June 2018 Update

Task/Resistor-mesh/Java/resistor-mesh.java

@@ -0,0 +1,107 @@
+import java.util.ArrayList;
+import java.util.List;
+
+public class ResistorMesh {
+    private static final int S = 10;
+
+    private static class Node {
+        double v;
+        int fixed;
+
+        Node(double v, int fixed) {
+            this.v = v;
+            this.fixed = fixed;
+        }
+    }
+
+    private static void setBoundary(List<List<Node>> m) {
+        m.get(1).get(1).v = 1.0;
+        m.get(1).get(1).fixed = 1;
+
+        m.get(6).get(7).v = -1.0;
+        m.get(6).get(7).fixed = -1;
+    }
+
+    private static double calcDiff(List<List<Node>> m, List<List<Node>> d, int w, int h) {
+        double total = 0.0;
+        for (int i = 0; i < h; ++i) {
+            for (int j = 0; j < w; ++j) {
+                double v = 0.0;
+                int n = 0;
+                if (i > 0) {
+                    v += m.get(i - 1).get(j).v;
+                    n++;
+                }
+                if (j > 0) {
+                    v += m.get(i).get(j - 1).v;
+                    n++;
+                }
+                if (i + 1 < h) {
+                    v += m.get(i + 1).get(j).v;
+                    n++;
+                }
+                if (j + 1 < w) {
+                    v += m.get(i).get(j + 1).v;
+                    n++;
+                }
+                v = m.get(i).get(j).v - v / n;
+                d.get(i).get(j).v = v;
+                if (m.get(i).get(j).fixed == 0) {
+                    total += v * v;
+                }
+            }
+        }
+        return total;
+    }
+
+    private static double iter(List<List<Node>> m, int w, int h) {
+        List<List<Node>> d = new ArrayList<>(h);
+        for (int i = 0; i < h; ++i) {
+            List<Node> t = new ArrayList<>(w);
+            for (int j = 0; j < w; ++j) {
+                t.add(new Node(0.0, 0));
+            }
+            d.add(t);
+        }
+
+        double[] cur = new double[3];
+        double diff = 1e10;
+
+        while (diff > 1e-24) {
+            setBoundary(m);
+            diff = calcDiff(m, d, w, h);
+            for (int i = 0; i < h; ++i) {
+                for (int j = 0; j < w; ++j) {
+                    m.get(i).get(j).v -= d.get(i).get(j).v;
+                }
+            }
+        }
+
+        for (int i = 0; i < h; ++i) {
+            for (int j = 0; j < w; ++j) {
+                int k = 0;
+                if (i != 0) k++;
+                if (j != 0) k++;
+                if (i < h - 1) k++;
+                if (j < w - 1) k++;
+                cur[m.get(i).get(j).fixed + 1] += d.get(i).get(j).v * k;
+            }
+        }
+
+        return (cur[2] - cur[0]) / 2.0;
+    }
+
+    public static void main(String[] args) {
+        List<List<Node>> mesh = new ArrayList<>(S);
+        for (int i = 0; i < S; ++i) {
+            List<Node> t = new ArrayList<>(S);
+            for (int j = 0; j < S; ++j) {
+                t.add(new Node(0.0, 0));
+            }
+            mesh.add(t);
+        }
+
+        double r = 2.0 / iter(mesh, S, S);
+        System.out.printf("R = %.15f", r);
+    }
+}