From cbc62f94c241b5567b1e1dfafad837710ed4961d Mon Sep 17 00:00:00 2001
From: Paul Jungeblut <paul.jungeblut@gmail.com>
Date: Wed, 14 Dec 2016 00:04:53 +0100
Subject: Adding 2-SAT code.

---
 graph/2sat.cpp | 66 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 66 insertions(+)
 create mode 100644 graph/2sat.cpp

(limited to 'graph/2sat.cpp')

diff --git a/graph/2sat.cpp b/graph/2sat.cpp
new file mode 100644
index 0000000..bc1572d
--- /dev/null
+++ b/graph/2sat.cpp
@@ -0,0 +1,66 @@
+struct sat2 {
+	vector<vector<int>> adjlist, sccs;
+	vector<bool> visited, inStack;
+	int n, sccCounter, dfsCounter;
+	vector<int> d, low, idx, sol;
+	stack<int> s;
+
+	sat2(int vars) : n(vars*2) { adjlist.resize(n);	};
+
+	static int var(int i) { return i << 1; }
+
+	void addImpl(int v1, int v2) {
+		adjlist[v1].push_back(v2);
+		adjlist[1^v2].push_back(1^v1);
+	}
+	void addEquiv(int v1, int v2) { addImpl(v1, v2); addImpl(v2, v1); }
+	void addOr(int v1, int v2) { addImpl(1^v1, v2); addImpl(1^v2, v1); }
+	void addXor(int v1, int v2) { addOr(v1, v2); addOr(1^v1, 1^v2); }
+	void addTrue(int v1) { addImpl(1^v1, v1); }
+	void addFalse(int v1) { addTrue(1^v1); }
+	void addAnd(int v1, int v2) {	addTrue(v1); addTrue(v2); }
+
+	void dfs(int v) {
+		visited[v] = true;
+		d[v] = low[v] = dfsCounter++;
+		s.push(v); inStack[v] = true;
+
+		for (auto w : adjlist[v]) {
+			if (!visited[w]) {
+				dfs(w);
+				low[v] = min(low[v], low[w]);
+			} else if (inStack[w]) low[v] = min(low[v], low[w]);
+		}
+
+		if (d[v] == low[v]) {
+			sccs.push_back(vector<int>());
+			int w;
+			do {
+				w = s.top(); s.pop(); inStack[w] = false;
+				idx[w] = sccCounter;
+				sccs[sccCounter].push_back(w);
+			} while (w != v);
+			sccCounter++;
+	}}
+
+	bool solvable() {
+		visited.assign(n, false);
+		inStack.assign(n, false);
+		d.assign(n, -1);
+		low.assign(n, -1);
+		idx.assign(n, -1);
+		sccCounter = dfsCounter = 0;
+		for (int i = 0; i < n; i++) if (!visited[i]) dfs(i);
+		for (int i = 0; i < n; i += 2) if (idx[i] == idx[i + 1]) return false;
+		return true;
+	}
+
+	void assign() {
+		sol.assign(n, -1);
+		for (int i = 0; i < sccCounter; i++) {
+			if (sol[sccs[i][0]] == -1) {
+				for (int v : sccs[i]) {
+					sol[v] = 1;
+					sol[1^v] = 0;
+	}}}}
+};
-- 
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