text: #include <fs/fat32.h>
  #include <lib/blib.h>
  #include <drivers/disk.h>
  #include <lib/libc.h>
  #include <lib/print.h>
  #include <stdbool.h>
  
  #define FAT32_LFN_MAX_ENTRIES 20
  #define FAT32_LFN_MAX_FILENAME_LENGTH (FAT32_LFN_MAX_ENTRIES * 13 + 1)
  
  #define FAT32_VALID_SIGNATURE_1 0x28
  #define FAT32_VALID_SIGNATURE_2 0x29
  #define FAT32_VALID_SYSTEM_IDENTIFIER "FAT32   "
  #define FAT32_SECTOR_SIZE 512
  #define FAT32_ATTRIBUTE_SUBDIRECTORY 0x10
  #define FAT32_LFN_ATTRIBUTE 0x0F
  
  struct fat32_bpb {
      uint8_t jump[3];
      char oem[8];
      uint16_t bytes_per_sector;
      uint8_t sectors_per_cluster;
      uint16_t reserved_sectors;
      uint8_t fats_count;
      uint16_t directory_entries_count;
      uint16_t sector_totals;
      uint8_t media_descriptor_type;
      uint16_t sectors_per_fat_16;
      uint16_t sectors_per_track;
      uint16_t heads_count;
      uint32_t hidden_sectors_count;
      uint32_t large_sectors_count;
      uint32_t sectors_per_fat_32;
      uint16_t flags;
      uint16_t fat_version_number;
      uint32_t root_directory_cluster;
      uint16_t fs_info_sector;
      uint16_t backup_boot_sector;
      uint8_t reserved[12];
      uint8_t drive_number;
      uint8_t nt_flags;
      uint8_t signature;
      uint32_t volume_serial_number;
      char label[11];
      char system_identifier[8];
  } __attribute__((packed));
  
  struct fat32_directory_entry {
      char file_name_and_ext[8 + 3];
      uint8_t attribute;
      uint8_t file_data_1[8];
      uint16_t cluster_num_high;
      uint8_t file_data_2[4];
      uint16_t cluster_num_low;
      uint32_t file_size_bytes;
  } __attribute__((packed));
  
  struct fat32_lfn_entry {
      uint8_t sequence_number;
      char name1[10];
      uint8_t attribute;
      uint8_t type;
      uint8_t dos_checksum;
      char name2[12];
      uint16_t first_cluster;
      char name3[4];
  } __attribute__((packed));
  
  static int fat32_init_context(struct fat32_context* context, struct part *part) {
      context->part  = *part;
      context->drive = part->drive;
  
      struct fat32_bpb bpb;
      part_read(&context->part, &bpb, 0, sizeof(struct fat32_bpb));
  
      if (bpb.signature != FAT32_VALID_SIGNATURE_1 && bpb.signature != FAT32_VALID_SIGNATURE_2) {
          return 1;
      }
  
      if (strncmp(bpb.system_identifier, FAT32_VALID_SYSTEM_IDENTIFIER, SIZEOF_ARRAY(bpb.system_identifier)) != 0) {
          return 1;
      }
  
      context->sectors_per_cluster = bpb.sectors_per_cluster;
      context->reserved_sectors = bpb.reserved_sectors;
      context->number_of_fats = bpb.fats_count;
      context->hidden_sectors = bpb.hidden_sectors_count;
      context->sectors_per_fat = bpb.sectors_per_fat_32;
      context->root_directory_cluster = bpb.root_directory_cluster;
      context->fat_start_lba = bpb.reserved_sectors;
      context->data_start_lba = context->fat_start_lba + bpb.fats_count * bpb.sectors_per_fat_32;
  
      return 0;
  }
  
  static int fat32_read_cluster_from_map(struct fat32_context* context, uint32_t cluster, uint32_t* out) {
      const uint32_t sector = cluster / (FAT32_SECTOR_SIZE / 4);
      const uint32_t offset = cluster % (FAT32_SECTOR_SIZE / 4);
  
      uint32_t clusters[FAT32_SECTOR_SIZE / sizeof(uint32_t)];
      int r = part_read(&context->part, &clusters[0], (context->fat_start_lba + sector) * FAT32_SECTOR_SIZE, sizeof(clusters));
  
      if (r) {
          return r;
      }
  
      *out = clusters[offset] & 0x0FFFFFFF;
      return 0;
  }
  
  static int fat32_load_fat_cluster_to_memory(struct fat32_context* context, uint32_t cluster_number, void* buffer, uint32_t offset, uint32_t limit) {
      const uint32_t sector = context->data_start_lba + (cluster_number - 2) * context->sectors_per_cluster;
      return part_read(&context->part, buffer, ((uint64_t) sector) * FAT32_SECTOR_SIZE + offset, limit);
  }
  
  // Copy ucs-2 characters to char*
  static void fat32_lfncpy(char* destination, const void* source, unsigned int size) {
      for (unsigned int i = 0; i < size; i++) {
          // ignore high bytes
          *(((uint8_t*) destination) + i) = *(((uint8_t*) source) + (i * 2));
      }
  }
  
  static bool fat32_filename_to_8_3(char *dest, const char *src) {
      int i = 0, j = 0;
      bool ext = false;
  
      while (src[i]) {
          if (src[i] == '.') {
              if (ext) {
                  // This is a double extension here, just give up.
                  return false;
              }
              ext = true;
              // Pad the rest of the base filename with spaces
              while (j < 8)
                  dest[j++] = ' ';
              i++;
              continue;
          }
          if (j >= 8+3 || (j >= 8 && !ext)) {
              // Filename too long, give up.
              return false;
          }
          dest[j++] = toupper(src[i++]);
      }
  
      return true;
  }
  
  static int fat32_open_in(struct fat32_context* context, struct fat32_directory_entry* directory, struct fat32_directory_entry* file, const char* name) {
      int error;
      uint32_t current_cluster_number = directory->cluster_num_high << 16 | directory->cluster_num_low;
  
      char current_lfn[FAT32_LFN_MAX_FILENAME_LENGTH] = {0};
  
      do {
          for (size_t sector_in_cluster = 0; sector_in_cluster < context->sectors_per_cluster; sector_in_cluster++) {
              struct fat32_directory_entry directory_entries[FAT32_SECTOR_SIZE / sizeof(struct fat32_directory_entry)];
              error = fat32_load_fat_cluster_to_memory(context, current_cluster_number, directory_entries, sector_in_cluster * FAT32_SECTOR_SIZE, sizeof(directory_entries));
              if (error != 0) {
                  return error;
              }
  
              for (unsigned int i = 0; i < SIZEOF_ARRAY(directory_entries); i++) {
                  if (directory_entries[i].file_name_and_ext[0] == 0x00) {
                      // no more entries here
                      break;
                  }
  
                  if (directory_entries[i].attribute == FAT32_LFN_ATTRIBUTE) {
                      struct fat32_lfn_entry* lfn = (struct fat32_lfn_entry*) &directory_entries[i];
  
                      if (lfn->sequence_number & 0b01000000) {
                          // this lfn is the first entry in the table, clear the lfn buffer
                          memset(current_lfn, ' ', sizeof(current_lfn));
                      }
  
                      const unsigned int lfn_index = ((lfn->sequence_number & 0b00011111) - 1U) * 13U;
                      if (lfn_index >= FAT32_LFN_MAX_ENTRIES * 13) {
                          continue;
                      }
  
                      fat32_lfncpy(current_lfn + lfn_index + 00, lfn->name1, 5);
                      fat32_lfncpy(current_lfn + lfn_index + 05, lfn->name2, 6);
                      fat32_lfncpy(current_lfn + lfn_index + 11, lfn->name3, 2);
  
                      if (lfn_index != 0)
                          continue;
  
                      // remove trailing spaces
                      for (int j = SIZEOF_ARRAY(current_lfn) - 2; j >= -1; j--) {
                          if (j == -1 || current_lfn[j] != ' ') {
                              current_lfn[j + 1] = 0;
                              break;
                          }
                      }
  
                      if (!strcmp(current_lfn, name)) {
                          *file = directory_entries[i+1];
                          return 0;
                      }
                  } else {
                      char fn[8+3];
                      if (fat32_filename_to_8_3(fn, name) && !strncmp(directory_entries[i].file_name_and_ext, fn, 8+3)) {
                          *file = directory_entries[i];
                          return 0;
                      }
                  }
              }
          }
  
          error = fat32_read_cluster_from_map(context, current_cluster_number, &current_cluster_number);
  
          if (error != 0) {
              return error;
          }
      } while (current_cluster_number >= 0x00000002 && current_cluster_number <= 0x0FFFFFEF);
  
      // file not found
      return -1;
  }
  
  int fat32_check_signature(struct part *part) {
      struct fat32_context context;
      return fat32_init_context(&context, part) == 0;
  }
  
  int fat32_open(struct fat32_file_handle* ret, struct part *part, const char* path) {
      struct fat32_context context;
      int r = fat32_init_context(&context, part);
  
      if (r) {
          print("fat32: context init failure (%d)\n", r);
          return r;
      }
  
      struct fat32_directory_entry current_directory;
      struct fat32_directory_entry current_file;
      unsigned int current_index = 0;
      char current_part[FAT32_LFN_MAX_FILENAME_LENGTH];
  
      // skip trailing slashes
      while (path[current_index] == '/') {
          current_index++;
      }
  
      // walk down the directory tree
      current_directory.cluster_num_low = context.root_directory_cluster & 0xFFFF;
      current_directory.cluster_num_high = context.root_directory_cluster >> 16;
  
      for (;;) {
          bool expect_directory = false;
  
          for (unsigned int i = 0; i < SIZEOF_ARRAY(current_part); i++) {
              if (path[i + current_index] == 0) {
                  memcpy(current_part, path + current_index, i);
                  current_part[i] = 0;
                  expect_directory = false;
                  break;
              }
  
              if (path[i + current_index] == '/') {
                  memcpy(current_part, path + current_index, i);
                  current_part[i] = 0;
                  current_index += i + 1;
                  expect_directory = true;
                  break;
              }
          }
  
          if ((r = fat32_open_in(&context, &current_directory, &current_file, current_part)) != 0) {
              print("fat32: file %s not found\n", path);
              return r;
          }
  
          if (expect_directory) {
              current_directory = current_file;
          } else {
              ret->context = context;
              ret->first_cluster = current_file.cluster_num_high << 16 | current_file.cluster_num_low;
              ret->size_clusters = DIV_ROUNDUP(current_file.file_size_bytes, FAT32_SECTOR_SIZE);
              ret->size_bytes = current_file.file_size_bytes;
              return 0;
          }
      }
  }
  
  int fat32_read(struct fat32_file_handle* file, void* buf, uint64_t loc, uint64_t count) {
      int r;
      uint32_t cluster_size = file->context.sectors_per_cluster * FAT32_SECTOR_SIZE;
      uint32_t current_cluster_number = file->first_cluster;
  
      // skip first clusters
      while (loc >= cluster_size) {
          r = fat32_read_cluster_from_map(&file->context, current_cluster_number, &current_cluster_number);
  
          if (r != 0) {
              print("fat32: failed to read cluster %x from map\n", current_cluster_number);
              return r;
          }
  
          loc -= cluster_size;
      }
  
      uint64_t read_total = 0;
  
      do {
          // find non-fragmented cluster chains to improve read performance
          uint32_t non_fragmented_clusters = 1;
          for (size_t i = 0 ; i < count / cluster_size; i++) {
              uint32_t next_cluster;
  
              r = fat32_read_cluster_from_map(&file->context, current_cluster_number + i, &next_cluster);
  
              if (r != 0) {
                  print("fat32: failed to read cluster %x from map\n", current_cluster_number);
                  return r;
              }
  
              if (next_cluster != current_cluster_number + i + 1) {
                  break;
              }
  
              non_fragmented_clusters++;
          }
  
          // find largest read size
          uint64_t current_read = count;
          if (current_read > non_fragmented_clusters * cluster_size - loc) {
              current_read = non_fragmented_clusters * cluster_size - loc;
          }
  
          r = fat32_load_fat_cluster_to_memory(&file->context, current_cluster_number, buf + read_total, loc, current_read);
  
          if (r != 0) {
              print("fat32: failed to load cluster %x to memory\n", current_cluster_number);
              return r;
          }
  
          loc = 0;
          count -= current_read;
          read_total += current_read;
  
          if (count == 0) {
              return 0;
          }
  
          // fetch next cluster number
          r = fat32_read_cluster_from_map(&file->context, current_cluster_number, &current_cluster_number);
  
          if (r != 0) {
              print("fat32: failed to read cluster %x from map\n", current_cluster_number);
              return r;
          }
      } while (current_cluster_number >= 0x00000002 && current_cluster_number <= 0x0FFFFFEF);
  
      print("fat32: read failed, unexpected end of cluster chain\n");
      return 0;
  }
  