supercaracal/ruby-memory-alloc.md

## ruby-memory-alloc.md

      
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              ruby-memory-alloc.md
            
          
    Array vs StringArray

This use case is under conditions that the array size is huge and the kind of elements are limited.
irb(main):001:0> require 'objspace'
=> true

irb(main):002:0> ObjectSpace.memsize_of(Array.new(16384))
=> 131112

irb(main):003:0> s = '0' * 16384
=> "00000000000000000000000000000000000...

irb(main):004:0> ObjectSpace.memsize_of(String.new(s, encoding: Encoding::BINARY, capacity: 16384))
=> 16425

irb(main):005:0> ObjectSpace.memsize_of(Array.new(256))
=> 2088

StringArray = Struct.new('StringArray', :string, :elements, keyword_init: true) do
  def [](index)
    raise IndexError if index < 0
    return if index >= string.bytesize

    elements[string.getbyte(index)]
  end

  def []=(index, element)
    raise IndexError if index < 0
    return if index >= string.bytesize

    pos = elements.find_index(element) # O(N)
    if pos.nil?
      raise(RangeError, 'full of elements') if elements.size >= 256

      pos = elements.size
      elements << element
    end

    string.setbyte(index, pos)
  end
end

FIXED_ARRAY_SIZE = 16384
BASE_STRING = '0' * FIXED_ARRAY_SIZE

base_elements = %w[a b c d e f g]

arr = if base_elements.size > 256
        Array.new(FIXED_ARRAY_SIZE)
      else
        StringArray.new(
          string: String.new(BASE_STRING, encoding: Encoding::BINARY, capacity: FIXED_ARRAY_SIZE),
          elements: base_elements
        )
      end

FIXED_ARRAY_SIZE.times do { |i| arr[i] = base_elements.sample }
Hash vs Struct

In certain use cases, it would be better to use struct instead of hash for memory efficiency.
irb(main):006:0> ObjectSpace.memsize_of({a:1, b:2, c:3, d:4, e:5})
=> 168

irb(main):007:0> Foo = Struct.new('Foo', :a, :b, :c, :d, :e, keyword_init: true)
=> Struct::Foo(keyword_init: true)

irb(main):008:0> ObjectSpace.memsize_of(Foo.new(a:1, b:2, c:3, d:4, e:5))
=> 80