Adds bounded priority queue

Author: kelvinlauKL

SwiftAlgorithmClub/BoundedPriorityQueue.playground/Contents.swift

@@ -0,0 +1,98 @@
+struct Message: Comparable, CustomStringConvertible {
+  let name: String
+  let priority: Int
+
+  var description: String {
+    return "\(name):\(priority)"
+  }
+}
+
+func == (m1: Message, m2: Message) -> Bool {
+  return m1.priority == m2.priority
+}
+
+func < (m1: Message, m2: Message) -> Bool {
+  return m1.priority < m2.priority
+}
+
+let queue = BoundedPriorityQueue<Message>(maxElements: 5)
+queue.count
+
+queue.enqueue(Message(name: "hello", priority: 100))
+queue.count
+queue.peek()
+print(queue)
+
+queue.enqueue(Message(name: "there", priority: 99))
+queue.count
+queue.peek()
+print(queue)
+
+queue.enqueue(Message(name: "world", priority: 150))
+queue.count
+queue.peek()
+print(queue)
+
+queue.enqueue(Message(name: "swift", priority: 110))
+queue.count
+queue.peek()
+print(queue)
+
+queue.enqueue(Message(name: "is", priority: 30))
+queue.count
+queue.peek()
+print(queue)
+
+// At this point, the queue is:
+// <world:150, swift:110, hello:100, there:99, is:30, >
+
+// Try to insert an item with a really low priority. This should not get added.
+queue.enqueue(Message(name: "very", priority: -1))
+queue.count    // 5
+queue.peek()
+print(queue)   // still same as before
+
+// Try to insert an item with medium priority. This gets added and the lowest
+// priority item is removed.
+queue.enqueue(Message(name: "cool", priority: 120))
+queue.count
+queue.peek()
+print(queue)
+
+// Try to insert an item with very high priority. This gets added and the
+// lowest priority item is removed.
+queue.enqueue(Message(name: "!!!", priority: 500))
+queue.count
+queue.peek()
+print(queue)
+
+// Test dequeuing
+queue.dequeue()
+queue.count
+queue.peek()
+print(queue)
+
+queue.dequeue()
+queue.count
+queue.peek()
+print(queue)
+
+queue.dequeue()
+queue.count
+queue.peek()
+print(queue)
+
+queue.dequeue()
+queue.count
+queue.peek()
+print(queue)
+
+queue.dequeue()
+queue.count
+queue.peek()
+print(queue)
+
+queue.dequeue()
+queue.count
+queue.peek()
+print(queue)

SwiftAlgorithmClub/BoundedPriorityQueue.playground/Sources/BoundedPriorityQueue.swift

@@ -0,0 +1,137 @@
+public class LinkedListNode<T: Comparable> {
+  var value: T
+  var next: LinkedListNode?
+  var previous: LinkedListNode?
+
+  public init(value: T) {
+    self.value = value
+  }
+}
+
+public class BoundedPriorityQueue<T: Comparable> {
+  fileprivate typealias Node = LinkedListNode<T>
+
+  private(set) public var count = 0
+  fileprivate var head: Node?
+  private var tail: Node?
+  private var maxElements: Int
+
+  public init(maxElements: Int) {
+    self.maxElements = maxElements
+  }
+
+  public var isEmpty: Bool {
+    return count == 0
+  }
+
+  public func peek() -> T? {
+    return head?.value
+  }
+
+  public func enqueue(_ value: T) {
+    if let node = insert(value, after: findInsertionPoint(value)) {
+      // If the newly inserted node is the last one in the list, then update
+      // the tail pointer.
+      if node.next == nil {
+        tail = node
+      }
+
+      // If the queue is full, then remove an element from the back.
+      count += 1
+      if count > maxElements {
+        removeLeastImportantElement()
+      }
+    }
+  }
+
+  private func insert(_ value: T, after: Node?) -> Node? {
+    if let previous = after {
+
+      // If the queue is full and we have to insert at the end of the list,
+      // then there's no reason to insert the new value.
+      if count == maxElements && previous.next == nil {
+        print("Queue is full and priority of new object is too small")
+        return nil
+      }
+
+      // Put the new node in between previous and previous.next (if exists).
+      let node = Node(value: value)
+      node.next = previous.next
+      previous.next?.previous = node
+      previous.next = node
+      node.previous = previous
+      return node
+
+    } else if let first = head {
+      // Have to insert at the head, so shift the existing head up once place.
+      head = Node(value: value)
+      head!.next = first
+      first.previous = head
+      return head
+
+    } else {
+      // This is the very first item in the queue.
+      head = Node(value: value)
+      return head
+    }
+  }
+
+  /* Find the node after which to insert the new value. If this returns nil,
+     the new value should be inserted at the head of the list. */
+  private func findInsertionPoint(_ value: T) -> Node? {
+    var node = head
+    var prev: Node?
+
+    while let current = node, value < current.value {
+      prev = node
+      node = current.next
+    }
+    return prev
+  }
+
+  private func removeLeastImportantElement() {
+    if let last = tail {
+      tail = last.previous
+      tail?.next = nil
+      count -= 1
+    }
+
+    // Note: Instead of using a tail pointer, we could just scan from the new
+    // node until the end. Then nodes also don't need a previous pointer. But
+    // this is much slower on large lists.
+  }
+
+  public func dequeue() -> T? {
+    if let first = head {
+      count -= 1
+      if count == 0 {
+        head = nil
+        tail = nil
+      } else {
+        head = first.next
+        head!.previous = nil
+      }
+      return first.value
+    } else {
+      return nil
+    }
+  }
+}
+
+extension LinkedListNode: CustomStringConvertible {
+  public var description: String {
+    return "\(value)"
+  }
+}
+
+extension BoundedPriorityQueue: CustomStringConvertible {
+  public var description: String {
+    var s = "<"
+    var node = head
+    while let current = node {
+      s += "\(current), "
+      node = current.next
+    }
+    return s + ">"
+  }
+}

SwiftAlgorithmClub/BoundedPriorityQueue.playground/contents.xcplayground

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='5.0' target-platform='osx' last-migration='1000'>
+    <timeline fileName='timeline.xctimeline'/>
+</playground>

SwiftAlgorithmClub/BoundedPriorityQueue.playground/playground.xcworkspace/contents.xcworkspacedata

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>IDEDidComputeMac32BitWarning</key>
+	<true/>
+</dict>
+</plist>