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Improve GroupIndexes() performance with #1178

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BobLd
2025-10-12 20:17:34 +01:00
parent b14f45f59f
commit ffba176060
1 changed files with 20 additions and 54 deletions
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74
src/UglyToad.PdfPig.DocumentLayoutAnalysis/Clustering.cs
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@@ -216,25 +216,26 @@
yield return group.Select(i => elements[i]).ToList();
}
}
/// <summary>
/// Group elements using Depth-first search.
/// <para>https://en.wikipedia.org/wiki/Depth-first_search</para>
/// </summary>
/// <param name="edges">The graph. edges[i] = j indicates that there is an edge between i and j.</param>
/// <returns>A List of HashSets containing the grouped indexes.</returns>
internal static List<HashSet<int>> GroupIndexes(int[] edges)
{
int[][] adjacency = new int[edges.Length][];
// Improved thanks to https://github.com/UglyToad/PdfPig/issues/1178
var adjacency = new List<int>[edges.Length];
for (int i = 0; i < edges.Length; i++)
{
HashSet<int> matches = new HashSet<int>();
if (edges[i] != -1) matches.Add(edges[i]);
for (int j = 0; j < edges.Length; j++)
adjacency[i] = new List<int>();
}
// one pass O(n)
for (int i = 0; i < edges.Length; i++)
{
int j = edges[i];
if (j != -1)
{
if (edges[j] == i) matches.Add(j);
// i <-> j
adjacency[i].Add(j);
adjacency[j].Add(i);
}
adjacency[i] = matches.ToArray();
}
List<HashSet<int>> groupedIndexes = new List<HashSet<int>>();
@@ -242,55 +243,20 @@
for (int p = 0; p < edges.Length; p++)
{
if (isDone[p]) continue;
if (isDone[p])
{
continue;
}
groupedIndexes.Add(DfsIterative(p, adjacency, ref isDone));
}
return groupedIndexes;
}
/// <summary>
/// Group elements using Depth-first search.
/// <para>https://en.wikipedia.org/wiki/Depth-first_search</para>
/// </summary>
/// <param name="edges">The graph. edges[i] = [j, k, l, ...] indicates that there is an edge between i and each element j, k, l, ...</param>
/// <returns>A List of HashSets containing the grouped indexes.</returns>
internal static List<HashSet<int>> GroupIndexes(int[][] edges)
{
int[][] adjacency = new int[edges.Length][];
for (int i = 0; i < edges.Length; i++)
{
HashSet<int> matches = new HashSet<int>();
for (int j = 0; j < edges[i].Length; j++)
{
if (edges[i][j] != -1) matches.Add(edges[i][j]);
}
for (int j = 0; j < edges.Length; j++)
{
for (int k = 0; k < edges[j].Length; k++)
{
if (edges[j][k] == i) matches.Add(j);
}
}
adjacency[i] = matches.ToArray();
}
List<HashSet<int>> groupedIndexes = new List<HashSet<int>>();
bool[] isDone = new bool[edges.Length];
for (int p = 0; p < edges.Length; p++)
{
if (isDone[p]) continue;
groupedIndexes.Add(DfsIterative(p, adjacency, ref isDone));
}
return groupedIndexes;
}
/// <summary>
/// Depth-first search
/// <para>https://en.wikipedia.org/wiki/Depth-first_search</para>
/// </summary>
private static HashSet<int> DfsIterative(int s, int[][] adj, ref bool[] isDone)
private static HashSet<int> DfsIterative(int s, List<int>[] adj, ref bool[] isDone)
{
HashSet<int> group = new HashSet<int>();
Stack<int> S = new Stack<int>();