Task: Resistor mesh



C#: using System;
using System.Collections.Generic;

namespace ResistorMesh {
    class Node {
        public Node(double v, int fixed_) {
            V = v;
            Fixed = fixed_;
        }

        public double V { get; set; }
        public int Fixed { get; set; }
    }

    class Program {
        static void SetBoundary(List<List<Node>> m) {
            m[1][1].V = 1.0;
            m[1][1].Fixed = 1;

            m[6][7].V = -1.0;
            m[6][7].Fixed = -1;
        }

        static double CalcuateDifference(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[i - 1][j].V;
                        n++;
                    }
                    if (j > 0) {
                        v += m[i][j - 1].V;
                        n++;
                    }
                    if (i + 1 < h) {
                        v += m[i + 1][j].V;
                        n++;
                    }
                    if (j + 1 < w) {
                        v += m[i][j + 1].V;
                        n++;
                    }
                    v = m[i][j].V - v / n;
                    d[i][j].V = v;
                    if (m[i][j].Fixed == 0) {
                        total += v * v;
                    }
                }
            }
            return total;
        }

        static double Iter(List<List<Node>> m, int w, int h) {
            List<List<Node>> d = new List<List<Node>>(h);
            for (int i = 0; i < h; i++) {
                List<Node> t = new List<Node>(w);
                for (int j = 0; j < w; j++) {
                    t.Add(new Node(0.0, 0));
                }
                d.Add(t);
            }

            double[] curr = new double[3];
            double diff = 1e10;

            while (diff > 1e-24) {
                SetBoundary(m);
                diff = CalcuateDifference(m, d, w, h);
                for (int i = 0; i < h; i++) {
                    for (int j = 0; j < w; j++) {
                        m[i][j].V -= d[i][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++;
                    curr[m[i][j].Fixed + 1] += d[i][j].V * k;
                }
            }

            return (curr[2] - curr[0]) / 2.0;
        }

        const int S = 10;
        static void Main(string[] args) {
            List<List<Node>> mesh = new List<List<Node>>(S);
            for (int i = 0; i < S; i++) {
                List<Node> t = new List<Node>(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);
            Console.WriteLine("R = {0:F15}", r);
        }
    }
}









Ada: with Ada.Text_IO; use Ada.Text_IO;
procedure ResistMesh is
   H, W : constant Positive := 10;
   rowA, colA : constant Positive := 2; 
   rowB : constant Positive := 7;
   colB : constant Positive := 8;

   type Ntype is (A, B, Free);
   type Vtype is digits 15;
   type Node is record
      volt : Vtype := 0.0;
      name : Ntype := Free;
   end record;
   type NodeMesh is array (Positive range <>, Positive range <>) of Node;
   package IIO is new Ada.Text_IO.Float_IO (Vtype);
   mesh, dmesh : NodeMesh (1 .. H, 1 .. W);
   curA, curB, diff : Vtype;

   procedure set_AB (mesh : in out NodeMesh) is begin
      mesh (rowA, colA).volt :=  1.0;  mesh (rowA, colA).name := A;
      mesh (rowB, colB).volt := -1.0;  mesh (rowB, colB).name := B;
   end set_AB;

   function sides (i : Positive; j : Positive) return Vtype is
      s : Integer := 0;
   begin
      if i /= 1 and i /= H then s := s + 2; else s := s + 1; end if;
      if j /= 1 and j /= W then s := s + 2; else s := s + 1; end if;
      return Vtype (s);
   end sides;

   procedure calc_diff (mesh : NodeMesh; dmesh : out NodeMesh;
      total : out Vtype)  is
      n : Natural;
      v : Vtype := 0.0;
   begin
      total := 0.0;
      for i in Positive range 1 .. H loop
         for j in Positive range 1 .. W loop
            n := 0;    v := 0.0;
            if  i /= 1 then v := v + mesh (i - 1, j).volt; n := n + 1; end if;
            if j /= 1 then  v := v + mesh (i, j - 1).volt; n := n + 1; end if;
            if i < H then v := v + mesh (i + 1, j).volt; n := n + 1; end if;
            if j < W then v := v + mesh (i, j + 1).volt; n := n + 1; end if;
            v := mesh (i, j).volt - v / Vtype (n);
            dmesh (i, j).volt := v;
            if mesh (i, j).name = Free then total := total + v ** 2; end if;
         end loop;
      end loop;
   end calc_diff;

begin

   loop
      set_AB (mesh);
      calc_diff (mesh, dmesh, diff);
      exit when diff < 1.0e-40;
      for i in Positive range 1 .. H loop
         for j in Positive range 1 .. W loop
            mesh (i, j).volt := mesh (i, j).volt - dmesh (i, j).volt;
         end loop;
      end loop;
   end loop;

   curA := dmesh (rowA, colA).volt * sides (rowA, colA);
   curB := dmesh (rowB, colB).volt * sides (rowB, colB);
   diff := 4.0 / (curA - curB);
   IIO.Put (diff, Exp => 0); New_Line;
end ResistMesh;