2020-08-10_build-a-fixed-size-array-from-scratch.md

Build a fixed size array from scratch

Using the Fiddle library, we can use malloc and build an array from Scract, which provides indexed access in O(1)

The biggest hurdle was dealing with the Fiddle::Pointer class given its limited documentation. The following uses the Marshal library to deal with any types, at the cost of what is probably a terrible runtime performance combined with a wasteful storage system

require 'fiddle'

class BYOArray < BasicObject

  PTR_SIZE = ::Fiddle::SIZEOF_LONG

  def initialize(max_size)
    @max_size = max_size
    @current_size = 0
    @beginning_address = ::Fiddle::Pointer.malloc(PTR_SIZE)
  end

  def add(str)
    ::Kernel.raise 'Array is full' if @current_size == @max_size

    ptr = ::Fiddle::Pointer.to_ptr(::Marshal.dump(str))
    offset = @current_size * PTR_SIZE # 0 at first, then 8, 16, etc ...

    @beginning_address[offset, PTR_SIZE] = ptr.ref

    @current_size += 1
    self
  end

  def get(i)
    return nil if i < 0 || i >= @current_size

    address = @beginning_address[i * PTR_SIZE, PTR_SIZE].unpack('Q')[0]
    ::Marshal.load(::Fiddle::Pointer.new(address).to_s)
  end

  def to_s
    "Size: #{ @current_size }"
  end
end

See: https://gist.github.com/pjambet/01b54af23790b9a279c45ae07c7ec4a5