specify the expected behavior of the main loops

in particular, the TagsHandler interface
also, rename the Reader/Writer classes to Decoder/Encoder for clarity

src/actions.cc

@@ -9,7 +9,7 @@ enum StreamSelection {
 	FIRST_STREAM = -2,
 };
 
-void list_tags(ogg::Reader *reader, long select)
+void list_tags(ogg::Decoder *reader, long select)
 {
 	ogg::Stream *s;
 	while ((s = reader->read_page()) != NULL) {
@@ -21,7 +21,7 @@ void list_tags(ogg::Reader *reader, long select)
 	}
 }
 
-void delete_tags(ogg::Reader *reader, opustags::ogg::Writer *writer, long select)
+void delete_tags(ogg::Decoder *reader, opustags::ogg::Encoder *writer, long select)
 {
 	ogg::Stream *s;
 	while ((s = reader->read_page()) != NULL) {

src/actions.h

@@ -0,0 +1,46 @@
+#pragma once
+
+#include "ogg.h"
+
+namespace opustags {
+
+    // To be defined properly somewhere.
+    class Tags;
+
+    // TagsHandler define various operations related to tags and stream in
+    // order to control the main loop.
+    // In its implementation, it is expected to receive an option structure.
+    struct TagsHandler {
+
+        // Irrelevant streams don't even need to be parsed, so we can save some
+        // effort with this method.
+        // Returns true if the stream should be parsed, false if it should be
+        // ignored (list) or copied identically (edit).
+        bool relevant(int streamno) { return false; }
+
+        // The list method is called by list_tags every time it has
+        // successfully parsed an OpusTags header.
+        void list(int streamno, const Tags&) {}
+
+        // Transform the tags at will.
+        // Returns true if the tags were indeed modified, false to cancel any
+        // kind of modification.
+        bool edit(int streamno, Tags&) { return false; }
+
+        // The work is done.
+        // When listing tags, once we've caught the streams we wanted, it's no
+        // use keeping reading the file for new streams. In that case, a true
+        // return value would abort any further processing.
+        bool done() { return false; }
+
+    };
+
+    // Decode a file and call the handler's list method every time a tags
+    // header is read.
+    void list_tags(ogg::Decoder&, TagsHandler&);
+
+    // Forward the input data to the output stream, transforming tags on-the-go
+    // with the handler's edit method.
+    void edit_tags(ogg::Decoder &in, ogg::Encoder &out, TagsHandler&);
+
+}

src/ogg.h

@@ -32,11 +32,15 @@ namespace ogg
         Stream(int serialno);
         ~Stream();
 
-        // Called by Reader once a page was read.
-        // Return true if it's ready, false if it expects more data.
-        // In the latter case, Reader::read_page will keep reading.
+        // Called by Decoder once a page was read.
+        // Returns true if it's ready, false if it expects more data.
+        // In the latter case, Decoder::read_page will keep reading.
         bool page_in(const ogg_page&);
 
+        // Make the stream behave as if it were unknown.
+        // As a consequence, no more effort would be made in extracting data.
+        void downgrade();
+
         StreamState state;
         StreamType type;
         Tags tags;
@@ -44,10 +48,10 @@ namespace ogg
         ogg_stream_state stream;
     };
 
-    struct Reader
+    struct Decoder
     {
-        Reader(std::istream&&);
-        ~Reader();
+        Decoder(std::istream&&);
+        ~Decoder();
 
         // Read a page, dispatch it, and return the stream it belongs to.
         // The read page is given to Stream::page_in before this function
@@ -62,10 +66,10 @@ namespace ogg
         std::map<int, Stream> streams;
     };
 
-    struct Writer
+    struct Encoder
     {
-        Writer(std::ostream&&);
-        ~Writer();
+        Encoder(std::ostream&&);
+        ~Encoder();
 
         void write_page(const ogg_page&);