encoder/Interface: move instance methods to abstract class

Rename struct Encoder to PreparedEncoder, and add a new (abstract)
class Encoder which represents one encoder instance.

src/output/plugins/httpd/HttpdOutputPlugin.cxx

@@ -63,8 +63,8 @@ HttpdOutput::~HttpdOutput()
 	if (metadata != nullptr)
 		metadata->Unref();
 
-	if (encoder != nullptr)
-		encoder->Dispose();
+	if (prepared_encoder != nullptr)
+		prepared_encoder->Dispose();
 
 }
 
@@ -123,12 +123,12 @@ HttpdOutput::Configure(const ConfigBlock &block, Error &error)
 
 	/* initialize encoder */
 
-	encoder = encoder_init(*encoder_plugin, block, error);
-	if (encoder == nullptr)
+	prepared_encoder = encoder_init(*encoder_plugin, block, error);
+	if (prepared_encoder == nullptr)
 		return false;
 
 	/* determine content type */
-	content_type = encoder_get_mime_type(encoder);
+	content_type = encoder_get_mime_type(prepared_encoder);
 	if (content_type == nullptr)
 		content_type = "application/octet-stream";
 
@@ -169,7 +169,7 @@ inline void
 HttpdOutput::AddClient(int fd)
 {
 	auto *client = new HttpdClient(*this, fd, GetEventLoop(),
-				       encoder->plugin.tag == nullptr);
+				       !encoder->ImplementsTag());
 	clients.push_front(*client);
 
 	/* pass metadata to client */
@@ -250,15 +250,14 @@ HttpdOutput::ReadPage()
 		/* we have fed a lot of input into the encoder, but it
 		   didn't give anything back yet - flush now to avoid
 		   buffer underruns */
-		encoder_flush(encoder, IgnoreError());
+		encoder->Flush(IgnoreError());
 		unflushed_input = 0;
 	}
 
 	size_t size = 0;
 	do {
-		size_t nbytes = encoder_read(encoder,
-					     buffer + size,
-					     sizeof(buffer) - size);
+		size_t nbytes = encoder->Read(buffer + size,
+					      sizeof(buffer) - size);
 		if (nbytes == 0)
 			break;
 
@@ -292,7 +291,8 @@ httpd_output_disable(AudioOutput *ao)
 inline bool
 HttpdOutput::OpenEncoder(AudioFormat &audio_format, Error &error)
 {
-	if (!encoder->Open(audio_format, error))
+	encoder = prepared_encoder->Open(audio_format, error);
+	if (encoder == nullptr)
 		return false;
 
 	/* we have to remember the encoder header, i.e. the first
@@ -351,7 +351,7 @@ HttpdOutput::Close()
 	if (header != nullptr)
 		header->Unref();
 
-	encoder->Close();
+	delete encoder;
 }
 
 static void
@@ -441,7 +441,7 @@ HttpdOutput::BroadcastFromEncoder()
 inline bool
 HttpdOutput::EncodeAndPlay(const void *chunk, size_t size, Error &error)
 {
-	if (!encoder_write(encoder, chunk, size, error))
+	if (!encoder->Write(chunk, size, error))
 		return false;
 
 	unflushed_input += size;
@@ -491,18 +491,18 @@ httpd_output_pause(AudioOutput *ao)
 inline void
 HttpdOutput::SendTag(const Tag &tag)
 {
-	if (encoder->plugin.tag != nullptr) {
+	if (encoder->ImplementsTag()) {
 		/* embed encoder tags */
 
 		/* flush the current stream, and end it */
 
-		encoder_pre_tag(encoder, IgnoreError());
+		encoder->PreTag(IgnoreError());
 		BroadcastFromEncoder();
 
 		/* send the tag to the encoder - which starts a new
 		   stream now */
 
-		encoder_tag(encoder, tag, IgnoreError());
+		encoder->SendTag(tag, IgnoreError());
 
 		/* the first page generated by the encoder will now be
 		   used as the new "header" page, which is sent to all