update

Task/Resistor-mesh/Go/resistor-mesh.go

@@ -0,0 +1,102 @@
+package main
+
+import "fmt"
+
+const (
+	S = 10
+)
+
+type node struct {
+	v     float64
+	fixed int
+}
+
+func alloc2(w, h int) [][]node {
+	a := make([][]node, h)
+
+	for i := range a {
+		a[i] = make([]node, w)
+	}
+	return a
+}
+
+func set_boundary(m [][]node) {
+	m[1][1].fixed = 1
+	m[1][1].v = 1
+	m[6][7].fixed = -1
+	m[6][7].v = -1
+}
+
+func calc_diff(m [][]node, d [][]node, w, h int) float64 {
+	total := 0.0
+	for i := 0; i < h; i++ {
+		for j := 0; j < w; j++ {
+			v := 0.0
+			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/float64(n)
+			d[i][j].v = v
+			if m[i][j].fixed == 0 {
+				total += v * v
+			}
+		}
+	}
+	return total
+}
+
+func iter(m [][]node, w, h int) float64 {
+	d := alloc2(w, h)
+	diff := 1.0e10
+	cur := []float64{0, 0, 0}
+
+	for diff > 1e-24 {
+		set_boundary(m)
+		diff = calc_diff(m, d, w, h)
+		for i := 0; i < h; i++ {
+			for j := 0; j < w; j++ {
+				m[i][j].v -= d[i][j].v
+			}
+		}
+	}
+
+	for i := 0; i < h; i++ {
+		for j := 0; j < w; j++ {
+			t := 0
+			if i != 0 {
+				t += 1
+			}
+			if j != 0 {
+				t += 1
+			}
+			if i < h-1 {
+				t += 1
+			}
+			if j < w-1 {
+				t += 1
+			}
+			cur[m[i][j].fixed+1] += d[i][j].v * float64(t)
+		}
+	}
+	return (cur[2] - cur[0]) / 2
+}
+
+func main() {
+	mesh := alloc2(S, S)
+	fmt.Printf("R = %g\n", 2/iter(mesh, S, S))
+}