[Mod] M15: The heuristics in r20524 were a bit too loose, also identi…

…fying some GMC files as SoundTracker modules. Tighten the song title check again to previous values, but ignore it if a sufficent amount of sample headers has "real-looking" sample names (starting with a disk name like st-01:...).

git-svn-id: https://source.openmpt.org/svn/openmpt/trunk/OpenMPT@20538 56274372-70c3-4bfc-bfc3-4c3a0b034d27

soundlib/Load_mod.cpp

@@ -322,6 +322,11 @@ struct MODSampleHeader
 		       + ((loopStart > length * 2) ? 1 : 0);
 	}
 
+	bool HasDiskName() const
+	{
+		return (!memcmp(name, "st-", 3) || !memcmp(name, "ST-", 3)) && name[5] == ':';
+	}
+
 	// Suggested threshold for rejecting invalid files based on cumulated score returned by GetInvalidByteScore
 	static constexpr uint32 INVALID_BYTE_THRESHOLD = 40;
 
@@ -1410,15 +1415,13 @@ static bool ValidateHeader(const M15FileHeaders &fileHeaders)
 	// However, there are quite a few SoundTracker modules in the wild with random
 	// characters. To still be able to distguish them from other formats, we just reject
 	// files with *too* many bogus characters. Arbitrary threshold: 48 bogus characters in total
-	// or more than 11 invalid characters just in the title alone (just enough to make scramble_2.mod load).
-	uint32 invalidChars = CountInvalidChars(fileHeaders.songname);
-	if(invalidChars > 11)
-	{
-		return false;
-	}
+	// or more than 5 invalid characters just in the title alone
+	uint32 invalidCharsInTitle = CountInvalidChars(fileHeaders.songname);
+	uint32 invalidChars = invalidCharsInTitle;
 
 	SmpLength totalSampleLen = 0;
 	uint8 allVolumes = 0;
+	uint8 diskNameCount = 0;
 
 	for(SAMPLEINDEX smp = 0; smp < 15; smp++)
 	{
@@ -1429,6 +1432,8 @@ static bool ValidateHeader(const M15FileHeaders &fileHeaders)
 		// schmokk.mod has a non-zero value here but it should not be treated as finetune
 		if(sampleHeader.finetune != 0)
 			invalidChars += 16;
+		if(sampleHeader.HasDiskName())
+			diskNameCount++;
 
 		// Sanity checks - invalid character count adjusted for ata.mod (MD5 937b79b54026fa73a1a4d3597c26eace, SHA1 3322ca62258adb9e0ae8e9afe6e0c29d39add874)
 		if(invalidChars > 48
@@ -1442,6 +1447,12 @@ static bool ValidateHeader(const M15FileHeaders &fileHeaders)
 		allVolumes |= sampleHeader.volume;
 	}
 
+	// scramble_2.mod has a lot of garbage in the song title, but it has lots of sample names starting with st-01, so we consider those to be more important than the garbage bytes.
+	if(invalidCharsInTitle > 5 && diskNameCount < 4)
+	{
+		return false;
+	}
+
 	// Reject any files with no (or only silent) samples at all, as this might just be a random binary file (e.g. ID3 tags with tons of padding)
 	if(totalSampleLen == 0 || allVolumes == 0)
 	{
@@ -1548,7 +1559,7 @@ bool CSoundFile::ReadM15(FileReader &file, ModLoadingFlags loadFlags)
 
 		totalSampleLen += mptSmp.nLength;
 
-		if(m_szNames[smp][0] && ((memcmp(m_szNames[smp].buf, "st-", 3) && memcmp(m_szNames[smp].buf, "ST-", 3)) || m_szNames[smp][5] != ':'))
+		if(m_szNames[smp][0] && sampleHeader.HasDiskName())
 		{
 			// Ultimate Soundtracker 1.8 and D.O.C. SoundTracker IX always have sample names containing disk names.
 			hasDiskNames = false;