoded from whatever encoding is used..
    var jpgBytes []byte
    jpgBytes = mime.GetBodyBinary()

    // To get the body in base64 format, first make sure
    // the Content-Transfer-Encoding is base64, then call GetBodyEncoded.
    mime.SetEncoding("base64")
    jpgBase64 := mime.GetBodyEncoded()
    fmt.Println(*jpgBase64)
    fmt.Println("-")

    // Let's go back to "binary" MIME..
    mime.SetEncoding("binary")

    // Let's say we have MIME, and it was loaded directly from a file, or from
    // a byte array.  (It was not loaded from the contents of a string variable.)
    // We don't know whether the MIME contains binary or 8bit encodings, and thus
    // we dont' know if the MIME is safe to get as a string.
    // The Convert8Bit method can be called to recursively traverse the MIME and set
    // all 8bit or binary encodings to "base64".  This makes the MIME safe for storing in
    // a string.
    mime.Convert8Bit()
    mimeStr := mime.GetMime()
    fmt.Println(*mimeStr)
    fmt.Println("-")

    sbJpgBase64.DisposeStringBuilder()
    mime.DisposeMime()