lib/xiph/OggSyncState: keep track of the Reader offset

src/lib/xiph/OggSyncState.cxx

@@ -40,8 +40,13 @@ OggSyncState::ExpectPage(ogg_page &page)
 {
 	while (true) {
 		int r = ogg_sync_pageout(&oy, &page);
-		if (r != 0)
+		if (r != 0) {
+			if (r > 0) {
+				start_offset = offset;
+				offset += r;
+			}
 			return r > 0;
+		}
 
 		if (!Feed(1024))
 			return false;
@@ -66,12 +71,16 @@ OggSyncState::ExpectPageSeek(ogg_page &page)
 
 	while (true) {
 		int r = ogg_sync_pageseek(&oy, &page);
-		if (r > 0)
+		if (r > 0) {
+			start_offset = offset;
+			offset += r;
 			return true;
+		}
 
 		if (r < 0) {
 			/* skipped -r bytes */
 			size_t nbytes = -r;
+			offset += nbytes;
 			if (nbytes > remaining_skipped)
 				/* still no ogg page - we lost our
 				   patience, abort */

src/lib/xiph/OggSyncState.hxx

@@ -23,6 +23,7 @@
 #include <ogg/ogg.h>
 
 #include <stddef.h>
+#include <stdint.h>
 
 class Reader;
 
@@ -34,6 +35,17 @@ class OggSyncState {
 
 	Reader &reader;
 
+	/**
+	 * Keeps track of the end offset of the most recently returned
+	 * page.
+	 */
+	uint64_t offset = 0;
+
+	/**
+	 * The start offset of the most recently returned page.
+	 */
+	uint64_t start_offset = 0;
+
 public:
 	explicit OggSyncState(Reader &_reader)
 		:reader(_reader) {
@@ -51,6 +63,14 @@ public:
 		ogg_sync_reset(&oy);
 	}
 
+	void SetOffset(uint64_t _offset) noexcept {
+		offset = _offset;
+	}
+
+	uint64_t GetStartOffset() const noexcept {
+		return start_offset;
+	}
+
 	bool Feed(size_t size);
 
 	bool ExpectPage(ogg_page &page);