//! FAT table: cluster allocation and chains.

use super::Fat16;
use super::SECTOR_SIZE;
use port_block_device::{BlockDevice, SectorId};

impl<B: BlockDevice> Fat16<B> {
    pub(super) fn read_fat_entry(&mut self, cluster: u16) -> u16 {
        let fat_offset = (cluster as u32) * 2;
        let sector = self.bpb.fat_start() + fat_offset / (SECTOR_SIZE as u32);
        let offset = (fat_offset % (SECTOR_SIZE as u32)) as usize;
        let mut buf = [0u8; SECTOR_SIZE];
        if !self.device.read_sector(SectorId::new(sector), &mut buf) {
            return 0xFFFF;
        }
        u16::from_le_bytes([buf[offset], buf[offset + 1]])
    }

    pub(super) fn write_fat_entry(&mut self, cluster: u16, value: u16) {
        let fat_offset = (cluster as u32) * 2;
        let sector = self.bpb.fat_start() + fat_offset / (SECTOR_SIZE as u32);
        let offset = (fat_offset % (SECTOR_SIZE as u32)) as usize;
        let mut buf = [0u8; SECTOR_SIZE];
        self.device.read_sector(SectorId::new(sector), &mut buf);
        buf[offset] = value as u8;
        buf[offset + 1] = (value >> 8) as u8;
        self.device.write_sector(SectorId::new(sector), &buf);
    }

    pub(super) fn alloc_cluster(&mut self) -> Option<u16> {
        let max = self.bpb.total_sectors as u16 / self.bpb.sectors_per_cluster as u16;
        for i in 2..max {
            if self.read_fat_entry(i) == 0 {
                self.write_fat_entry(i, 0xFFFF);
                return Some(i);
            }
        }
        None
    }

    pub(super) fn free_chain(&mut self, start: u16) {
        let mut cluster = start;
        loop {
            if cluster < 2 || cluster >= 0xFFF8 {
                break;
            }
            let next = self.read_fat_entry(cluster);
            self.write_fat_entry(cluster, 0);
            cluster = next;
        }
    }

    pub(super) fn follow_chain(&mut self, start: u16, skip_clusters: usize) -> u16 {
        let mut cluster = start;
        for _ in 0..skip_clusters {
            let next = self.read_fat_entry(cluster);
            if next >= 0xFFF8 {
                return cluster;
            }
            cluster = next;
        }
        cluster
    }
}