From 161e84598d65f0718540fb745e2358c4d1f9956b Mon Sep 17 00:00:00 2001
From: "Christoph \"baka0815\" Schwerdtfeger" <git@baka0815.de>
Date: Sat, 3 Nov 2018 19:24:53 +0100
Subject: [PATCH] Respect the offset of the padding

The StartHunk logic was flawed and while working with CHD files before v5 didn't work for v5 (and above).
The extraframes logic did work in some cases (read 3 track CHDv5) but didn't work in all cases and was merely a hack in retrospective.

Thanks to the comment of @flyinghead (see also https://github.com/libretro/reicast-emulator/commit/b6c2508922084c294bd57f275ef644f2ca6ddb28#diff-cb507bb17d74342aa45b6e4d10d607af) the logic is now correct and works for v4 and v5 files.
---
 core/imgread/chd.cpp | 45 +++++++++++++++-----------------------------
 1 file changed, 15 insertions(+), 30 deletions(-)

diff --git a/core/imgread/chd.cpp b/core/imgread/chd.cpp
index f8fa8ee6f..58c3175f3 100644
--- a/core/imgread/chd.cpp
+++ b/core/imgread/chd.cpp
@@ -35,40 +35,31 @@ struct CHDTrack : TrackFile
 {
 	CHDDisc* disc;
 	u32 StartFAD;
-	u32 StartHunk;
+	u32 Offset;
 	u32 fmt;
 
-	u32 extraframes; /* number of "spillage" frames in previous tracks */
-
-	CHDTrack(CHDDisc* disc, u32 StartFAD,u32 StartHunk, u32 fmt, u32 extraframes)
+	CHDTrack(CHDDisc* disc, u32 StartFAD, u32 Offset, u32 fmt)
 	{
 		this->disc=disc;
 		this->StartFAD=StartFAD;
-		this->StartHunk=StartHunk;
+		this->Offset=Offset;
 		this->fmt=fmt;
-		this->extraframes = extraframes;
 	}
 
-	virtual void Read(u32 FAD,u8* dst,SectorFormat* sector_type,u8* subcode,SubcodeFormat* subcode_type)
+	virtual void Read(u32 FAD, u8* dst, SectorFormat* sector_type, u8* subcode, SubcodeFormat* subcode_type)
 	{
-		s32 fad_offs = FAD - (StartFAD + extraframes);
-		u32 hunk=(fad_offs)/(s32)disc->sph + StartHunk;
+		s32 fad_offs = FAD + Offset;
+		u32 hunk = fad_offs / disc->sph;
 
-		if (fad_offs < 0)
+		if (disc->old_hunk != hunk)
 		{
-			hunk--;
-			fad_offs += disc->sph;
-		}
-
-		if (disc->old_hunk!=hunk)
-		{
-			chd_read(disc->chd,hunk,disc->hunk_mem); //CHDERR_NONE
+			chd_read(disc->chd, hunk, disc->hunk_mem); //CHDERR_NONE
 			disc->old_hunk = hunk;
 		}
 
-		u32 hunk_ofs=fad_offs%disc->sph;
+		u32 hunk_ofs = fad_offs%disc->sph;
 
-		memcpy(dst,disc->hunk_mem+hunk_ofs*(2352+96),fmt);
+		memcpy(dst, disc->hunk_mem + hunk_ofs * (2352+96), fmt);
 
 		*sector_type=fmt==2352?SECFMT_2352:SECFMT_2048_MODE1;
 
@@ -108,8 +99,7 @@ bool CHDDisc::TryOpen(const wchar* file)
 	u32 total_frames = 150;
 
 	u32 total_secs = 0;
-	u32 total_hunks = 0;
-	int extraframes = 0;
+	u32 Offset = 0;
 
 	for(;;)
 	{
@@ -157,16 +147,11 @@ bool CHDDisc::TryOpen(const wchar* file)
 		t.EndFAD = total_frames - 1;
 		t.ADDR = 0;
 		t.CTRL = strcmp(type,"AUDIO") == 0 ? 0 : 4;
-		t.file = new CHDTrack(this, t.StartFAD, total_hunks, strcmp(type,"MODE1") ? 2352 : 2048, extraframes);
-		if (head->version >= 5)
-		{
-			int padded = (frames + CD_TRACK_PADDING - 1) / CD_TRACK_PADDING;
-			extraframes += (padded * CD_TRACK_PADDING) - frames;
-		}
+		t.file = new CHDTrack(this, t.StartFAD, Offset - t.StartFAD, strcmp(type,"MODE1") ? 2352 : 2048);
 
-		total_hunks += frames / sph;
-		if (frames % sph)
-			total_hunks++;
+		// CHD files are padded, so we have to respect the offset
+		int padded = (frames + CD_TRACK_PADDING - 1) / CD_TRACK_PADDING;
+		Offset += padded * CD_TRACK_PADDING;
 
 		tracks.push_back(t);
 	}