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Merged
BobLd merged 2 commits into from
Oct 13, 2025
Merged

Feature/improve group indexes #1181

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5f6cf46
Improve GroupIndexes() performance with #1178
BobLd Oct 12, 2025
4dfbf20
Improve DfsIterative() performance
BobLd Oct 12, 2025
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91 changes: 34 additions & 57 deletions src/UglyToad.PdfPig.DocumentLayoutAnalysis/Clustering.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -216,71 +216,37 @@ public static IEnumerable<IReadOnlyList<T>> NearestNeighbours<T>(IReadOnlyList<T
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)

internal static List<List<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++)
{
if (edges[j] == i) matches.Add(j);
}
adjacency[i] = matches.ToArray();
adjacency[i] = new List<int>();
}

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>
/// 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][];
// one pass O(n)
for (int i = 0; i < edges.Length; i++)
{
HashSet<int> matches = new HashSet<int>();
for (int j = 0; j < edges[i].Length; j++)
int j = edges[i];
if (j != -1)
{
if (edges[i][j] != -1) matches.Add(edges[i][j]);
// i <-> j
adjacency[i].Add(j);
adjacency[j].Add(i);
}

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>>();
List<List<int>> groupedIndexes = new List<List<int>>();
bool[] isDone = new bool[edges.Length];

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;
Expand All @@ -290,22 +256,33 @@ internal static List<HashSet<int>> GroupIndexes(int[][] edges)
/// 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 List<int> DfsIterative(int s, List<int>[] adj, ref bool[] isDone)
{
HashSet<int> group = new HashSet<int>();
Stack<int> S = new Stack<int>();
List<int> group = new List<int>();
Stack<int> S = new Stack<int>(4);
S.Push(s);

isDone[s] = true;
while (S.Count > 0)
{
var u = S.Pop();
if (!isDone[u])
group.Add(u);

#if NET
var currentAdj = System.Runtime.InteropServices.CollectionsMarshal.AsSpan(adj[u]);
int count = currentAdj.Length;
#else
var currentAdj = adj[u];
int count = currentAdj.Count;
#endif
for (int i = 0; i < count; ++i)
{
group.Add(u);
isDone[u] = true;
foreach (var v in adj[u])
var v = currentAdj[i];
ref bool done = ref isDone[v];
if (!done)
{
S.Push(v);
done = true;
}
}
}
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