Solve Day 8

Signed-off-by: Alexander Kurbatov <sir.alkurbatov@yandex.ru>

Author: alkurbatov

2023/8/main.go

@@ -0,0 +1,149 @@
+// --- Day 8: Haunted Wasteland ---
+// https://adventofcode.com/2023/day/8
+package main
+
+import (
+	"bufio"
+	"errors"
+	"log"
+	"os"
+	"regexp"
+	"strings"
+)
+
+var errNoNodes = errors.New("no nodes found")
+
+var nodeRe = regexp.MustCompile(`[\dA-Z]{3}`)
+
+type Node struct {
+	id    string
+	left  string
+	right string
+}
+
+func parseRoute(scanner *bufio.Scanner) ([]rune, error) {
+	scanner.Scan()
+
+	if err := scanner.Err(); err != nil {
+		return nil, err
+	}
+
+	return []rune(scanner.Text()), nil
+}
+
+func parseNodes(scanner *bufio.Scanner) (map[string]Node, error) {
+	nodes := make(map[string]Node, 0)
+
+	for scanner.Scan() {
+		src := scanner.Text()
+
+		if src == "" {
+			continue
+		}
+
+		fields := nodeRe.FindAllString(src, -1)
+		if fields == nil || len(fields) < 3 {
+			return nil, errNoNodes
+		}
+
+		nodes[fields[0]] = Node{fields[0], fields[1], fields[2]}
+	}
+
+	if err := scanner.Err(); err != nil {
+		log.Printf("Failed to scan seeds from input.txt: %v", err)
+		return nil, err
+	}
+
+	if len(nodes) == 0 {
+		return nil, errNoNodes
+	}
+
+	return nodes, nil
+}
+
+func calcSteps(route []rune, from, to string, nodes map[string]Node) int {
+	var steps, pos int
+
+	for {
+		if strings.HasSuffix(nodes[from].id, to) {
+			return steps
+		}
+
+		direction := route[pos]
+		if direction == 'L' {
+			from = nodes[from].left
+		} else {
+			from = nodes[from].right
+		}
+
+		pos++
+		if pos == len(route) {
+			pos = 0
+		}
+
+		steps++
+	}
+}
+
+// Find greatest common divisor (GCD) via Euclidean algorithm.
+func FindGCD(a, b int) int {
+	for b != 0 {
+		t := b
+		b = a % b
+		a = t
+	}
+
+	return a
+}
+
+// Find Least Common Multiple (LCM) via Greatest Common Divisor (GCD).
+func FindLCM(a, b int, values ...int) int {
+	result := a * b / FindGCD(a, b)
+
+	for i := 0; i < len(values); i++ {
+		result = FindLCM(result, values[i])
+	}
+
+	return result
+}
+
+func calcGhostSteps(route []rune, nodes map[string]Node) int {
+	steps := make([]int, 0)
+
+	for from := range nodes {
+		if !strings.HasSuffix(from, "A") {
+			continue
+		}
+
+		steps = append(steps, calcSteps(route, from, "Z", nodes))
+	}
+
+	return FindLCM(steps[0], steps[1], steps...)
+}
+
+func main() {
+	input, err := os.Open("./2023/8/input.txt")
+	if err != nil {
+		log.Printf("Failed to open input.txt: %v", err)
+		return
+	}
+	defer input.Close()
+
+	scanner := bufio.NewScanner(input)
+
+	route, err := parseRoute(scanner)
+	if err != nil {
+		log.Printf("Failed to parse routes from input: %v", err)
+	}
+
+	nodes, err := parseNodes(scanner)
+	if err != nil {
+		log.Printf("Failed to parse nodes from input: %v", err)
+	}
+
+	firstPartResult := calcSteps(route, "AAA", "ZZZ", nodes)
+	log.Printf("Part 1 result: %d", firstPartResult)
+
+	secondPartResult := calcGhostSteps(route, nodes)
+	log.Printf("Part 2 result: %d", secondPartResult)
+}