solve day8 part 2

2023/day08_left_right.go

@@ -44,11 +44,66 @@ func move(current string, step int, leftRight string, nodes map[string][2]string
 
 func Day08Part2(input io.Reader) (int, error) {
 	scanner := bufio.NewScanner(input)
+	scanner.Scan()
+	leftRight := scanner.Text()
+
+	scanner.Scan() // skip blank line
+
+	nodes := make(map[string][2]string)
+	var positions []string
 
 	for scanner.Scan() {
-		line := scanner.Text()
-		_ = line
+		line := strings.Split(scanner.Text(), " = ") // AAA = (BBB, CCC)
+		node := line[0]
+		rest := strings.Split(strings.NewReplacer("(", "", " ", "", ")", "").Replace(line[1]), ",")
+		nodes[node] = [2]string{rest[0], rest[1]}
+		if strings.HasSuffix(node, "A") {
+			positions = append(positions, node)
+		}
 	}
 
-	return 0, nil
+	step := 0
+	reachedZ := make([]int, len(positions))
+	for !allFinished(positions, reachedZ, step) {
+		for i := 0; i < len(positions); i++ {
+			positions[i] = move(positions[i], step, leftRight, nodes)
+		}
+		step++
+	}
+
+	return LCM(reachedZ[0], reachedZ[1], reachedZ[2:]...), nil
+}
+
+func allFinished(positions []string, reachedZ []int, step int) bool {
+	for i, pos := range positions {
+		if strings.HasSuffix(pos, "Z") && reachedZ[i] == 0 {
+			reachedZ[i] = step
+		}
+	}
+	for _, z := range reachedZ {
+		if z == 0 {
+			return false
+		}
+	}
+	return true
+}
+
+// GCD and LCM from https://siongui.github.io/2017/06/03/go-find-lcm-by-gcd/
+func GCD(a, b int) int {
+	for b != 0 {
+		t := b
+		b = a % b
+		a = t
+	}
+	return a
+}
+
+func LCM(a, b int, integers ...int) int {
+	result := a * b / GCD(a, b)
+
+	for i := 0; i < len(integers); i++ {
+		result = LCM(result, integers[i])
+	}
+
+	return result
 }

2023/day08_left_right_test.go

@@ -17,8 +17,8 @@ func TestDay08Part1(t *testing.T) {
 
 func TestDay08Part2(t *testing.T) {
 	tests := []testCase{
-		{"inputs/day08_test2", 0},
-		{"inputs/day08", 0},
+		{"inputs/day08_test3", 6},
+		{"inputs/day08", 14265111103729},
 	}
 	for _, test := range tests {
 		t.Run(test.filename, check(test, Day08Part2))

2023/inputs/day08_test3

@@ -0,0 +1,10 @@
+LR
+
+11A = (11B, XXX)
+11B = (XXX, 11Z)
+11Z = (11B, XXX)
+22A = (22B, XXX)
+22B = (22C, 22C)
+22C = (22Z, 22Z)
+22Z = (22B, 22B)
+XXX = (XXX, XXX)