From 817e654e745ebf34399db7264c0816c72192b1bf Mon Sep 17 00:00:00 2001
From: Yann MAGNIN <yann.magnin@protonmail.com>
Date: Fri, 11 Apr 2025 12:46:45 +0200
Subject: [PATCH] fygue: fix FAT12 cluster handling + fix file seek() handling

---
 src/fs/fygue/fat/cluster.c       | 22 ++++++++++------------
 src/fs/fygue/fat/fat_file_read.c |  7 +++++++
 src/fs/fygue/fat/fat_file_seek.c | 16 ++++++++--------
 3 files changed, 25 insertions(+), 20 deletions(-)

diff --git a/src/fs/fygue/fat/cluster.c b/src/fs/fygue/fat/cluster.c
index 71f11a0..409177b 100644
--- a/src/fs/fygue/fat/cluster.c
+++ b/src/fs/fygue/fat/cluster.c
@@ -34,7 +34,6 @@ static int _fygue_fat16_cluster_get_next(
 }
 
 /* _fygue_fat12_cluster_get_next(): FAT12 cluster handling */
-// TODO: assume fixed sector size to 512
 // TODO: better calculus
 static int _fygue_fat12_cluster_get_next(
     struct fygue_fat *fat,
@@ -51,39 +50,38 @@ static int _fygue_fat12_cluster_get_next(
 
     fat_index0  = *cluster + (*cluster / 2);
     fat_index1  = fat_index0 + 1;
-    fat_sector0 = fat->FAT0SectorID + (fat_index0 / fat->SectorSize);
-    fat_sector1 = fat->FAT0SectorID + (fat_index1 / fat->SectorSize);
+    fat_sector0 = fat->FAT0SectorID + (fat_index0 / 512);
+    fat_sector1 = fat->FAT0SectorID + (fat_index1 / 512);
 
     if (fat_sector0 == fat_sector1) {
         if (fygue_fat_sector_get_addr(fat, &fat_data0, fat_sector0) != 0)
             return -1;
         fat_data1 = fat_data0;
-        fat_offset0 = fat_index0 - (fat_sector0 * fat->SectorSize);
+        fat_offset0 = fat_index0 - ((fat_index0 / 512) * 512);
         fat_offset1 = fat_offset0 + 1;
     } else {
         if (fygue_fat_sector_get_addr(fat, &fat_data0, fat_sector0) != 0)
             return -1;
         if (fygue_fat_sector_get_addr(fat, &fat_data1, fat_sector1) != 0)
             return -1;
-        fat_offset0 = fat_index0 - (fat_sector0 * fat->SectorSize);
-        fat_offset1 = fat_index1 - (fat_sector1 * fat->SectorSize);
+        fat_offset0 = fat_index0 - ((fat_index0 / 512) * 512);
+        fat_offset1 = fat_index1 - ((fat_index1 / 512) * 512);
     }
 
     if (*cluster & 1) {
         *cluster = (
-            ((((uint8_t*)fat_data0)[fat_offset0] & 0xf0) >> 4) |
-            ((((uint8_t*)fat_data1)[fat_offset1] & 0xff) << 4)
+            (((((uint8_t*)fat_data0)[fat_offset0] & 0xf0) >> 4) << 0) |
+            (((((uint8_t*)fat_data1)[fat_offset1] & 0xff) >> 0) << 4)
         );
     } else {
         *cluster = (
-            ((((uint8_t*)fat_data0)[fat_offset0] & 0xff) >> 0) |
-            ((((uint8_t*)fat_data1)[fat_offset1] & 0xf0) << 8)
+            (((((uint8_t*)fat_data0)[fat_offset0] & 0xff) >> 0) << 0) |
+            (((((uint8_t*)fat_data1)[fat_offset1] & 0x0f) >> 0) << 8)
         );
     }
-
     if (*cluster == 0x0fff)
         *cluster = 0xffff;
-    return -1;
+    return 0;
 }
 
 //---
diff --git a/src/fs/fygue/fat/fat_file_read.c b/src/fs/fygue/fat/fat_file_read.c
index 034714f..07639b9 100644
--- a/src/fs/fygue/fat/fat_file_read.c
+++ b/src/fs/fygue/fat/fat_file_read.c
@@ -2,6 +2,10 @@
 #include <string.h>
 #include "fat.h"
 
+uintptr_t debug_read_addr = 0;
+size_t debug_read_size = 0;
+int debug_read_off = 0;
+
 /* fygue_fat_file_read(): read primitive */
 int fygue_fat_file_read(
     struct fygue_fat *fat,
@@ -30,6 +34,9 @@ int fygue_fat_file_read(
         chunk_data_available -= chunk_data_offset;
         if (read + chunk_data_available > size)
         {
+            debug_read_addr = chunk_data_addr;
+            debug_read_size = size - read;
+            debug_read_off = chunk_data_offset;
             chunk_data_offset += size - read;
             memcpy(buffer, (void*)chunk_data_addr, size - read);
             read += size - read;
diff --git a/src/fs/fygue/fat/fat_file_seek.c b/src/fs/fygue/fat/fat_file_seek.c
index c9b3c37..b8da951 100644
--- a/src/fs/fygue/fat/fat_file_seek.c
+++ b/src/fs/fygue/fat/fat_file_seek.c
@@ -7,20 +7,20 @@ int fygue_fat_file_seek(
     struct fygue_fat_file *file,
     off_t offset
 ) {
-    off_t offset_test;
+    off_t offset_current;
 
-    if (fat == NULL || file == NULL) {
-        errno = EIO;
+    if (fat == NULL || file == NULL)
         return -1;
-    }
-    offset_test = 0;
+    offset_current = 0;
     file->chunk_rd_index = 0;
     file->chunk_rd_offset = 0;
     for (int i = 0; i < file->chunk_count; i++)
     {
-        offset_test += file->chunk[i].size;
-        if (offset_test > offset) {
-            file->chunk_rd_offset = offset - offset_test;
+        offset_current += file->chunk[i].size;
+        if (offset_current > offset) {
+            file->chunk_rd_offset = (
+                file->chunk[i].size - (offset_current - offset)
+            );
             file->cursor = offset;
             return 0;
         }