Add test to validate we can survive giving the fountain sink the wrong data

... since we're using that as the detection mechanism for auto-detect.

Also, a simplification to Decoder::do_decode()

Author: sz3

src/lib/encoder/Decoder.h

@@ -80,7 +80,6 @@ inline unsigned Decoder::do_decode(CimbReader& reader, STREAM& ostream, bool leg
 	colorPositions.resize(reader.num_reads()); // the number of cells == reader.num_reads(). Can we calculate this from config at compile time? Do we care?
 
 	unsigned bitsPerSymbol = cimbar::Config::symbol_bits();
-	unsigned bytesDecoded = 0;
 	{
 		bitbuffer symbolBits(cimbar::Config::capacity(bitsPerSymbol));
 		// read symbols first
@@ -116,8 +115,8 @@ inline unsigned Decoder::do_decode(CimbReader& reader, STREAM& ostream, bool leg
 	}
 
 	reed_solomon_stream rss(ostream, _eccBytes, _eccBlockSize);
-	bytesDecoded += colorBits.flush(rss);  // will return the pos after this flush(), which includes the previous flush()...
-	return bytesDecoded;
+	// flush() will return the (good) cumulative bytes written to the underlying stream
+	return colorBits.flush(rss);
 }
 
 template <typename STREAM>

src/lib/encoder/test/EncoderRoundTripTest.cpp

@@ -46,6 +46,42 @@ TEST_CASE( "EncoderRoundTripTest/testFountain.Pad", "[unit]" )
 
 	std::string decodedContents = File(tempdir.path() / "0.626").read_all();
 	assertEquals( "hello", decodedContents );
+
+	assertEquals( 1, fds.num_done() );
+}
+
+TEST_CASE( "EncoderRoundTripTest/testFountain.SinkMismatch", "[unit]" )
+{
+	MakeTempDirectory tempdir;
+
+	std::string inputFile = tempdir.path() / "hello.txt";
+	std::string outPrefix = tempdir.path() / "encoder.fountain";
+
+	{
+		std::ofstream f(inputFile);
+		f << "hello"; // 5 bytes!
+	}
+
+	// will be padded so the fountain encoding is happy. The encoded image looks suspiciously non-random!
+	Encoder enc(30, 4, 2);
+	assertEquals( 1, enc.encode_fountain(inputFile, outPrefix) );
+
+	uint64_t hash = 0xeecc8800efce8c48;
+	std::string path = fmt::format("{}_0.png", outPrefix);
+	cv::Mat encodedImg = cv::imread(path);
+	cv::cvtColor(encodedImg, encodedImg, cv::COLOR_BGR2RGB);
+	assertEquals( hash, image_hash::average_hash(encodedImg) );
+
+	// decoder
+	Decoder dec(30);
+	// sink with a mismatched fountain_chunk_size
+	// importantly, the sink expects a *larger* chunk than we'll give it...
+	fountain_decoder_sink<cimbar::zstd_decompressor<std::ofstream>> fds(tempdir.path(), cimbar::Config::fountain_chunk_size(30, 6, true));
+
+	unsigned bytesDecoded = dec.decode_fountain(encodedImg, fds, 1);
+	assertEquals( 7500, bytesDecoded );
+
+	assertEquals( 0, fds.num_done() );
 }
 
 TEST_CASE( "EncoderRoundTripTest/testStreaming", "[unit]" )