# on disk layout for the 16 mib test disk (32768 sectors):
#   sector 0        superblock (magic + total sectors + root dir block)
#   sectors 1..128  fat, one 2 byte entry per block on the whole disk
#   sectors 129..   data, used for both file bytes and directory entry blocks
#
# a directory is just a file whose data blocks hold 32 byte entries. the
# superblock points at the root dir, paths are walked from there. a fat entry
# points at the next block in a chain, 0 is free, END is the last

const SB_LBA = 0
const FAT_LBA = 1
const FAT_SECTORS = 128
const DATA_LBA = 129
const BLOCK = 512
const ENTS = 16          # 32 byte entries per 512 byte block
const NAME_MAX = 24
const MAGIC = 0x32465652 # "RVF2" little endian
const END = 0xffff       # end of chain marker in the fat
const T_FILE = 1
const T_DIR = 2

fn split_path(path) do
  const raw_parts = path:split("/")
  let parts = {}
  for index in 0..raw_parts:len() do
    if raw_parts[index]:len() > 0 parts:push(raw_parts[index])
  end
  return parts
end

# scratch is a spare sector for superblocks/dirs/data, fat is a separate scratch
# so fat lookups dont trample a dir block were mid edit.
global fs = {
  scratch = 0, fat = 0, ready = :false, total = 0, root = 0,

  fat_get = fn(self, block) do
    disk_read(FAT_LBA + (block shr 8), self.fat)
    mem_read(self.fat + (block band 255) * 2, 2)
  end,

  fat_set = fn(self, block, value) do
    const sector = FAT_LBA + (block shr 8)
    disk_read(sector, self.fat)
    mem_write(self.fat + (block band 255) * 2, value, 2)
    disk_write(sector, self.fat)
  end,

  fat_alloc = fn(self) do
    let block = DATA_LBA
    while block < self.total do
      disk_read(FAT_LBA + (block shr 8), self.fat)
      let offset = block band 255
      while (offset < 256 and block < self.total) do
        if mem_read(self.fat + offset * 2, 2) == 0 return block
        offset += 1
        block += 1
      end
    end
    0
  end,

  free_chain = fn(self, first) do
    let block = first
    while (block != 0 and block != END) do
      const next_block = self:fat_get(block)
      self:fat_set(block, 0)
      block = next_block
    end
  end,

  # entry is 32 bytes:
  # +0 type (0 free/1 file/2 dir)
  # +1 name len
  # +2 first block
  # +4 size
  # +8 name bytes
  ent_name = fn(self, entry_offset) do
    const name_length = mem_read(self.scratch + entry_offset + 1, 1)
    let result = ""
    for index in 0..name_length do result = result ~ string.of_ascii(mem_read(self.scratch + entry_offset + 8 + index, 1)) end
    result
  end,

  ent_write = fn(self, entry_offset, type, name, first, size) do
    mem_write(self.scratch + entry_offset, type, 1)
    mem_write(self.scratch + entry_offset + 1, name:len(), 1)
    mem_write(self.scratch + entry_offset + 2, first, 2)
    mem_write(self.scratch + entry_offset + 4, size, 4)
    for index in 0..name:len() do mem_write(self.scratch + entry_offset + 8 + index, name[index]:ascii(), 1) end
  end,

  dir_lookup = fn(self, dir_first, name) do
    let block = dir_first
    while (block != 0 and block != END) do
      const next_block = self:fat_get(block)
      disk_read(block, self.scratch)
      for slot in 0..ENTS do
        const entry_offset = slot * 32
        if mem_read(self.scratch + entry_offset, 1) != 0 and self:ent_name(entry_offset) == name do
          return { type = mem_read(self.scratch + entry_offset, 1), first = mem_read(self.scratch + entry_offset + 2, 2), size = mem_read(self.scratch + entry_offset + 4, 4) }
        end
      end
      block = next_block
    end
    return :nil
  end,

  # add or overwrite `name` in the dir
  # grows a block if every slot is taken
  dir_set = fn(self, dir_first, name, type, first, size) do
    let block = dir_first
    let found_block = 0
    let found_offset = 0
    let free_block = 0
    let free_offset = -1
    let last_block = dir_first
    while (block != 0 and block != END) do
      last_block = block
      const next_block = self:fat_get(block)
      disk_read(block, self.scratch)
      for slot in 0..ENTS do
        const entry_offset = slot * 32
        if mem_read(self.scratch + entry_offset, 1) == 0 do
          if free_offset < 0 do free_block = block free_offset = entry_offset end
        end
        else do
          if self:ent_name(entry_offset) == name do found_block = block found_offset = entry_offset end
        end
      end
      block = next_block
    end

    let target_block = 0
    let target_offset = 0
    if found_block != 0 do target_block = found_block target_offset = found_offset end
    else if free_offset >= 0 do target_block = free_block target_offset = free_offset end
    else do
      const new_block = self:fat_alloc()
      if new_block == 0 return :false
      self:fat_set(new_block, END)
      self:fat_set(last_block, new_block)
      mem_zero(self.scratch, BLOCK)
      disk_write(new_block, self.scratch)
      target_block = new_block
      target_offset = 0
    end

    disk_read(target_block, self.scratch)
    self:ent_write(target_offset, type, name, first, size)
    disk_write(target_block, self.scratch)
    return :true
  end,

  dir_clear = fn(self, dir_first, name) do
    let block = dir_first
    while (block != 0 and block != END) do
      const next_block = self:fat_get(block)
      disk_read(block, self.scratch)
      for slot in 0..ENTS do
        const entry_offset = slot * 32
        if mem_read(self.scratch + entry_offset, 1) != 0 and self:ent_name(entry_offset) == name do
          const info = { type = mem_read(self.scratch + entry_offset, 1), first = mem_read(self.scratch + entry_offset + 2, 2) }
          mem_write(self.scratch + entry_offset, 0, 1)
          disk_write(block, self.scratch)
          return info
        end
      end
      block = next_block
    end
    return :nil
  end,

  dir_empty = fn(self, dir_first) do
    let block = dir_first
    while (block != 0 and block != END) do
      const next_block = self:fat_get(block)
      disk_read(block, self.scratch)
      for slot in 0..ENTS do
        if mem_read(self.scratch + slot * 32, 1) != 0 return :false
      end
      block = next_block
    end
    return :true
  end,

  resolve = fn(self, path) do
    const parts = split_path(path)
    let current = self.root
    let entry = { type = T_DIR, first = self.root, size = 0 }
    for index in 0..parts:len() do
      if entry.type != T_DIR return :nil
      const found = self:dir_lookup(current, parts[index])
      if found == :nil return :nil
      entry = found
      current = found.first
    end
    return entry
  end,

  resolve_parent = fn(self, path) do
    const parts = split_path(path)
    if parts:len() == 0 return :nil
    let parent_dir = self.root
    for index in 0..(parts:len() - 1) do
      const found = self:dir_lookup(parent_dir, parts[index])
      if found == :nil or found.type != T_DIR return :nil
      parent_dir = found.first
    end
    return { dir = parent_dir, name = parts[parts:len() - 1] }
  end,

  mount = fn(self) do
    self.scratch = frame_alloc()
    self.fat = frame_alloc()
    self.total = disk_sectors()
    if self.total == 0 do self.ready = :false return :nil end
    disk_read(SB_LBA, self.scratch)
    if mem_read(self.scratch, 4) == MAGIC do
      self.root = mem_read(self.scratch + 8, 2)
      self.ready = :true
    end
    else do self.ready = :false end
  end,

  ensure = fn(self) do
    if self.ready return :true
    puts("no filesystem, run mkfs first\n")
    :false
  end,

  format = fn(self) do
    if disk_sectors() == 0 do puts("mkfs: no disk\n") return :nil end
    mem_zero(self.scratch, BLOCK)
    for sector in 0..FAT_SECTORS do disk_write(FAT_LBA + sector, self.scratch) end
    const root = self:fat_alloc()
    self:fat_set(root, END)
    mem_zero(self.scratch, BLOCK)
    disk_write(root, self.scratch)
    self.root = root
    mem_zero(self.scratch, BLOCK)
    mem_write(self.scratch, MAGIC, 4)
    mem_write(self.scratch + 4, self.total, 4)
    mem_write(self.scratch + 8, root, 2)
    disk_write(SB_LBA, self.scratch)
    self.ready = :true
    puts("formatted #{self.total} sectors, #{(self.total shr 1)} KiB\n")
  end,

  mkdir = fn(self, path) do
    if self:ensure() == :false return :nil
    const parent = self:resolve_parent(path)
    if parent == :nil do puts("mkdir: #{path}: bad path\n") return :nil end
    if parent.name:len() > NAME_MAX do puts("name too long (max #{NAME_MAX})\n") return :nil end
    if self:dir_lookup(parent.dir, parent.name) != :nil do puts("mkdir: #{parent.name}: already exists\n") return :nil end
    const new_block = self:fat_alloc()
    if new_block == 0 do puts("disk full\n") return :nil end
    self:fat_set(new_block, END)
    mem_zero(self.scratch, BLOCK)
    disk_write(new_block, self.scratch)
    self:dir_set(parent.dir, parent.name, T_DIR, new_block, 0)
  end,

  store = fn(self, path, text) do
    if self:ensure() == :false return 0
    const parent = self:resolve_parent(path)
    if parent == :nil do puts("#{path}: bad path\n") return 0 end
    if parent.name:len() > NAME_MAX do puts("name too long (max #{NAME_MAX})\n") return 0 end
    const existing = self:dir_lookup(parent.dir, parent.name)
    if existing != :nil do
      if existing.type == T_DIR do puts("#{parent.name}: is a directory\n") return 0 end
      self:free_chain(existing.first)
    end

    const size = text:len()
    let first = 0
    let previous = 0
    let written = 0
    while written < size do
      const block = self:fat_alloc()
      if block == 0 do puts("disk full\n") return 0 end
      self:fat_set(block, END)
      if first == 0 do first = block end
      else do self:fat_set(previous, block) end
      mem_zero(self.scratch, BLOCK)
      # blit a whole block of the string at once, per byte was O(n^2) and choked
      # the second files got into the kilobytes
      let chunk = size - written
      if chunk > BLOCK do chunk = BLOCK end
      blit_str(self.scratch, text[written..(written + chunk)])
      disk_write(block, self.scratch)
      written += chunk
      previous = block
    end
    self:dir_set(parent.dir, parent.name, T_FILE, first, size)
    return 1
  end,

  write = fn(self, path, text) do
    if self:store(path, text) == 1 puts("wrote #{path} (#{text:len()} bytes)\n")
  end,

  size = fn(self, path) do
    const entry = self:resolve(path)
    if entry == :nil or entry.type == T_DIR return -1
    return entry.size
  end,

  read_bytes = fn(self, path, offset, length) do
    const entry = self:resolve(path)
    if entry == :nil or entry.type == T_DIR return ""
    let block = entry.first
    let stop = offset + length
    if stop > entry.size do stop = entry.size end
    let position = 0
    let result = ""
    while (block != 0 and block != END and position < stop) do
      const next_block = self:fat_get(block)
      if (position + BLOCK > offset and position < stop) do
        disk_read(block, self.scratch)
        # pull this blocks slice in one go, per byte concat was O(n^2)
        let from = 0
        if offset > position do from = offset - position end
        let to = BLOCK
        if stop - position < BLOCK do to = stop - position end
        result = result ~ phys_to_str(self.scratch + from, to - from)
      end
      position += BLOCK
      block = next_block
    end
    return result
  end,

  cat = fn(self, path) do
    if self:ensure() == :false return :nil
    const entry = self:resolve(path)
    if entry == :nil do puts("cat: #{path}: no such file\n") return :nil end
    if entry.type == T_DIR do puts("cat: #{path}: is a directory\n") return :nil end
    let block = entry.first
    let remaining = entry.size
    let result = ""
    while (block != 0 and block != END and remaining > 0) do
      const next_block = self:fat_get(block)
      disk_read(block, self.scratch)
      const chunk = if remaining > BLOCK BLOCK else remaining
      for index in 0..chunk do result = result ~ string.of_ascii(mem_read(self.scratch + index, 1)) end
      remaining -= chunk
      block = next_block
    end
    puts(result)
    puts("\n")
  end,

  ls = fn(self, path) do
    if self:ensure() == :false return -1
    const entry = self:resolve(path)
    if entry == :nil do puts("ls: #{path}: no such file\n") return -1 end
    if entry.type != T_DIR do puts("  #{path}  #{entry.size} bytes\n") return -1 end
    let count = 0
    let block = entry.first
    while (block != 0 and block != END) do
      const next_block = self:fat_get(block)
      disk_read(block, self.scratch)
      for slot in 0..ENTS do
        const entry_offset = slot * 32
        const entry_type = mem_read(self.scratch + entry_offset, 1)
        if entry_type != 0 do
          const name = self:ent_name(entry_offset)
          if entry_type == T_DIR do puts("  #{name}/\n") end
          else do puts("  #{name}  #{mem_read(self.scratch + entry_offset + 4, 4)} bytes\n") end
          count += 1
        end
      end
      block = next_block
    end
    return count
  end,

  rm = fn(self, path) do
    if self:ensure() == :false return :nil
    const parent = self:resolve_parent(path)
    if parent == :nil do puts("rm: #{path}: no such file\n") return :nil end
    const entry = self:dir_lookup(parent.dir, parent.name)
    if entry == :nil do puts("rm: #{path}: no such file\n") return :nil end
    if entry.type == T_DIR and self:dir_empty(entry.first) == :false do
      puts("rm: #{parent.name}: directory not empty\n")
      return :nil
    end
    self:free_chain(entry.first)
    self:dir_clear(parent.dir, parent.name)
    puts("removed #{parent.name}\n")
  end,

  is_dir = fn(self, path) do
    const entry = self:resolve(path)
    return entry != :nil and entry.type == T_DIR
  end,

  entries = fn(self, path) do
    const entry = self:resolve(path)
    if entry == :nil or entry.type != T_DIR return :nil
    let result = {}
    let block = entry.first
    while (block != 0 and block != END) do
      const next_block = self:fat_get(block)
      disk_read(block, self.scratch)
      for slot in 0..ENTS do
        const entry_offset = slot * 32
        const entry_type = mem_read(self.scratch + entry_offset, 1)
        if entry_type != 0 result:push({ name = self:ent_name(entry_offset), type = entry_type, size = mem_read(self.scratch + entry_offset + 4, 4) })
      end
      block = next_block
    end
    return result
  end,

  unlink = fn(self, path) do
    const parent = self:resolve_parent(path)
    if parent == :nil return :false
    const entry = self:dir_lookup(parent.dir, parent.name)
    if entry == :nil return :false
    if (entry.type == T_DIR and self:dir_empty(entry.first) == :false) return :false
    self:free_chain(entry.first)
    self:dir_clear(parent.dir, parent.name)
    return :true
  end,
}

global fs_store = fn(name, text) fs:store(name, text)
global fs_size = fn(name) fs:size(name)
global fs_read_bytes = fn(name, offset, length) fs:read_bytes(name, offset, length)