Fix SPFA negative cycle detection across all vertices

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 We follow SemVer as most of the Julia ecosystem. Below you might see the "breaking" label even for minor version bumps -- we use it a bit more loosely to denote things that are not breaking by SemVer's definition but might cause breakage to people using internal or experimental APIs or undocumented implementation details. 
 
+## Unreleased
+- `has_negative_edge_cycle_spfa` now detects negative cycles unreachable from vertex 1
+
 ## v1.15.0 - 2026-09-07
 - `is_articulation(g, v)` for checking whether a single vertex is an articulation point
 - The iFUB algorithm is used for faster diameter calculation and now supports weighted graph diameter calculation

src/shortestpaths/spfa.jl

@@ -51,15 +51,21 @@ function spfa_shortest_paths(
 
     (source in 1:nvg) ||
         throw(DomainError(source, "source should be in between 1 and $nvg"))
+    return _spfa_shortest_paths(graph, [source], distmx)
+end
+
+function _spfa_shortest_paths(
+    graph::AbstractGraph{U}, sources::AbstractVector{<:Integer}, distmx::AbstractMatrix{T}
+) where {T<:Number} where {U<:Integer}
+    nvg = nv(graph)
     dists = fill(typemax(T), nvg)
-    dists[source] = 0
+    dists[sources] .= zero(T)
 
     count = zeros(U, nvg)           # Vector to store the count of number of times a vertex goes in the queue.
 
-    queue = Vector{U}()             # Vector used to implement queue
+    queue = Vector{U}(sources)      # Vector used to implement queue
     inqueue = falses(nvg, 1)         # BitArray to mark which vertices are in queue
-    push!(queue, source)
-    inqueue[source] = true
+    inqueue[sources] .= true
 
     @inbounds while !isempty(queue)
         v = popfirst!(queue)
@@ -117,7 +123,7 @@ function has_negative_edge_cycle_spfa(
     g::AbstractGraph{U}, distmx::AbstractMatrix{T}
 ) where {T<:Number} where {U<:Integer}
     try
-        spfa_shortest_paths(g, 1, distmx)
+        _spfa_shortest_paths(g, collect_if_not_vector(vertices(g)), distmx)
     catch e
         isa(e, NegativeCycleError) && return true
     end

test/shortestpaths/spfa.jl

@@ -197,6 +197,16 @@
     end
 
     @testset "Negative Cycle" begin
+        @testset "Negative cycle unreachable from vertex 1" begin
+            gx = SimpleDiGraph(3)
+            add_edge!(gx, 2, 3)
+            add_edge!(gx, 3, 2)
+            d = [0 0 0; 0 0 -2; 0 1 0]
+            for g in test_generic_graphs(gx)
+                @test has_negative_edge_cycle_spfa(g, d)
+            end
+        end
+
         # Negative Cycle 1
         gx = complete_graph(3)
         for g in test_generic_graphs(gx)