Decoder: when we known there's a mismatch between the fountain chunk size...

... and the fountain *stream* (sink)'s chunk size, we shouldn't send it
the decoded bytes. It's not going to be able to do anything useful with
them. Instead, we'll have a /dev/null style "null_stream" that just
tracks how many bytes were written to it.

Author: sz3

src/lib/encoder/Decoder.h

@@ -8,6 +8,7 @@
 #include "cimb_translator/Config.h"
 #include "cimb_translator/Interleave.h"
 #include "util/File.h"
+#include "util/null_stream.h"
 
 #include <opencv2/opencv.hpp>
 #include <functional>
@@ -166,8 +167,22 @@ template <typename MAT, typename FOUNTAINSTREAM>
 inline unsigned Decoder::decode_fountain(const MAT& img, FOUNTAINSTREAM& ostream,  unsigned color_mode, bool should_preprocess, int color_correction)
 {
 	CimbReader reader(img, _decoder, color_mode, should_preprocess, color_correction);
-	aligned_stream aligner(ostream, ostream.chunk_size(), 0, std::bind(&CimbReader::update_metadata, &reader, std::placeholders::_1, std::placeholders::_2));
-	return do_decode(reader, aligner, color_mode==0);
+	bool legacy_mode = color_mode == 0;
+	unsigned chunk_size = cimbar::Config::fountain_chunk_size(_eccBytes, _bitsPerOp, legacy_mode);
+	auto update_md_fun = std::bind(&CimbReader::update_metadata, &reader, std::placeholders::_1, std::placeholders::_2);
+
+	// we don't want to feed the fountain stream bad data, so we eat the decode if we have a mismatch
+	// we still might succeed the decode, in which case (hopefully) the positive bytes we return will
+	// tell our caller to fix the underlying ostream so the next round will work.
+	if (ostream.chunk_size() != chunk_size)
+	{
+		null_stream devnull;
+		aligned_stream aligner(devnull, chunk_size, 0, update_md_fun);
+		return do_decode(reader, aligner, legacy_mode);
+	}
+
+	aligned_stream aligner(ostream, ostream.chunk_size(), 0, update_md_fun);
+	return do_decode(reader, aligner, legacy_mode);
 }
 
 inline unsigned Decoder::decode(std::string filename, std::string output, unsigned color_mode)

src/lib/encoder/reed_solomon_stream.h

@@ -88,7 +88,7 @@ class reed_solomon_stream
 	bool _good;
 };
 
-inline std::ifstream& operator<<(std::ifstream& s, const ReedSolomon::BadChunk& chunk)
+inline std::ifstream& operator<<(std::ifstream& s, const ReedSolomon::BadChunk&)
 {
 	return s;
 }

src/lib/encoder/test/EncoderRoundTripTest.cpp

@@ -64,9 +64,10 @@ TEST_CASE( "EncoderRoundTripTest/testFountain.SinkMismatch", "[unit]" )
 
 	// will be padded so the fountain encoding is happy. The encoded image looks suspiciously non-random!
 	Encoder enc(30, 4, 2);
+	enc.set_legacy_mode();
 	assertEquals( 1, enc.encode_fountain(inputFile, outPrefix) );
 
-	uint64_t hash = 0xeecc8800efce8c48;
+	uint64_t hash = 0xaecc8c00efce8c28;
 	std::string path = fmt::format("{}_0.png", outPrefix);
 	cv::Mat encodedImg = cv::imread(path);
 	cv::cvtColor(encodedImg, encodedImg, cv::COLOR_BGR2RGB);
@@ -75,10 +76,11 @@ TEST_CASE( "EncoderRoundTripTest/testFountain.SinkMismatch", "[unit]" )
 	// 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));
+	// importantly, the sink expects a *smaller* chunk than we'll give it...
+	// because that's a more interesting test...
+	fountain_decoder_sink<cimbar::zstd_decompressor<std::ofstream>> fds(tempdir.path(), cimbar::Config::fountain_chunk_size(30, 6, false));
 
-	unsigned bytesDecoded = dec.decode_fountain(encodedImg, fds, 1);
+	unsigned bytesDecoded = dec.decode_fountain(encodedImg, fds, 0);
 	assertEquals( 7500, bytesDecoded );
 
 	assertEquals( 0, fds.num_done() );

src/lib/util/null_stream.h

@@ -0,0 +1,28 @@
+/* This code is subject to the terms of the Mozilla Public License, v.2.0. http://mozilla.org/MPL/2.0/. */
+#pragma once
+
+class null_stream
+{
+public:
+	null_stream()
+	{}
+
+	null_stream& write(const char*, unsigned length)
+	{
+		_count += length;
+		return *this;
+	}
+
+	bool good() const
+	{
+		return true;
+	}
+
+	long tellp() const
+	{
+		return _count;
+	}
+
+protected:
+	long _count = 0;
+};