From 086ec74767a44df5beed64ccc35e67d0534a0552 Mon Sep 17 00:00:00 2001
From: pjs <pjs@alum.mit.edu>
Date: Fri, 19 Aug 2016 15:02:00 -0700
Subject: [PATCH] Array-based priority queue
---
lib/priority_queue.rb | 89 +++++++++++++++++++++++++++++++++++++++++++
lib/simplekit.rb | 13 +++----
2 files changed, 94 insertions(+), 8 deletions(-)
create mode 100644 lib/priority_queue.rb
diff --git a/lib/priority_queue.rb b/lib/priority_queue.rb
new file mode 100644
index 0000000..d638406
--- /dev/null
+++ b/lib/priority_queue.rb
@@ -0,0 +1,89 @@
+# dis is a PriorityQueue
+class PriorityQueue
+ def initialize
+ clear
+ end
+
+ def <<(element)
+ @elements << element
+ # bubble up the element that we just added
+ bubble_up(@elements.size - 1)
+ end
+
+ alias push <<
+
+ def peek
+ # the first element will always be the min, because of the heap constraint
+ @elements[1]
+ end
+
+ def pop
+ # remove the last element of the list
+ min = @elements[1]
+
+ # and make sure the tree is ordered again
+ bubble_down(1)
+ min
+ end
+
+ def clear
+ @elements = [nil]
+ end
+
+ def empty?
+ @elements.length < 2
+ end
+
+ private
+
+ def bubble_up(index)
+ target = @elements[index]
+ loop do
+ parent_index = (index / 2)
+
+ # return if we reach the root or the parent is less than the child
+ if parent_index < 1 || @elements[parent_index] <= target
+ @elements[index] = target
+ return
+ end
+
+ # otherwise we exchange the child with the parent
+ @elements[index] = @elements[parent_index]
+
+ # and keep bubbling up
+ index = parent_index
+ end
+ end
+
+ def bubble_down(index)
+ target = @elements.pop
+ loop do
+ child_index = (index * 2)
+
+ # stop if we reach the bottom of the tree
+ if child_index > @elements.size - 1
+ @elements[index] = target
+ return
+ end
+
+ # make sure we get the smallest child
+ not_the_last_element = child_index < @elements.size - 1
+ left_element = @elements[child_index]
+ right_element = @elements[child_index + 1]
+ child_index += 1 if not_the_last_element && right_element < left_element
+
+ # there is no need to continue if the parent element is already smaller
+ # then its children
+ if target <= @elements[child_index]
+ @elements[index] = target
+ return
+ end
+
+ @elements[index] = @elements[child_index]
+
+ # repeat the process until we reach a point where the parent
+ # is larger than its children
+ index = child_index
+ end
+ end
+end
diff --git a/lib/simplekit.rb b/lib/simplekit.rb
index 07cdc7f..cf428b4 100644
--- a/lib/simplekit.rb
+++ b/lib/simplekit.rb
@@ -1,5 +1,4 @@
-require 'rubygems' if RUBY_VERSION =~ /^1\.8/
-require 'skewheap'
+require_relative 'priority_queue'
# The +SimpleKit+ module provides basic event scheduling capabilities.
#
@@ -10,7 +9,7 @@ require 'skewheap'
module SimpleKit
# The set of module methods to be passed to the EventScheduler
# if not found in the model class.
- DELEGATED_METHODS = [:model_time, :schedule, :halt]
+ DELEGATED_METHODS = [:model_time, :schedule, :halt].freeze
# Run your model by creating a new +EventScheduler+ and invoking its
# +run+ method.
@@ -29,8 +28,8 @@ module SimpleKit
end
end
- # Class +EventScheduler+ provides the computation engine for a discrete
- # event simulation model. It uses the +SkewHeap+ RubyGem as a priority
+ # Class +EventScheduler+ provides the computation engine for a
+ # discrete event simulation model. It uses an array-based priority
# queue implementation for the pending events list.
#
# Users must create a model class which:
@@ -43,7 +42,7 @@ module SimpleKit
# and setting up an empty event list.
def initialize(the_model)
@user_model = the_model
- @event_list = SkewHeap.new
+ @event_list = PriorityQueue.new
end
# Add an event to the pending events list.
@@ -80,8 +79,6 @@ module SimpleKit
end
end
- private
-
# This is a private helper Struct for the EventScheduler class.
# Users should never try to access this directly.
EventNotice = Struct.new(:event, :time, *:args) do
--
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