Bugfix (maybe): properly clean up old decodes

This requires some more thoughts -- the shape of the problem is that the
encoder and decoder are running out of band with each other (very
intentionally -- it's the whole point). We want to "keep up" with what
the encoder throws at us, but we also need to clean up our list of
in-progress (and finished!) decodes at some point -- we don't want it to
be unbounded.

The bug this fixes should manifest as files refusing to
progress/download -- the decoder (app) has already reserved the "stream
slot", finished a download, but failed to clean up the mess to let it be
reclaimed. The workaround for CFC is to close and re-open the app, which
will throw out all in progress downloads and start from a clean slate.

src/exe/cimbar_send/send.cpp

@@ -70,7 +70,7 @@ int main(int argc, char** argv)
 	if (!initialize_GL(window_size, window_size))
 	{
 		std::cerr << "failed to create window :(" << std::endl;
-		return 50;
+		return 70;
 	}
 
 	configure(colorBits, ecc, compressionLevel);

src/lib/fountain/FountainMetadata.h

@@ -33,7 +33,7 @@ class FountainMetadata
 		to_uint8_arr(encode_id, size, d);
 	}
 
-	unsigned id() const
+	uint64_t id() const
 	{
 		return _data;
 	}

src/lib/fountain/fountain_decoder_sink.h

@@ -39,10 +39,10 @@ class fountain_decoder_sink
 		return true;
 	}
 
-	void mark_done(uint64_t id)
+	void mark_done(const FountainMetadata& md)
 	{
-		_done.insert(id);
-		auto it = _streams.find(id);
+		_done.insert(md.id());
+		auto it = _streams.find(stream_slot(md));
 		if (it != _streams.end())
 			_streams.erase(it);
 	}
@@ -106,7 +106,7 @@ class fountain_decoder_sink
 			return false;
 
 		if (store(md, *finished))
-			mark_done(md.id());
+			mark_done(md);
 		return true;
 	}
 
@@ -122,7 +122,7 @@ class fountain_decoder_sink
 	}
 
 protected:
-	// streams is limited to at most 8 decoders at a time. Current, we just use the lower bits of the encode_id.
+	// streams is limited to at most 8 decoders at a time. Currently, we just use the lower bits of the encode_id.
 	uint8_t stream_slot(const FountainMetadata& md) const
 	{
 		return md.encode_id() & 0x7;
@@ -137,6 +137,9 @@ class fountain_decoder_sink
 	std::string _dataDir;
 	unsigned _chunkSize;
 
+	// maybe instead of unordered_map+set, something where we can "age out" old streams?
+	// e.g. most recent 16/8, or something?
+	// question is what happens to _done/_streams when we wrap for continuous data streaming...
 	std::unordered_map<uint8_t, fountain_decoder_stream> _streams;
 	// track the uint64_t combo of (encode_id,size) to avoid redundant work
 	std::set<uint64_t> _done;