Merge pull request #609 from simonwhitaker/master

Multiset

Author: kelvinlauKL

Multiset/Multiset.playground/Contents.swift

@@ -0,0 +1,26 @@
+//: Playground - noun: a place where people can play
+
+#if swift(>=4.0)
+  print("Hello, Swift 4!")
+#endif
+
+import Cocoa
+
+var set = Multiset<String>()
+
+set.add("Foo")
+set.add("Foo")
+set.add("Bar")
+set.add("Baz")
+
+set.count
+set.count(for: "Foo")
+
+set.allItems
+
+var set2 = Multiset<String>()
+set2.add("Foo")
+set2.add("Foo")
+
+set2.isSubSet(of: set) // true
+set.isSubSet(of: set2) // false

Multiset/Multiset.playground/Sources/Multiset.swift

@@ -0,0 +1,71 @@
+//
+//  Multiset.swift
+//  Multiset
+//
+//  Created by Simon Whitaker on 28/08/2017.
+//
+
+import Foundation
+
+public struct Multiset<Element: Hashable> {
+  private var storage: [Element: UInt] = [:]
+  public private(set) var count: UInt = 0
+
+  public init() {}
+
+  public mutating func add (_ elem: Element) {
+    storage[elem, default: 0] += 1
+    count += 1
+  }
+
+  public mutating func remove (_ elem: Element) {
+    if let currentCount = storage[elem] {
+      if currentCount > 1 {
+        storage[elem] = currentCount - 1
+      } else {
+        storage.removeValue(forKey: elem)
+      }
+      count -= 1
+    }
+  }
+
+  public func isSubSet (of superset: Multiset<Element>) -> Bool {
+    for (key, count) in storage {
+      let supersetcount = superset.storage[key] ?? 0
+      if count > supersetcount {
+        return false
+      }
+    }
+    return true
+  }
+
+  public func count(for key: Element) -> UInt {
+    return storage[key] ?? 0
+  }
+
+  public var allItems: [Element] {
+    var result = [Element]()
+    for (key, count) in storage {
+      for _ in 0 ..< count {
+        result.append(key)
+      }
+    }
+    return result
+  }
+}
+
+// MARK: - Equatable
+extension Multiset: Equatable {
+  public static func == (lhs: Multiset<Element>, rhs: Multiset<Element>) -> Bool {
+    if lhs.storage.count != rhs.storage.count {
+      return false
+    }
+    for (lkey, lcount) in lhs.storage {
+      let rcount = rhs.storage[lkey] ?? 0
+      if lcount != rcount {
+        return false
+      }
+    }
+    return true
+  }
+}

Multiset/Multiset.playground/contents.xcplayground

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

Multiset/README.markdown

@@ -0,0 +1,91 @@
+# Multiset
+
+A multiset (also known as a bag) is a data structure similar to a regular set, but it can store multiple instances of the same element.
+
+The following all represent the same *set*, but do not represent the same *multiset*.
+
+```
+[1, 3, 2]
+[1, 3, 2, 2]
+```
+
+In this multiset implementation, ordering is unimportant. So these two multisets are identical:
+
+```
+[1, 2, 2]
+[2, 1, 2]
+```
+
+Typical operations on a multiset are:
+
+- Add an element
+- Remove an element
+- Get the count for an element (the number of times it's been added)
+- Get the count for the whole set (the number of items that have been added)
+- Check whether it is a subset of another multiset
+
+## Implementation
+
+Under the hood, this implementation of Multiset uses a dictionary to store a mapping of elements to the number of times they've been added.
+
+Here's the essence of it:
+
+``` swift
+public struct Multiset<Element: Hashable> {
+  fileprivate var storage: [Element: UInt]
+  
+  public init() {
+    storage = [:]
+  }
+```
+
+And here's how you'd use this class to create a multiset of strings:
+
+``` swift
+var set = Multiset<String>()
+```
+
+Adding an element is a case of incrementing the counter for that element, or setting it to 1 if it doesn't already exist:
+
+``` swift
+public mutating func add (_ elem: Element) {
+  if let currentCount = storage[elem] {
+    storage[elem] = currentCount + 1;
+  } else {
+    storage[elem] = 1
+  }
+}
+```
+
+Here's how you'd use this method to add to the set we created earlier:
+
+```swift
+set.add("foo")
+set.add("foo") 
+```
+
+Our set now contains two elements, both the string "foo".
+
+Removing an element works much the same way as adding; decrement the counter for the element, or remove it from the underlying dictionary if its value is 1 before removal.
+
+``` swift
+public mutating func remove (_ elem: Element) {
+  if let currentCount = storage[elem] {
+    if currentCount > 1 {
+      storage[elem] = currentCount - 1
+    } else {
+      storage.removeValue(forKey: elem)
+    }
+  }
+}
+```
+
+Getting the count for an item is simple: we just return the value for the given item in the internal dictionary.
+
+``` swift
+public func count(for key: Element) -> UInt {
+  return storage[key] ?? 0
+}
+```
+
+*Written for the Swift Algorithm Club by Simon Whitaker*

README.markdown

@@ -172,7 +172,7 @@ Most of the time using just the built-in `Array`, `Dictionary`, and `Set` types
 
 - [Bloom Filter](Bloom%20Filter/). A constant-memory data structure that probabilistically tests whether an element is in a set.
 - [Hash Set](Hash%20Set/). A set implemented using a hash table.
-- Multiset
+- [Multiset](Multiset/). A set where the number of times an element is added matters. (Also known as a bag.)
 - [Ordered Set](Ordered%20Set/). A set where the order of items matters.
 
 ### Graphs