From db69aca1b49733da51f64ee857ac9b861b1c468c Mon Sep 17 00:00:00 2001
From: Pk11 <epicpkmn11@outlook.com>
Date: Mon, 21 Feb 2022 23:43:50 -0600
Subject: [PATCH] Fix save file FAT header generation

---
 arm9/src/sav.c | 154 ++++++++++++++++++++-----------------------------
 1 file changed, 62 insertions(+), 92 deletions(-)

diff --git a/arm9/src/sav.c b/arm9/src/sav.c
index 9eaba01..ed27cd2 100644
--- a/arm9/src/sav.c
+++ b/arm9/src/sav.c
@@ -2,51 +2,7 @@
 #include <string.h>
 #include <malloc.h>
 
-static u32 _getClusterSize(u32 sizebytes)
-{
-	if (sizebytes < 573440)
-		return 512;
-
-	else if (sizebytes < 5472256)
-		return 2048;
-
-	else if (sizebytes < 17301504)
-		return 4096;
-
-	else
-		return 2048;
-}
-
-static u16 _getMaxFiles(u32 sizebytes)
-{
-	if (sizebytes < 573440)
-		return 16;
-
-	else
-		return 256;
-}
-
-//wip
-static u16 _getFatz(u32 sizebytes)
-{
-	if (sizebytes <= 0x4000)		//16 kb
-		return 1;
-
-	else if (sizebytes <= 0x200000)	//2 mb
-		return 3;
-
-	else
-		return 6;
-}
-
-//wip
-static u16 _getTotSec16(u32 sizebytes, u16 bytesPerSec)
-{
-	if (sizebytes == 0x4000)	//16 kb
-		return 27;
-
-	return sizebytes / bytesPerSec;
-}
+#define align(v, a) (((v) % (a)) ? ((v) + (a) - ((v) % (a))) : (v))
 
 bool initFatHeader(FILE* f)
 {
@@ -57,61 +13,75 @@ bool initFatHeader(FILE* f)
 	fseek(f, 0, SEEK_END);
 	u32 size = ftell(f);
 
+	//based on GodMode9
+	//https://github.com/d0k3/GodMode9/blob/d8d43c14f3317423c677b1c7e0987bcb9bbd7299/arm9/source/game/nds.c#L47-L105
+	const u16 sectorSize = 0x200;
+
+	//fit maximum sectors for the size
+	const u16 maxSectors = size / sectorSize;
+	u16 sectorCount = 1;
+	u16 secPerTrk = 1;
+	u16 numHeads = 1;
+	u16 sectorCountNext = 0;
+	while (sectorCountNext <= maxSectors)
+	{
+		sectorCountNext = secPerTrk * (numHeads + 1) * (numHeads + 1);
+		if (sectorCountNext <= maxSectors)
+		{
+			numHeads++;
+			sectorCount = sectorCountNext;
+
+			secPerTrk++;
+			sectorCountNext = secPerTrk * numHeads * numHeads;
+			if (sectorCountNext <= maxSectors)
+			{
+				sectorCount = sectorCountNext;
+			}
+		}
+	}
+	sectorCountNext = (secPerTrk + 1) * numHeads * numHeads;
+	if (sectorCountNext <= maxSectors)
+	{
+		secPerTrk++;
+		sectorCount = sectorCountNext;
+	}
+
+	u8 secPerCluster = (sectorCount > (8 << 10)) ? 8 : (sectorCount > (1 << 10) ? 4 : 1);
+
+	u16 rootEntryCount = size < 0x8C000 ? 0x20 : 0x200;
+
+	u16 totalClusters = align(sectorCount, secPerCluster) / secPerCluster;
+	u32 fatBytes = (align(totalClusters, 2) / 2) * 3; // 2 sectors -> 3 byte
+	u16 fatSize = align(fatBytes, sectorSize) / sectorSize;
+
+
 	FATHeader* h = (FATHeader*)malloc(sizeof(FATHeader));
 
 	h->BS_JmpBoot[0] = 0xE9;
 	h->BS_JmpBoot[1] = 0;
 	h->BS_JmpBoot[2] = 0;
 
-	h->BS_OEMName[0] = 'M';
-	h->BS_OEMName[1] = 'S';
-	h->BS_OEMName[2] = 'W';
-	h->BS_OEMName[3] = 'I';
-	h->BS_OEMName[4] = 'N';
-	h->BS_OEMName[5] = '4';
-	h->BS_OEMName[6] = '.';
-	h->BS_OEMName[7] = '1';
+	memcpy(h->BS_OEMName, "MSWIN4.1", 8);
 
-	h->BPB_BytesPerSec = 512;
-	h->BPB_SecPerClus = _getClusterSize(size) / h->BPB_BytesPerSec;
-	h->BPB_RsvdSecCnt = 1;
-	h->BPB_NumFATs = 2;
-	h->BPB_RootEntCnt = _getMaxFiles(size) * 2;
-	h->BPB_TotSec16 = _getTotSec16(size, h->BPB_BytesPerSec);	//
-	h->BPB_Media = 0xF8;
-	h->BPB_FATSz16 = _getFatz(size);			//
-	h->BPB_SecPerTrk = 0;
-	h->BPB_NumHeads = 0;
-	h->BPB_HiddSec = 0;
-	h->BPB_TotSec32 = 0;
-	h->BS_DrvNum = 0x07;
-	h->BS_Reserved1 = 0;
+	h->BPB_BytesPerSec = sectorSize;
+	h->BPB_SecPerClus = secPerCluster;
+	h->BPB_RsvdSecCnt = 0x0001;
+	h->BPB_NumFATs = 0x02;
+	h->BPB_RootEntCnt = rootEntryCount;
+	h->BPB_TotSec16 = sectorCount;
+	h->BPB_Media = 0xF8; // "hard drive"
+	h->BPB_FATSz16 = fatSize;
+	h->BPB_SecPerTrk = secPerTrk;
+	h->BPB_NumHeads = numHeads;
+	h->BPB_HiddSec = 0x00000000;
+	h->BPB_TotSec32 = 0x00000000;
+	h->BS_DrvNum = 0x05;
+	h->BS_Reserved1 = 0x00;
 	h->BS_BootSig = 0x29;
-	h->BS_VolID = 305419896;
-
-	h->BS_VolLab[0] = 'V';
-	h->BS_VolLab[1] = 'O';
-	h->BS_VolLab[2] = 'L';
-	h->BS_VolLab[3] = 'U';
-	h->BS_VolLab[4] = 'M';
-	h->BS_VolLab[5] = 'E';
-	h->BS_VolLab[6] = 'L';
-	h->BS_VolLab[7] = 'A';
-	h->BS_VolLab[8] = 'B';
-	h->BS_VolLab[9] = 'E';
-	h->BS_VolLab[10] = 'L';
-
-	h->BS_FilSysType[0] = 'F';
-	h->BS_FilSysType[1] = 'A';
-	h->BS_FilSysType[2] = 'T';
-	h->BS_FilSysType[3] = '1';
-	h->BS_FilSysType[4] = '2';
-	h->BS_FilSysType[5] = ' ';
-	h->BS_FilSysType[6] = ' ';
-	h->BS_FilSysType[7] = ' ';
-
-	memset(h->BS_BootCode, 0, 448);
-
+	h->BS_VolID = 0x12345678;
+	memcpy(h->BS_VolLab, "VOLUMELABEL", 11);
+	memcpy(h->BS_FilSysType,"FAT12   ", 8);
+	memset(h->BS_BootCode, 0, sizeof(h->BS_BootCode));
 	h->BS_BootSign = 0xAA55;
 
 	fseek(f, 0, SEEK_SET);