Fix #301: [Model] MinimumDummyActivitiesPert (#751)

* Add plan for #301: [Model] MinimumDummyActivitiesPert

* Implement #301: [Model] MinimumDummyActivitiesPert

* chore: remove plan file after implementation

* Fix formatting after merge with main

* Fix dummy arc overcount on transitive arcs, add regression test, fix paper solve command

- Subtract task-arc intersections from dummy count: when a non-merged
  arc's dummy coincides with a task arc, it doesn't add a new arc to the
  event network and shouldn't be counted.
- Add regression test for DAG 0→1, 1→2, 0→2 (optimal = 0 dummies).
- Fix paper's pred solve command to include --solver brute-force.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Fix formatting after merge with main

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: GiggleLiu

docs/paper/reductions.typ

@@ -141,6 +141,7 @@
   "MinMaxMulticenter": [Min-Max Multicenter],
   "FlowShopScheduling": [Flow Shop Scheduling],
   "MinimumCutIntoBoundedSets": [Minimum Cut Into Bounded Sets],
+  "MinimumDummyActivitiesPert": [Minimum Dummy Activities in PERT Networks],
   "MinimumSumMulticenter": [Minimum Sum Multicenter],
   "MinimumTardinessSequencing": [Minimum Tardiness Sequencing],
   "MultipleChoiceBranching": [Multiple Choice Branching],
@@ -2126,6 +2127,58 @@ is feasible: each set induces a connected subgraph, the component weights are $2
   ]
 }
 
+#{
+  let x = load-model-example("MinimumDummyActivitiesPert")
+  let nv = x.instance.graph.num_vertices
+  let arcs = x.instance.graph.arcs
+  let ne = arcs.len()
+  let sol = (config: x.optimal_config, metric: x.optimal_value)
+  let merged = arcs.enumerate().filter(((i, _)) => sol.config.at(i) == 1).map(((i, arc)) => arc)
+  let dummy = arcs.enumerate().filter(((i, _)) => sol.config.at(i) == 0).map(((i, arc)) => arc)
+  let opt = sol.metric.Valid
+  let blue = graph-colors.at(0)
+  [
+    #problem-def("MinimumDummyActivitiesPert")[
+      Given a precedence DAG $G = (V, A)$, find an activity-on-arc PERT event network with one real activity arc for each task $v in V$, minimizing the number of dummy activity arcs, such that for every ordered pair of tasks $(u, v)$ there is a path from the finish event of $u$ to the start event of $v$ if and only if $v$ is reachable from $u$ in $G$.
+    ][
+    The decision version of minimum dummy activities appears as ND44 in Garey and Johnson's compendium @garey1979. It arises when an activity-on-node precedence DAG must be converted into an activity-on-arc PERT chart: merging compatible finish/start events removes dummy activities, but an over-aggressive merge creates spurious precedence relations between unrelated tasks. The implementation here enumerates, for each direct precedence arc, whether it is realized as an event merge or left as a dummy activity, so brute-force over the $m = #ne$ direct precedences yields a worst-case bound of $O^*(2^m)$. #footnote[No exact algorithm improving on the direct-precedence merge encoding implemented in the codebase is claimed here.]
+
+    *Example.* Consider the canonical precedence DAG on $n = #nv$ tasks with direct precedences #arcs.map(a => $(v_#(a.at(0)), v_#(a.at(1)))$).join(", "). The optimal encoding merges the predecessor-finish/successor-start pairs #merged.map(a => $(v_#(a.at(0)), v_#(a.at(1)))$).join(", "), so those handoffs need no dummy activity at all. The remaining direct precedences #dummy.map(a => $(v_#(a.at(0)), v_#(a.at(1)))$).join(" and ") still require dummy activities, so the optimum is $#opt$. Both unresolved precedences enter $v_3$, and merging either of them would identify unrelated task completions, creating spurious reachability between the two source tasks.
+
+    #pred-commands(
+      "pred create --example " + problem-spec(x) + " -o minimum-dummy-activities-pert.json",
+      "pred solve minimum-dummy-activities-pert.json --solver brute-force",
+      "pred evaluate minimum-dummy-activities-pert.json --config " + x.optimal_config.map(str).join(","),
+    )
+
+    #figure({
+      let positions = ((0, 1.0), (0, -0.3), (2.0, 1.3), (2.0, 0.35), (2.0, -0.95), (4.0, 1.3))
+      canvas(length: 1cm, {
+        for (k, pos) in positions.enumerate() {
+          g-node(pos, name: "v" + str(k),
+            fill: white,
+            label: [$v_#k$])
+        }
+        for arc in dummy {
+          let (u, v) = arc
+          draw.line("v" + str(u), "v" + str(v),
+            stroke: (paint: luma(140), thickness: 1pt, dash: "dashed"),
+            mark: (end: "straight", scale: 0.4))
+        }
+        for arc in merged {
+          let (u, v) = arc
+          draw.line("v" + str(u), "v" + str(v),
+            stroke: 1.7pt + blue,
+            mark: (end: "straight", scale: 0.45))
+        }
+      })
+    },
+    caption: [Canonical Minimum Dummy Activities in PERT Networks instance. Blue precedence arcs are encoded by merging the predecessor finish event with the successor start event; dashed gray arcs still require dummy activities. The optimal encoding leaves exactly #opt dummy activities.],
+    ) <fig:minimum-dummy-activities-pert>
+    ]
+  ]
+}
+
 #{
   let x = load-model-example("MinimumFeedbackVertexSet")
   let nv = graph-num-vertices(x.instance)

problemreductions-cli/src/cli.rs

@@ -302,6 +302,7 @@ Flags by problem type:
   SCS                             --strings, --bound [--alphabet-size]
   StringToStringCorrection         --source-string, --target-string, --bound [--alphabet-size]
   D2CIF                           --arcs, --capacities, --source-1, --sink-1, --source-2, --sink-2, --requirement-1, --requirement-2
+  MinimumDummyActivitiesPert      --arcs [--num-vertices]
   CBQ                              --domain-size, --relations, --conjuncts-spec
   ILP, CircuitSAT                 (via reduction only)
 
@@ -331,6 +332,7 @@ Examples:
   pred create ConsistencyOfDatabaseFrequencyTables --num-objects 6 --attribute-domains \"2,3,2\" --frequency-tables \"0,1:1,1,1|1,1,1;1,2:1,1|0,2|1,1\" --known-values \"0,0,0;3,0,1;1,2,1\"
   pred create BiconnectivityAugmentation --graph 0-1,1-2,2-3 --potential-edges 0-2:3,0-3:4,1-3:2 --budget 5
   pred create FVS --arcs \"0>1,1>2,2>0\" --weights 1,1,1
+  pred create MinimumDummyActivitiesPert --arcs \"0>2,0>3,1>3,1>4,2>5\" --num-vertices 6
   pred create UndirectedTwoCommodityIntegralFlow --graph 0-2,1-2,2-3 --capacities 1,1,2 --source-1 0 --sink-1 3 --source-2 1 --sink-2 3 --requirement-1 1 --requirement-2 1
   pred create IntegralFlowHomologousArcs --arcs \"0>1,0>2,1>3,2>3,1>4,2>4,3>5,4>5\" --capacities 1,1,1,1,1,1,1,1 --source 0 --sink 5 --requirement 2 --homologous-pairs \"2=5;4=3\"
   pred create X3C --universe 9 --sets \"0,1,2;0,2,4;3,4,5;3,5,7;6,7,8;1,4,6;2,5,8\"

problemreductions-cli/src/commands/create.rs

@@ -14,8 +14,9 @@ use problemreductions::models::formula::Quantifier;
 use problemreductions::models::graph::{
     DisjointConnectingPaths, GeneralizedHex, GraphPartitioning, HamiltonianCircuit,
     HamiltonianPath, IntegralFlowBundles, LengthBoundedDisjointPaths, LongestCircuit, LongestPath,
-    MinimumCutIntoBoundedSets, MinimumMultiwayCut, MixedChinesePostman, MultipleChoiceBranching,
-    PathConstrainedNetworkFlow, SteinerTree, SteinerTreeInGraphs, StrongConnectivityAugmentation,
+    MinimumCutIntoBoundedSets, MinimumDummyActivitiesPert, MinimumMultiwayCut, MixedChinesePostman,
+    MultipleChoiceBranching, PathConstrainedNetworkFlow, SteinerTree, SteinerTreeInGraphs,
+    StrongConnectivityAugmentation,
 };
 use problemreductions::models::misc::{
     AdditionalKey, BinPacking, BoyceCoddNormalFormViolation, CbqRelation, ConjunctiveBooleanQuery,
@@ -617,6 +618,7 @@ fn example_for(canonical: &str, graph_type: Option<&str>) -> &'static str {
             "--arcs \"0>2,0>3,1>2,1>3,2>4,2>5,3>4,3>5\" --capacities 1,1,1,1,1,1,1,1 --source-1 0 --sink-1 4 --source-2 1 --sink-2 5 --requirement-1 1 --requirement-2 1"
         }
         "MinimumFeedbackArcSet" => "--arcs \"0>1,1>2,2>0\"",
+        "MinimumDummyActivitiesPert" => "--arcs \"0>2,0>3,1>3,1>4,2>5\" --num-vertices 6",
         "StrongConnectivityAugmentation" => {
             "--arcs \"0>1,1>2\" --candidate-arcs \"2>0:1\" --bound 1"
         }
@@ -3608,6 +3610,22 @@ pub fn create(args: &CreateArgs, out: &OutputConfig) -> Result<()> {
             )
         }
 
+        // MinimumDummyActivitiesPert
+        "MinimumDummyActivitiesPert" => {
+            let usage = "Usage: pred create MinimumDummyActivitiesPert --arcs \"0>2,0>3,1>3,1>4,2>5\" [--num-vertices N]";
+            let arcs_str = args.arcs.as_deref().ok_or_else(|| {
+                anyhow::anyhow!(
+                    "MinimumDummyActivitiesPert requires --arcs\n\n\
+                     {usage}"
+                )
+            })?;
+            let (graph, _) = parse_directed_graph(arcs_str, args.num_vertices)?;
+            (
+                ser(MinimumDummyActivitiesPert::try_new(graph).map_err(|e| anyhow::anyhow!(e))?)?,
+                resolved_variant.clone(),
+            )
+        }
+
         // MixedChinesePostman
         "MixedChinesePostman" => {
             let usage = "Usage: pred create MixedChinesePostman --graph 0-2,1-3,0-4,4-2 --arcs \"0>1,1>2,2>3,3>0\" --edge-weights 2,3,1,2 --arc-costs 2,3,1,4 --bound 24 [--num-vertices N]";
@@ -7166,6 +7184,56 @@ mod tests {
         assert!(err.contains("--num-vertices (5) is too small for the arcs"));
     }
 
+    #[test]
+    fn test_create_minimum_dummy_activities_pert_json() {
+        use crate::dispatch::ProblemJsonOutput;
+        use problemreductions::models::graph::MinimumDummyActivitiesPert;
+
+        let mut args = empty_args();
+        args.problem = Some("MinimumDummyActivitiesPert".to_string());
+        args.num_vertices = Some(6);
+        args.arcs = Some("0>2,0>3,1>3,1>4,2>5".to_string());
+
+        let output_path = temp_output_path("minimum_dummy_activities_pert");
+        let out = OutputConfig {
+            output: Some(output_path.clone()),
+            quiet: true,
+            json: false,
+            auto_json: false,
+        };
+
+        create(&args, &out).unwrap();
+
+        let json = fs::read_to_string(&output_path).unwrap();
+        let created: ProblemJsonOutput = serde_json::from_str(&json).unwrap();
+        assert_eq!(created.problem_type, "MinimumDummyActivitiesPert");
+        assert!(created.variant.is_empty());
+
+        let problem: MinimumDummyActivitiesPert = serde_json::from_value(created.data).unwrap();
+        assert_eq!(problem.num_vertices(), 6);
+        assert_eq!(problem.num_arcs(), 5);
+
+        let _ = fs::remove_file(output_path);
+    }
+
+    #[test]
+    fn test_create_minimum_dummy_activities_pert_rejects_cycles() {
+        let mut args = empty_args();
+        args.problem = Some("MinimumDummyActivitiesPert".to_string());
+        args.num_vertices = Some(3);
+        args.arcs = Some("0>1,1>2,2>0".to_string());
+
+        let out = OutputConfig {
+            output: None,
+            quiet: true,
+            json: false,
+            auto_json: false,
+        };
+
+        let err = create(&args, &out).unwrap_err().to_string();
+        assert!(err.contains("requires the input graph to be a DAG"));
+    }
+
     #[test]
     fn test_create_balanced_complete_bipartite_subgraph() {
         use crate::dispatch::ProblemJsonOutput;

src/lib.rs

@@ -59,12 +59,12 @@ pub mod prelude {
     pub use crate::models::graph::{
         KColoring, LongestCircuit, MaxCut, MaximalIS, MaximumClique, MaximumIndependentSet,
         MaximumMatching, MinMaxMulticenter, MinimumCutIntoBoundedSets, MinimumDominatingSet,
-        MinimumFeedbackArcSet, MinimumFeedbackVertexSet, MinimumMultiwayCut, MinimumSumMulticenter,
-        MinimumVertexCover, MultipleChoiceBranching, MultipleCopyFileAllocation,
-        OptimalLinearArrangement, PartitionIntoPathsOfLength2, PartitionIntoTriangles,
-        PathConstrainedNetworkFlow, RuralPostman, ShortestWeightConstrainedPath,
-        SteinerTreeInGraphs, TravelingSalesman, UndirectedFlowLowerBounds,
-        UndirectedTwoCommodityIntegralFlow,
+        MinimumDummyActivitiesPert, MinimumFeedbackArcSet, MinimumFeedbackVertexSet,
+        MinimumMultiwayCut, MinimumSumMulticenter, MinimumVertexCover, MultipleChoiceBranching,
+        MultipleCopyFileAllocation, OptimalLinearArrangement, PartitionIntoPathsOfLength2,
+        PartitionIntoTriangles, PathConstrainedNetworkFlow, RuralPostman,
+        ShortestWeightConstrainedPath, SteinerTreeInGraphs, TravelingSalesman,
+        UndirectedFlowLowerBounds, UndirectedTwoCommodityIntegralFlow,
     };
     pub use crate::models::misc::{
         AdditionalKey, BinPacking, BoyceCoddNormalFormViolation, CbqRelation,