ring_buffer.rb

class RingBuffer
  attr_reader :capacity, :size

  def initialize(capacity)
    @array = Array.new(capacity)
    @capacity = capacity
    @read_index = 0
    @size = 0
  end

  def push(elem)
    unless writeable?
      return false
    end

    @size += 1
    @array[next_index(@size)] = elem

    return true
  end

  def peek
    if empty?
      return nil
    end

    return @array[next_index]
  end

  def pop
    if empty?
      return nil
    end

    @size -= 1
    @read_index = next_index

    return @array[@read_index]
  end

  def empty?
    return @size == 0
  end

  private

  def next_index(offset = 1)
    return (@read_index + offset) % @capacity
  end

  def writeable?
    return @read_index + @size < @capacity
  end
end

ring_buffer_test.rb

require "cutest"
require "ring_buffer"

setup { RingBuffer.new(5) }

test "push" do |buffer|
  assert_equal buffer.push(0), true
end

test "push when full" do |buffer|

  5.times { |i| buffer.push(i) }

  assert_equal buffer.push(5), false
end

test "pop" do |buffer|
  buffer.push(0)

  assert_equal buffer.pop, 0
end

test "pop when empty" do |buffer|
  assert_equal buffer.pop, nil
  buffer.push(0)
  assert_equal buffer.pop, 0
end

test "fifo" do |buffer|
  5.times { |i| buffer.push(i) }

  5.times { |i| assert_equal buffer.pop, i }
  assert_equal buffer.pop, nil
end

test "peek" do |buffer|
  buffer.push(0)

  assert_equal buffer.peek, 0
  assert_equal buffer.peek, 0
end

test "peek when empty" do |buffer|
  assert_equal buffer.peek, nil
  assert_equal buffer.peek, nil
end

test "size" do |buffer|
  assert_equal buffer.size, 0
  
  buffer.push(0)
  assert_equal buffer.size, 1
  
  buffer.pop
  assert_equal buffer.size, 0
end

test "empty?" do |buffer|
  assert_equal buffer.empty?, true
  
  buffer.push(0)
  assert_equal buffer.empty?, false
  
  buffer.pop
  assert_equal buffer.empty?, true
end