From f8643f98c9a091cd9b56fc9468197ae58df0364b Mon Sep 17 00:00:00 2001
From: Ben Culkin <scorpress@gmail.com>
Date: Sat, 24 Sep 2022 12:35:09 -0400
Subject: Implement basic Petri nets

---
 src/main/java/bjc/data/PetriTransition.java | 127 ++++++++++++++++++++++++++++
 1 file changed, 127 insertions(+)
 create mode 100644 src/main/java/bjc/data/PetriTransition.java

(limited to 'src/main/java/bjc/data/PetriTransition.java')

diff --git a/src/main/java/bjc/data/PetriTransition.java b/src/main/java/bjc/data/PetriTransition.java
new file mode 100644
index 0000000..d1d20f9
--- /dev/null
+++ b/src/main/java/bjc/data/PetriTransition.java
@@ -0,0 +1,127 @@
+package bjc.data;
+
+import java.util.*;
+import java.util.Map.Entry;
+
+public class PetriTransition<Label> {
+	private final Label name;
+
+	private Map<Label, Integer> sources;
+	private Map<Label, Integer> destinations;
+
+	public PetriTransition(Label name) {
+		this.name = name;
+
+		sources = new HashMap<>();
+		destinations = new HashMap<>();
+	}
+
+	public void addSource(Label lab, int tokens) {
+		sources.merge(lab, tokens, (k, v) -> v + tokens);
+	}
+
+	public int removeSource(Label lab) {
+		return sources.remove(lab);
+	}
+
+	public void addDestination(Label lab, int tokens) {
+		destinations.merge(lab, tokens, (k, v) -> v + tokens);
+	}
+
+	public int removeDestination(Label lab, int tokens) {
+		return destinations.remove(lab);
+	}
+
+	public boolean apply(Map<Label, Integer> nodes) {
+		Set<Entry<Label, Integer>> debts = new HashSet<>();
+		boolean failed = false;
+
+		for (Entry<Label, Integer> source : sources.entrySet()) {
+			Label lab = source.getKey();
+			int cost = source.getValue();
+
+			if (nodes.containsKey(lab)) {
+				int count = nodes.get(lab);
+				if (count > cost) {
+					nodes.put(lab, count - cost);
+				} else {
+					failed = true;
+					break;
+				}
+			} else {
+				throw new IllegalArgumentException("Node " + lab + "missing; needed for transition " + name);
+			}
+		}
+
+		// If we failed, we may need to re-deposit some tokens
+		if (failed) {
+			for (Entry<Label, Integer> debt : debts) {
+				nodes.computeIfPresent(debt.getKey(), (k, v) -> v + debt.getValue());
+			}
+			return false;
+		}
+
+		// NOTE: One possibility would be to use the same loop for both of these, and
+		// just negate sources by default.
+		for (Entry<Label, Integer> destination : destinations.entrySet()) {
+			Label lab = destination.getKey();
+			int profit = destination.getValue();
+
+			if (nodes.containsKey(lab)) {
+				int count = nodes.get(lab);
+				if (count > profit) {
+					nodes.put(lab, count + profit);
+				} else {
+					failed = true;
+					break;
+				}
+			} else {
+				throw new IllegalArgumentException("Node " + lab + "missing; needed for transition " + name);
+			}
+		}
+
+		return true;
+	}
+	
+	public void merge(PetriTransition<Label> transition) {
+		for (Entry<Label, Integer> source : transition.sources.entrySet()) {
+			int val = source.getValue();
+			sources.merge(source.getKey(), val, (k, v) -> v + val);
+		}
+		for (Entry<Label, Integer> destination : transition.destinations.entrySet()) {
+			int val = destination.getValue();
+			sources.merge(destination.getKey(), val, (k, v) -> v + val);
+		}
+	}
+	@Override
+	public String toString() {
+		return String.format("PetriTransition [name=%s, sources=%s, destinations=%s]", name, sources, destinations);
+	}
+
+	@Override
+	public int hashCode() {
+		return Objects.hash(destinations, name, sources);
+	}
+
+	@Override
+	public boolean equals(Object obj) {
+		if (this == obj)
+			return true;
+		if (obj == null)
+			return false;
+		if (getClass() != obj.getClass())
+			return false;
+		PetriTransition<?> other = (PetriTransition<?>) obj;
+		return Objects.equals(destinations, other.destinations) && Objects.equals(name, other.name)
+				&& Objects.equals(sources, other.sources);
+	}
+
+	@SafeVarargs
+	public static <Label> PetriTransition<Label> merged(Label mergeName, PetriTransition<Label>... transitions) {
+		PetriTransition<Label> result = new PetriTransition<>(mergeName);
+		for (PetriTransition<Label> transition : transitions) {
+			result.merge(transition);
+		}
+		return result;
+	}
+}
\ No newline at end of file
-- 
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