Add Multiset

Author: Simon Whitaker

Multiset/Multiset.playground/Contents.swift

@@ -0,0 +1,19 @@
+//: 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()

Multiset/Multiset.playground/Sources/Multiset.swift

@@ -0,0 +1,82 @@
+//
+//  Multiset.swift
+//  Multiset
+//
+//  Created by Simon Whitaker on 28/08/2017.
+//
+
+import Foundation
+
+public struct Multiset<T: Hashable> {
+  fileprivate var storage: Dictionary<T, Int>
+  private var _count: Int = 0
+
+  public init() {
+    storage = Dictionary<T, Int>()
+  }
+
+  public mutating func add (_ elem: T) {
+    if let currentCount = self.storage[elem] {
+      self.storage[elem] = currentCount + 1;
+    } else {
+      self.storage[elem] = 1
+    }
+    _count += 1
+  }
+
+  public mutating func remove (_ elem: T) {
+    if let currentCount = self.storage[elem] {
+      if currentCount > 1 {
+        self.storage[elem] = currentCount - 1
+      } else {
+        self.storage.removeValue(forKey: elem)
+      }
+      _count -= 1
+    }
+  }
+
+  public func isSubSet (of superset: Multiset<T>) -> Bool {
+    for (key, count) in self.storage {
+      let supersetcount = superset.storage[key] ?? 0
+      if count > supersetcount {
+        return false
+      }
+    }
+    return true
+  }
+
+  public var count: Int {
+    get {
+      return _count
+    }
+  }
+
+  public func count(for key: T) -> Int {
+    return self.storage[key] ?? 0
+  }
+
+  public func allItems() -> [T] {
+    var result = Array<T>()
+    for (key, count) in self.storage {
+      for _ in 0 ..< count {
+        result.append(key)
+      }
+    }
+    return result
+  }
+}
+
+extension Multiset: Equatable {
+  public static func == (lhs: Multiset<T>, rhs: Multiset<T>) -> 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 +1,12 @@
+# An implementation of the multiset data stucture
+
+## Usage
+
+``` swift
+var b = Multiset<Character>();
+for c in "hello".characters {
+    b.add(c)
+}
+let count = b.count // count is 5
+let lcount = b.count(for: "l") // lcount is 2
+```

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