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// Copyright (c) StellaOps. All rights reserved.
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// Licensed under AGPL-3.0-or-later. See LICENSE in the project root.
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using System.Collections.Immutable;
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using System.Globalization;
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using System.Security.Cryptography;
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using System.Text;
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namespace StellaOps.BinaryIndex.Semantic.Internal;
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/// <summary>
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/// Weisfeiler-Lehman graph hashing for deterministic semantic fingerprints.
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/// Uses iterative label refinement to capture graph structure.
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/// </summary>
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internal sealed class WeisfeilerLehmanHasher
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{
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private readonly int _iterations;
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/// <summary>
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/// Creates a new Weisfeiler-Lehman hasher.
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/// </summary>
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/// <param name="iterations">Number of WL iterations (default: 3).</param>
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public WeisfeilerLehmanHasher(int iterations = 3)
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{
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ArgumentOutOfRangeException.ThrowIfLessThan(iterations, 1);
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_iterations = iterations;
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}
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/// <summary>
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/// Compute a deterministic hash of the semantic graph.
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/// </summary>
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/// <param name="graph">The semantic graph to hash.</param>
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/// <returns>SHA-256 hash of the graph.</returns>
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public byte[] ComputeHash(KeySemanticsGraph graph)
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{
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ArgumentNullException.ThrowIfNull(graph);
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if (graph.Nodes.IsEmpty)
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{
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return SHA256.HashData(Encoding.UTF8.GetBytes("EMPTY_GRAPH"));
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}
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// Build adjacency lists for efficient neighbor lookup
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var outEdges = BuildAdjacencyList(graph.Edges, e => e.SourceId, e => e.TargetId);
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var inEdges = BuildAdjacencyList(graph.Edges, e => e.TargetId, e => e.SourceId);
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// Initialize labels from node properties
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var labels = InitializeLabels(graph.Nodes);
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// WL iterations
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for (var i = 0; i < _iterations; i++)
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{
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labels = RefineLabels(graph.Nodes, labels, outEdges, inEdges, graph.Edges);
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}
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// Compute final hash from sorted labels
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return ComputeFinalHash(labels);
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}
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/// <summary>
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/// Compute canonical labels for all nodes (useful for graph comparison).
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/// </summary>
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/// <param name="graph">The semantic graph.</param>
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/// <returns>Array of canonical labels indexed by node ID.</returns>
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public ImmutableArray<string> ComputeCanonicalLabels(KeySemanticsGraph graph)
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{
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ArgumentNullException.ThrowIfNull(graph);
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if (graph.Nodes.IsEmpty)
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{
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return [];
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}
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var outEdges = BuildAdjacencyList(graph.Edges, e => e.SourceId, e => e.TargetId);
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var inEdges = BuildAdjacencyList(graph.Edges, e => e.TargetId, e => e.SourceId);
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var labels = InitializeLabels(graph.Nodes);
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for (var i = 0; i < _iterations; i++)
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{
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labels = RefineLabels(graph.Nodes, labels, outEdges, inEdges, graph.Edges);
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}
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// Return labels in node ID order
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var maxId = graph.Nodes.Max(n => n.Id);
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var result = new string[maxId + 1];
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foreach (var node in graph.Nodes)
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{
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result[node.Id] = labels.TryGetValue(node.Id, out var label) ? label : string.Empty;
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}
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return [.. result];
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}
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private static Dictionary<int, List<int>> BuildAdjacencyList(
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ImmutableArray<SemanticEdge> edges,
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Func<SemanticEdge, int> keySelector,
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Func<SemanticEdge, int> valueSelector)
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{
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var result = new Dictionary<int, List<int>>();
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foreach (var edge in edges)
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{
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var key = keySelector(edge);
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var value = valueSelector(edge);
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if (!result.TryGetValue(key, out var list))
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{
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list = [];
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result[key] = list;
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}
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list.Add(value);
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}
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return result;
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}
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private static Dictionary<int, string> InitializeLabels(ImmutableArray<SemanticNode> nodes)
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{
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var labels = new Dictionary<int, string>(nodes.Length);
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foreach (var node in nodes)
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{
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// Create initial label from node type and operation
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var label = string.Create(
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CultureInfo.InvariantCulture,
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$"{(int)node.Type}:{node.Operation}");
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labels[node.Id] = label;
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}
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return labels;
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}
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private static Dictionary<int, string> RefineLabels(
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ImmutableArray<SemanticNode> nodes,
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Dictionary<int, string> currentLabels,
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Dictionary<int, List<int>> outEdges,
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Dictionary<int, List<int>> inEdges,
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ImmutableArray<SemanticEdge> edges)
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{
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var newLabels = new Dictionary<int, string>(nodes.Length);
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var edgeLookup = BuildEdgeLookup(edges);
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foreach (var node in nodes)
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{
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var sb = new StringBuilder();
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sb.Append(currentLabels[node.Id]);
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sb.Append('|');
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// Append sorted outgoing neighbor labels with edge types
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if (outEdges.TryGetValue(node.Id, out var outNeighbors))
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{
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var neighborLabels = outNeighbors
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.Select(n =>
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{
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var edgeType = GetEdgeType(edgeLookup, node.Id, n);
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return string.Create(
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CultureInfo.InvariantCulture,
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$"O{(int)edgeType}:{currentLabels[n]}");
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})
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.OrderBy(l => l, StringComparer.Ordinal)
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.ToList();
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sb.AppendJoin(',', neighborLabels);
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}
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sb.Append('|');
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// Append sorted incoming neighbor labels with edge types
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if (inEdges.TryGetValue(node.Id, out var inNeighbors))
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{
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var neighborLabels = inNeighbors
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.Select(n =>
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{
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var edgeType = GetEdgeType(edgeLookup, n, node.Id);
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return string.Create(
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CultureInfo.InvariantCulture,
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$"I{(int)edgeType}:{currentLabels[n]}");
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})
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.OrderBy(l => l, StringComparer.Ordinal)
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.ToList();
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sb.AppendJoin(',', neighborLabels);
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}
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// Hash the combined string to create new label
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var combined = sb.ToString();
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var hash = SHA256.HashData(Encoding.UTF8.GetBytes(combined));
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newLabels[node.Id] = Convert.ToHexString(hash)[..16]; // Use first 16 hex chars
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}
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return newLabels;
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}
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private static Dictionary<(int, int), SemanticEdgeType> BuildEdgeLookup(ImmutableArray<SemanticEdge> edges)
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{
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var lookup = new Dictionary<(int, int), SemanticEdgeType>(edges.Length);
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foreach (var edge in edges)
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{
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lookup[(edge.SourceId, edge.TargetId)] = edge.Type;
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}
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return lookup;
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}
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private static SemanticEdgeType GetEdgeType(
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Dictionary<(int, int), SemanticEdgeType> lookup,
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int source,
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int target)
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{
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return lookup.TryGetValue((source, target), out var type) ? type : SemanticEdgeType.Unknown;
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}
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private static byte[] ComputeFinalHash(Dictionary<int, string> labels)
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{
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// Sort labels for deterministic output
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var sortedLabels = labels.Values
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.OrderBy(l => l, StringComparer.Ordinal)
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.ToList();
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var combined = string.Join("|", sortedLabels);
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return SHA256.HashData(Encoding.UTF8.GetBytes(combined));
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}
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}
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