DMDecoder: keep on decoding if a format error occurs

This is inspired by the discussion in zxing-cpp#774. The idea is to "get as much
information out of the barcode as possible", even if a format error (like
an invalid code word) or readerInit flag at the wrong position occurs. This
still results in an error condition but the user might still find value in
interpreting the (completely) decoded message.

core/src/datamatrix/DMDecoder.cpp

@@ -285,12 +285,18 @@ DecoderResult Decode(ByteArray&& bytes, const bool isDMRE)
 	int firstFNC1Position = 1;
 	Shift128 upperShift;
 
+	auto setError = [&error](Error&& e) {
+		// return only the first error but keep on decoding if possible
+		if (!error)
+			error = std::move(e);
+	};
+
 	// See ISO 16022:2006, 5.2.3 and Annex C, Table C.2
 	try {
 		while (!done && bits.available() >= 8) {
 			int oneByte = bits.readBits(8);
 			switch (oneByte) {
-			case 0: throw FormatError("invalid 0 code word");
+			case 0: setError(FormatError("invalid 0 code word")); break;
 			case 129: done = true; break; // Pad -> we are done, ignore the rest of the bits
 			case 230: DecodeC40OrTextSegment(bits, result, Mode::C40); break;
 			case 231: DecodeBase256Segment(bits, result); break;
@@ -307,13 +313,13 @@ DecoderResult Decode(ByteArray&& bytes, const bool isDMRE)
 				break;
 			case 233: // Structured Append
 				if (!firstCodeword) // Must be first ISO 16022:2006 5.6.1
-					throw FormatError("structured append tag must be first code word");
+					setError(FormatError("structured append tag must be first code word"));
 				ParseStructuredAppend(bits, sai);
 				firstFNC1Position = 5;
 				break;
 			case 234: // Reader Programming
 				if (!firstCodeword) // Must be first ISO 16022:2006 5.2.4.9
-					throw FormatError("reader programming tag must be first code word");
+					setError(FormatError("reader programming tag must be first code word"));
 				readerInit = true;
 				break;
 			case 235: upperShift.set = true; break; // Upper Shift (shift to Extended ASCII)
@@ -338,13 +344,14 @@ DecoderResult Decode(ByteArray&& bytes, const bool isDMRE)
 					// work around encoders that use unlatch to ASCII as last code word (ask upstream)
 					if (oneByte == 254 && bits.available() == 0)
 						break;
-					throw FormatError("invalid code word");
+					setError(FormatError("invalid code word"));
+					break;
 				}
 			}
 			firstCodeword = false;
 		}
 	} catch (Error e) {
-		error = std::move(e);
+		setError(std::move(e));
 	}
 
 	result.append(resultTrailer);

test/unit/datamatrix/DMDecodedBitStreamParserTest.cpp

@@ -204,9 +204,9 @@ TEST(DMDecodeTest, StructuredAppend)
 
 	// Structured Append "233" must be first ISO 16022:2006 5.6.1
 	EXPECT_FALSE(parse({50, 233, 42, 1, 1}).isValid());
-	EXPECT_EQ(info({50, 233, 42, 1, 1}).index, -1);
 
 	// ISO/IEC 16022:2006 5.6.2 sequence indicator example
+	EXPECT_TRUE(parse({233, 42, 1, 1, 50}).isValid());
 	EXPECT_EQ(info({233, 42, 1, 1, 50}).index, 2); // 1-based position 3 == index 2
 	EXPECT_EQ(info({233, 42, 1, 1, 50}).count, 7);
 	EXPECT_EQ(info({233, 42, 1, 1, 50}).id, "257");
@@ -247,18 +247,15 @@ TEST(DMDecodeTest, ReaderInit)
 	EXPECT_TRUE(parse({50}).isValid());
 
 	// Reader Programming "234" must be first ISO 16022:2006 5.2.4.9
-	EXPECT_FALSE(parse({50, 234}).readerInit());
 	EXPECT_FALSE(parse({50, 234}).isValid());
 
 	// Set
-	EXPECT_TRUE(parse({234, 50}).readerInit());
 	EXPECT_TRUE(parse({234, 50}).isValid());
+	EXPECT_TRUE(parse({234, 50}).readerInit());
 
-	EXPECT_FALSE(parse({235, 234, 50}).readerInit()); // Upper Shift first
 	EXPECT_FALSE(parse({235, 234, 50}).isValid());
 
 	// Can't be used with Structured Append "233"
 	EXPECT_TRUE(parse({233, 42, 1, 1, 50}).isValid()); // Null
-	EXPECT_FALSE(parse({233, 42, 1, 1, 234, 50}).readerInit());
 	EXPECT_FALSE(parse({233, 42, 1, 1, 234, 50}).isValid());
 }