Support FS_SEEK_END and negative offset for zipped files in FS_Seek

Use FS_SEEK_END in sound code instead of working around it.
If FS_SEEK_SET and going to current position, just return.

code/client/snd_codec_ogg.c

@@ -157,11 +157,8 @@ int S_OGG_Callback_seek(void *datasource, ogg_int64_t offset, int whence)
 
 		case SEEK_END :
 		{
-			// Quake 3 seems to have trouble with FS_SEEK_END 
-			// so we use the file length and FS_SEEK_SET
-
 			// set the file position in the actual file with the Q3 function
-			retVal = FS_Seek(stream->file, (long) stream->length + (long) offset, FS_SEEK_SET);
+			retVal = FS_Seek(stream->file, (long) offset, FS_SEEK_END);
 
 			// something has gone wrong, so we return here
 			if(retVal < 0)

code/client/snd_codec_opus.c

@@ -146,11 +146,8 @@ int S_OggOpus_Callback_seek(void *datasource, opus_int64 offset, int whence)
 
 		case SEEK_END :
 		{
-			// Quake 3 seems to have trouble with FS_SEEK_END 
-			// so we use the file length and FS_SEEK_SET
-
 			// set the file position in the actual file with the Q3 function
-			retVal = FS_Seek(stream->file, (long) stream->length + (long) offset, FS_SEEK_SET);
+			retVal = FS_Seek(stream->file, (long) offset, FS_SEEK_END);
 
 			// something has gone wrong, so we return here
 			if(retVal < 0)

code/qcommon/files.c

@@ -277,6 +277,7 @@ typedef struct {
 	qboolean	handleSync;
 	int			fileSize;
 	int			zipFilePos;
+	int			zipFileLen;
 	qboolean	zipFile;
 	qboolean	streamed;
 	char		name[MAX_ZPATH];
@@ -1261,6 +1262,7 @@ long FS_FOpenFileReadDir(const char *filename, searchpath_t *search, fileHandle_
 					// open the file in the zip
 					unzOpenCurrentFile(fsh[*file].handleFiles.file.z);
 					fsh[*file].zipFilePos = pakFile->pos;
+					fsh[*file].zipFileLen = pakFile->len;
 
 					if(fs_debug->integer)
 					{
@@ -1635,36 +1637,63 @@ int FS_Seek( fileHandle_t f, long offset, int origin ) {
 	}
 
 	if (fsh[f].zipFile == qtrue) {
-		//FIXME: this is incomplete and really, really
-		//crappy (but better than what was here before)
+		//FIXME: this is really, really crappy
+		//(but better than what was here before)
 		byte	buffer[PK3_SEEK_BUFFER_SIZE];
-		int		remainder = offset;
+		int		remainder;
+		int		currentPosition = FS_FTell( f );
+
+		// change negative offsets into FS_SEEK_SET
+		if ( offset < 0 ) {
+			switch( origin ) {
+				case FS_SEEK_END:
+					remainder = fsh[f].zipFileLen + offset;
+					break;
 
-		if( offset < 0 || origin == FS_SEEK_END ) {
-			Com_Error( ERR_FATAL, "Negative offsets and FS_SEEK_END not implemented "
-					"for FS_Seek on pk3 file contents" );
-			return -1;
+				case FS_SEEK_CUR:
+					remainder = currentPosition + offset;
+					break;
+
+				case FS_SEEK_SET:
+				default:
+					remainder = 0;
+					break;
+			}
+
+			if ( remainder < 0 ) {
+				remainder = 0;
+			}
+
+			origin = FS_SEEK_SET;
+		} else {
+			if ( origin == FS_SEEK_END ) {
+				remainder = fsh[f].zipFileLen - currentPosition + offset;
+			} else {
+				remainder = offset;
+			}
 		}
 
 		switch( origin ) {
 			case FS_SEEK_SET:
+				if ( remainder == currentPosition ) {
+					return offset;
+				}
 				unzSetOffset(fsh[f].handleFiles.file.z, fsh[f].zipFilePos);
 				unzOpenCurrentFile(fsh[f].handleFiles.file.z);
 				//fallthrough
 
+			case FS_SEEK_END:
 			case FS_SEEK_CUR:
 				while( remainder > PK3_SEEK_BUFFER_SIZE ) {
 					FS_Read( buffer, PK3_SEEK_BUFFER_SIZE, f );
 					remainder -= PK3_SEEK_BUFFER_SIZE;
 				}
 				FS_Read( buffer, remainder, f );
 				return offset;
-				break;
 
 			default:
 				Com_Error( ERR_FATAL, "Bad origin in FS_Seek" );
 				return -1;
-				break;
 		}
 	} else {
 		FILE *file;