#include "fat12.h"

#include <string.h>

namespace esphome {
namespace grub_boot_switch {

static inline uint8_t fat_checksum(const char *name) {
  uint8_t s = name[0];
  s = (s << 7) + (s >> 1) + name[1];
  s = (s << 7) + (s >> 1) + name[2];
  s = (s << 7) + (s >> 1) + name[3];
  s = (s << 7) + (s >> 1) + name[4];
  s = (s << 7) + (s >> 1) + name[5];
  s = (s << 7) + (s >> 1) + name[6];
  s = (s << 7) + (s >> 1) + name[7];
  s = (s << 7) + (s >> 1) + name[8];
  s = (s << 7) + (s >> 1) + name[9];
  s = (s << 7) + (s >> 1) + name[10];
  return s;
}

static uint8_t write_utf16_chars(const char *name, uint8_t *index, uint8_t length, uint8_t write_count,
                                 uint8_t *output) {
  for (uint8_t i = write_count; i != 0; --i) {
    if (*index <= length) {
      *output++ = name[*index];
      *output++ = 0x00;
      ++(*index);
    } else {
      *output++ = 0xFF;
      *output++ = 0xFF;
    }
  }
  return write_count * 2;
}

uint8_t fat_write_lfn(const char *name, const FatDirEntry *direntry, uint8_t *output) {
  constexpr uint8_t kCharsPerEntry = 13;

  const size_t length = strlen(name);
  const uint8_t num_entries = (length + (kCharsPerEntry - 1)) / kCharsPerEntry;
  const uint8_t checksum = fat_checksum(reinterpret_cast<const char *>(direntry));

  for (uint8_t ordinal = num_entries; ordinal != 0; --ordinal) {
    uint8_t index = (ordinal - 1) * kCharsPerEntry;

    *output = ordinal | (kFat_LastLFN * (ordinal == num_entries));
    ++output;

    output += write_utf16_chars(name, &index, length, 5, output);

    output[0] = kFatAttr_LongFileName;
    output[1] = 0x00;
    output[2] = checksum;
    output += 3;

    output += write_utf16_chars(name, &index, length, 6, output);

    output[0] = 0x00;
    output[1] = 0x00;
    output += 2;

    output += write_utf16_chars(name, &index, length, 2, output);
  }

  return sizeof(FatDirEntry) * num_entries;
}

uint8_t fat_write_dir(const FatDirEntry *direntry, uint8_t *output) {
  memcpy(output, direntry, sizeof(FatDirEntry));
  return sizeof(FatDirEntry);
}

uint16_t fat_date(uint16_t year, uint8_t month, uint8_t day) {
  return ((year - 1980) << 9) | (month << 5) | day;
}

}  // namespace grub_boot_switch
}  // namespace esphome