我对使用字符串拼接与元组在生成哈希表中表示边的键时,以 O(1) 的查找性能为目标的方法感到好奇。在处理无向边的要求时有两种可能:
规范化键,使其在边的描述中指定哪个节点是第一个组件时保持一致。在我的测试中,我只选择将具有最低序数比较值的节点作为键的第一个组件。
在哈希表中创建两个条目,一个用于每个方向的边。
这里的一个关键假设是字符串节点标识符不是很长,因此与查找相比,键规范化的成本较低。
字符串拼接和带键规范化元组版本似乎效果差不多:在 release 模式下,在 VirtualBox VM 中完成了约 200 万次随机查找,耗时约 3 秒。
为了确定键规范化是否掩盖了查找操作的影响,第三个实现不进行键规范化,但是对于边的两个可能方向保持对称条目。这似乎比查找慢30-40%左右,这让我稍微感到意外。也许基础哈希表桶由于有两倍的元素数量而具有更高的平均占用率,在每个哈希桶内需要更长的线性搜索(平均而言)?
interface IEdgeCollection
{
bool AddEdge(string node1, string node2);
bool ContainsEdge(string node1, string node2);
bool RemoveEdge(string node1, string node2);
}
class EdgeSet1 : IEdgeCollection
{
private HashSet<string> _edges = new HashSet<string>();
private static string MakeEdgeKey(string node1, string node2)
{
return StringComparer.Ordinal.Compare(node1, node2) < 0 ? node1 + node2 : node2 + node1;
}
public bool AddEdge(string node1, string node2)
{
var key = MakeEdgeKey(node1, node2);
return _edges.Add(key);
}
public bool ContainsEdge(string node1, string node2)
{
var key = MakeEdgeKey(node1, node2);
return _edges.Contains(key);
}
public bool RemoveEdge(string node1, string node2)
{
var key = MakeEdgeKey(node1, node2);
return _edges.Remove(key);
}
}
class EdgeSet2 : IEdgeCollection
{
private HashSet<Tuple<string, string>> _edges = new HashSet<Tuple<string, string>>();
private static Tuple<string, string> MakeEdgeKey(string node1, string node2)
{
return StringComparer.Ordinal.Compare(node1, node2) < 0
? new Tuple<string, string>(node1, node2)
: new Tuple<string, string>(node2, node1);
}
public bool AddEdge(string node1, string node2)
{
var key = MakeEdgeKey(node1, node2);
return _edges.Add(key);
}
public bool ContainsEdge(string node1, string node2)
{
var key = MakeEdgeKey(node1, node2);
return _edges.Contains(key);
}
public bool RemoveEdge(string node1, string node2)
{
var key = MakeEdgeKey(node1, node2);
return _edges.Remove(key);
}
}
class EdgeSet3 : IEdgeCollection
{
private HashSet<Tuple<string, string>> _edges = new HashSet<Tuple<string, string>>();
private static Tuple<string, string> MakeEdgeKey(string node1, string node2)
{
return new Tuple<string, string>(node1, node2);
}
public bool AddEdge(string node1, string node2)
{
var key1 = MakeEdgeKey(node1, node2);
var key2 = MakeEdgeKey(node2, node1);
return _edges.Add(key1) && _edges.Add(key2);
}
public bool ContainsEdge(string node1, string node2)
{
var key = MakeEdgeKey(node1, node2);
return _edges.Contains(key);
}
public bool RemoveEdge(string node1, string node2)
{
var key1 = MakeEdgeKey(node1, node2);
var key2 = MakeEdgeKey(node2, node1);
return _edges.Remove(key1) && _edges.Remove(key2);
}
}
class Program
{
static void Test(string[] nodes, IEdgeCollection edges, int edgeCount)
{
var rng = new Random(edgeCount);
var edgeList = new List<Tuple<string, string>>();
Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();
for (int i = 0; i < edgeCount; i++)
{
string node1 = nodes[rng.Next(nodes.Length)];
string node2 = nodes[rng.Next(nodes.Length)];
edges.AddEdge(node1, node2);
edgeList.Add(new Tuple<string, string>(node1, node2));
}
var addElapsed = stopwatch.Elapsed;
int nonRandomFound = 0;
stopwatch.Start();
foreach (var edge in edgeList)
{
if (edges.ContainsEdge(edge.Item1, edge.Item2))
nonRandomFound++;
}
var nonRandomLookupElapsed = stopwatch.Elapsed;
if (nonRandomFound != edgeList.Count)
{
Console.WriteLine("The edge collection {0} is not working right!", edges.GetType().FullName);
return;
}
int randomFound = 0;
stopwatch.Start();
for (int i = 0; i < edgeCount; i++)
{
string node1 = nodes[rng.Next(nodes.Length)];
string node2 = nodes[rng.Next(nodes.Length)];
if (edges.ContainsEdge(node1, node2))
randomFound++;
}
var randomLookupElapsed = stopwatch.Elapsed;
stopwatch.Start();
foreach (var edge in edgeList)
{
edges.RemoveEdge(edge.Item1, edge.Item2);
}
var removeElapsed = stopwatch.Elapsed;
Console.WriteLine("Test: {0} with {1} edges: {2}s addition, {3}s non-random lookup, {4}s random lookup, {5}s removal",
edges.GetType().FullName,
edgeCount,
addElapsed.TotalSeconds,
nonRandomLookupElapsed.TotalSeconds,
randomLookupElapsed.TotalSeconds,
removeElapsed.TotalSeconds);
}
static void Main(string[] args)
{
var rng = new Random();
var nodes = new string[5000];
for (int i = 0; i < nodes.Length; i++)
{
StringBuilder name = new StringBuilder();
int length = rng.Next(7, 15);
for (int j = 0; j < length; j++)
{
name.Append((char) rng.Next(32, 127));
}
nodes[i] = name.ToString();
}
IEdgeCollection edges1 = new EdgeSet1();
IEdgeCollection edges2 = new EdgeSet2();
IEdgeCollection edges3 = new EdgeSet3();
Test(nodes, edges1, 2000000);
Test(nodes, edges2, 2000000);
Test(nodes, edges3, 2000000);
Console.ReadLine();
}
}
string key = (node1 < node2) ? node1+node2 : node2+node1;
- Monroe ThomasouterSets.Contains(new HashSet<string>{"node1", "node2"})
不起作用。如果这样做,事情会变得混乱,因为这意味着更改集合中的项可能会破坏项唯一性的约定。HashSet仅按引用比较,而不是其中包含的值。在HashSet中使用.SetEquals
进行比较将是更好的方法。 - spender