simple_cache.rb

#!/usr/bin/env ruby18
# a simple cache class, used in my dzen2 script
# usage:
#   cache = Cache.new(command, time)
#     where 'command' is program executed with in a shell (using backticks)
#     and 'time' is the timeout in seconds
#   to fetch a value:
#    cache.get
#   use bang method to ignore cache:
#    cache.get!
#   or reset the cache (and optional pass it a new timeout)
#    cache.reset or cache.reset(10)

class Cache
  def initialize(command, time)
    @command = command
    @time = time
    @last_time = Time.now
    @value = nil
  end

  def get
    return exec if @value.nil?
    if time_diff < @time
      return @value 
    else
      exec
    end
  end

  def get!
    exec
  end

  def reset(time = nil)
    @time = time if time
    @value = nil
  end

  def exec
    @value = `#{@command}`
    @last_time = Time.now
    @value
  end

  def time_diff
    (Time.now - @last_time).round
  end
end

if $0 == __FILE__
  require 'rubygems'
  require 'test/unit'
  require 'shoulda'
  
  class TestCache < Test::Unit::TestCase
    context "A cache instance" do
      setup do
        @cache = Cache.new('sleep 2;echo foo', 3)
      end

      should "fetch value on first get" do
        t1 = Time.now
        value = @cache.get
        t2 = Time.now
        assert (t2 - t1) >= 2
        assert_equal "foo\n", value
      end

      should "use cached when in time" do
        t1 = Time.now
        value = @cache.get
        t2 = Time.now
        value2 = @cache.get
        t3 = Time.now
        assert (t2 - t1) >= 2
        assert (t3 - t2) <= 1
        assert_equal "foo\n", value
        assert_equal "foo\n", value2
      end

      should "refetch when timed out" do
        t1 = Time.now
        value = @cache.get
        t2 = Time.now
        sleep 4
        t3 = Time.now
        value2 = @cache.get
        t4 = Time.now
        assert (t2 - t1) >= 2
        assert (t4 - t3) >= 2
        assert_equal "foo\n", value
        assert_equal "foo\n", value2
      end

      should "refetch when reseted before" do
        t1 = Time.now
        value = @cache.get
        t2 = Time.now
        @cache.reset
        t3 = Time.now
        value2 = @cache.get
        t4 = Time.now
        assert (t2 - t1) >= 2
        assert (t4 - t3) >= 2
        assert_equal "foo\n", value
        assert_equal "foo\n", value2
      end
    end
  end
end