#include "fat.h"
#include "string.h"
#include <stdint.h>

typedef struct {
    char jmp[3];
    char oemname[8];

    uint16_t bytes_per_sect;
    uint8_t sect_per_cluster;
    uint16_t num_reserved_sectors;
    uint8_t num_FAT;
    uint16_t max_root_dir_entries;
    uint16_t total_log_sectors;
    uint8_t media_desc;
    uint16_t log_sect_per_fat;
} __attribute__((packed))  BOOT_SECT;

typedef struct {
    char name[8];
    char ext[3];
    uint8_t file_attr;
    uint8_t user_attr;
    uint8_t del_char;
    uint16_t create_time;
    uint16_t create_date;
    uint16_t last_access_date;
    uint16_t access_rights;
    uint16_t last_modified_time;
    uint16_t last_modified_date;
    uint16_t start_of_clusters;
    uint32_t file_size;
} __attribute__((packed)) DIR_ENTRY;

typedef struct {
    uint8_t seq_num;
    uint16_t part_1[5];
    uint8_t file_attr;
    uint8_t user_attr;
    uint8_t del_char;
    uint16_t part_2[6];
    uint16_t start_of_clusters;
    uint16_t part_3[2];
} __attribute__((packed)) VFAT_LFN_ENTRY;


void *FAT_addr;
BOOT_SECT *boot_sect;

uint32_t FAT1_lba;
uint32_t FAT2_lba;
uint32_t root_lba;
uint32_t data_lba;

void FAT_init() {
    // open image file
    FAT_addr = (void*)(0x10000);

    boot_sect = (BOOT_SECT*)(FAT_addr);

    FAT1_lba = boot_sect->num_reserved_sectors;
    FAT2_lba = FAT1_lba + boot_sect->log_sect_per_fat;
    root_lba = FAT1_lba + (boot_sect->log_sect_per_fat * boot_sect->num_FAT);

    long root_dir_size = boot_sect->max_root_dir_entries * sizeof(DIR_ENTRY);
    data_lba = root_lba + (root_dir_size / boot_sect->bytes_per_sect);
}

void *FAT_read_sector_offset(uint32_t lba, uint32_t *offset) {
    if (offset != NULL) {
        lba += (*offset) / boot_sect->bytes_per_sect;
        *offset = (*offset) % boot_sect->bytes_per_sect;
    }

    return (void*)((long)(FAT_addr) + (long)(lba * boot_sect->bytes_per_sect));
}

void *FAT_read_cluster(uint16_t cluster, void *buffer) {
    // TODO: perform memcpy
    void *addr = FAT_read_sector_offset(data_lba + ((cluster - 2) * boot_sect->sect_per_cluster), NULL);

    uint32_t cluster_size = boot_sect->bytes_per_sect * boot_sect->sect_per_cluster;

    memcpy(buffer, addr, cluster_size);

    return (void*)((uint8_t*)(buffer) + cluster_size);
}

int FAT_root_dir_next(int index, char *fname_buffer, uint16_t *first_cluster, uint32_t *file_size) {
    memset(fname_buffer, 0, sizeof(fname_buffer));

    while (1) {
        // index overflow
        if (index >= boot_sect->max_root_dir_entries) {
            return -1;
        }

        uint32_t offset = index * sizeof(DIR_ENTRY);
        void *sect = FAT_read_sector_offset(root_lba, &offset);
        DIR_ENTRY *dir_entry = (DIR_ENTRY *)((uint32_t)(sect) + offset);

        // if first character of name is 0, then no subsequent entry is in use
        if (dir_entry->name[0] == 0x00) { 
            return -1; 
        }

        // deleted file
        if (dir_entry->name[0] == (char)(0xE5)) {
            index++;
            continue;
        }

        // VFAT LFN entry
        if (dir_entry->file_attr == 0x0F && dir_entry->start_of_clusters == 0 && dir_entry->file_size != 0) {
            VFAT_LFN_ENTRY *lfn_entry = (VFAT_LFN_ENTRY*)(dir_entry);

            int offset = 13 * ((lfn_entry->seq_num & 0x3F) - 1);

            // read long file name
            for (int i = 0; i < 5; i++) {
                fname_buffer[offset++] = lfn_entry->part_1[i];
            }

            for (int i = 0; i < 6; i++) {
                fname_buffer[offset++] = lfn_entry->part_2[i];
            }

            for (int i = 0; i < 2; i++) {
                fname_buffer[offset++] = lfn_entry->part_3[i];
            }

            index++;
            continue;
        }

        if (index == 0 && (dir_entry->file_attr & 0x08) && dir_entry->file_size == 0) {
        // volume label

        } else if ((dir_entry->file_attr & 0x10) && dir_entry->file_size == 0) {
        // subdirectory

        } else {
        // regular file

        }

        *file_size = dir_entry->file_size;
        *first_cluster = dir_entry->start_of_clusters;

        // if no VFAT LFN exists, use DOS name
        if (fname_buffer[0] == 0) {
            for (int i = 0; i < 8; i++) {
                fname_buffer[i] = dir_entry->name[i];
            }
            fname_buffer[8] = '.';
            for (int i = 0; i < 3; i++) {
                fname_buffer[i + 9] = dir_entry->ext[i];
            }
            fname_buffer[12] = 0;
        }

        return index + 1;
    }
}

uint16_t FAT_next_cluster(uint16_t cluster) {
    // assuming FAT12
    uint32_t offset = (cluster >> 1) * 3;
    uint8_t *sect = FAT_read_sector_offset(FAT1_lba, &offset);
    sect += offset;

    if (cluster & 0x01) {
        uint16_t high = (uint16_t)(sect[2]);
        uint16_t low = (uint16_t)(sect[1] & 0xF0) >> 4;
        return (high << 4) | low;
    } else {
        uint16_t low = (uint16_t)(sect[0]);
        uint16_t high = (uint16_t)(sect[1] & 0x0F) << 8;
        return low | high;
    }
}