require 'fs/xfs/inode'

module XFS
  BMAP_BTREE_ROOT_NODE_HEADER = BinaryStruct.new([
    'S>',          'level',           # B+Tree Level
    'S>',          'entry_count',     # Number of Key and Pointer Array Elements
  ])

  SIZEOF_BMAP_BTREE_ROOT_NODE_HEADER  = BMAP_BTREE_ROOT_NODE_HEADER.size

  #
  # Note that the following 3 entries are used simply to compute the maximum
  # number of records possible in the on-disk inode.
  #
  BMAP_BTREE_ROOT_NODE_ENTRIES = BinaryStruct.new([
    'Q>',          'starting_offset', # starting offset of the block
    'Q>',          'block_number',    # block number of the entry
  ])
  SIZEOF_BMAP_BTREE_ROOT_NODE_ENTRIES   = BMAP_BTREE_ROOT_NODE_ENTRIES.size

  BMAP_BTREE_ROOT_NODE_OFFSET = BinaryStruct.new([
    'Q>',          'starting_offset',  # starting offset of the block
  ])
  SIZEOF_BMAP_BTREE_ROOT_NODE_OFFSET   = BMAP_BTREE_ROOT_NODE_OFFSET.size

  BMAP_BTREE_ROOT_NODE_BLOCK = BinaryStruct.new([
    'Q>',          'block_number',     # block number of the entry
  ])
  SIZEOF_BMAP_BTREE_ROOT_NODE_BLOCK    = BMAP_BTREE_ROOT_NODE_BLOCK.size

  #
  # Handle the BTree Root Node in the Disk Inode
  #
  class BmapBTreeRootNode
    attr_accessor   :level, :entry_count, :blocks

    def initialize(data, inode)
      raise "XFS::BmapBTreeRootNode: Nil buffer" if data.nil?
      @inode         = inode
      @header        = BMAP_BTREE_ROOT_NODE_HEADER.decode(data[0..inode.length])
      @level         = @header['level']
      @entry_count   = @header['entry_count']
      raise "XFS::BmapBTreeRootNode: Invalid Root Node Level" if @level.nil? || @level == 0
      header_size    = SIZEOF_BMAP_BTREE_ROOT_NODE_HEADER
      return if @entry_count == 0
      #
      # The Root Node contains an array of starting offsets followed by
      # an array of block numbers.  While only the required number of entries
      # are initialized, there is space left for the maximum number of entries
      # that may fit in the disk inode data fork (eliminating any space taken
      # up by extended attributes).
      # We only care about the block numbers.  The offsets of individual data blocks are
      # also kept in the BmapBTreeRecord
      #
      fork_size       = @inode.dfork_size(Inode::XFS_DATA_FORK)
      maximum_records = (fork_size - header_size) / SIZEOF_BMAP_BTREE_ROOT_NODE_ENTRIES
      return if maximum_records == 0
      @blocks         = []
      offset_size     = SIZEOF_BMAP_BTREE_ROOT_NODE_OFFSET
      block_size      = SIZEOF_BMAP_BTREE_ROOT_NODE_BLOCK
      size            = SIZEOF_BMAP_BTREE_ROOT_NODE_HEADER
      1.upto(@entry_count) do |i|
        start             = size + maximum_records * offset_size + (i - 1) * block_size
        block             = (data[start..start + block_size]).unpack('Q>').shift
        agbno             = inode.sb.fsb_to_agbno(block)
        agno              = inode.sb.fsb_to_agno(block)
        real_block        = inode.sb.agbno_to_real_block(agno, agbno)
        @blocks.concat([real_block])
      end
    end
  end
end