fix issue #1

Author: yosupo06

Diff for: atcoder/maxflow.hpp

@@ -21,8 +21,11 @@ template <class Cap> struct mf_graph {
         assert(0 <= cap);
         int m = int(pos.size());
         pos.push_back({from, int(g[from].size())});
-        g[from].push_back(_edge{to, int(g[to].size()), cap});
-        g[to].push_back(_edge{from, int(g[from].size()) - 1, 0});
+        int from_id = int(g[from].size());
+        int to_id = int(g[to].size());
+        if (from == to) to_id++;
+        g[from].push_back(_edge{to, to_id, cap});
+        g[to].push_back(_edge{from, from_id, 0});
         return m;
     }
 

Diff for: atcoder/mincostflow.hpp

@@ -19,8 +19,11 @@ template <class Cap, class Cost> struct mcf_graph {
         assert(0 <= to && to < _n);
         int m = int(pos.size());
         pos.push_back({from, int(g[from].size())});
-        g[from].push_back(_edge{to, int(g[to].size()), cap, cost});
-        g[to].push_back(_edge{from, int(g[from].size()) - 1, 0, -cost});
+        int from_id = int(g[from].size());
+        int to_id = int(g[to].size());
+        if (from == to) to_id++;
+        g[from].push_back(_edge{to, to_id, cap, cost});
+        g[to].push_back(_edge{from, from_id, 0, -cost});
         return m;
     }
 

Diff for: test/unittest/maxflow_test.cpp

@@ -169,3 +169,12 @@ TEST(MaxflowTest, BoundUint) {
     e = {0, 2, INF, INF};
     edge_eq(e, g.get_edge(2));
 }
+
+// https://github.com/atcoder/ac-library/issues/1
+TEST(MaxflowTest, SelfLoop) {
+    mf_graph<int> g(3);
+    ASSERT_EQ(0, g.add_edge(0, 0, 100));
+
+    mf_graph<int>::edge e = {0, 0, 100, 0};
+    edge_eq(e, g.get_edge(0));
+}

Diff for: test/unittest/mincostflow_test.cpp

@@ -70,3 +70,12 @@ TEST(MincostflowTest, OutOfRange) {
     EXPECT_DEATH(g.slope(-1, 3), ".*");
     EXPECT_DEATH(g.slope(3, 3), ".*");
 }
+
+// https://github.com/atcoder/ac-library/issues/1
+TEST(MaxflowTest, SelfLoop) {
+    mcf_graph<int, int> g(3);
+    ASSERT_EQ(0, g.add_edge(0, 0, 100, 123));
+
+    mcf_graph<int, int>::edge e = {0, 0, 100, 0, 123};
+    edge_eq(e, g.get_edge(0));
+}