2022 day 9

Author: pvainio

2022/go/day9/day9.go

@@ -0,0 +1,100 @@
+package main
+
+import (
+	"fmt"
+	"math"
+	"os"
+	"strings"
+)
+
+func main() {
+	input, _ := os.ReadFile("../../input/day9.txt")
+	moves := parseMoves(string(input))
+	head := tieKnots(10)
+	head = runMoves(head, moves)
+	fmt.Printf("part1 : %d\n", len(head.getNth(1).visited))
+	fmt.Printf("part2 : %d\n", len(head.getNth(9).visited))
+}
+
+func tieKnots(count int) *knot {
+	knot := newKnot()
+	prev := knot
+	for i := 1; i < count; i++ {
+		prev.tail = newKnot()
+		prev = prev.tail
+	}
+	return knot
+}
+
+func runMoves(knot *knot, moves []move) *knot {
+	for _, m := range moves {
+		for s := byte(0); s < m.steps; s++ {
+			knot.move(dirs[m.dir])
+		}
+	}
+	return knot
+}
+
+type pos struct {
+	x, y int
+}
+
+type knot struct {
+	pos     pos
+	visited map[pos]bool
+	tail    *knot
+}
+
+type move struct {
+	dir, steps byte
+}
+
+func (k *knot) getNth(count int) *knot {
+	if count == 0 {
+		return k
+	}
+	return k.tail.getNth(count - 1)
+}
+
+func (k *knot) move(v pos) {
+	k.pos = k.pos.move(v)
+	k.visited[k.pos] = true
+	if k.tail != nil {
+		k.tail.follow(k)
+	}
+}
+
+func (p pos) move(v pos) pos {
+	return pos{p.x + v.x, p.y + v.y}
+}
+
+func (tail *knot) follow(head *knot) {
+	if tail.pos.touching(head.pos) {
+		return
+	}
+	dx := (head.pos.x-tail.pos.x)/2 + (head.pos.x-tail.pos.x)%2
+	dy := (head.pos.y-tail.pos.y)/2 + (head.pos.y-tail.pos.y)%2
+	tail.move(pos{dx, dy})
+}
+
+func (k1 pos) touching(k2 pos) bool {
+	return math.Abs(float64(k1.x)-float64(k2.x)) < 1.5 && math.Abs(float64(k1.y)-float64(k2.y)) < 1.5
+}
+
+func parseMoves(input string) (moves []move) {
+	lines := strings.Split(input, "\n")
+	for _, line := range lines {
+		var m move
+		fmt.Sscanf(line, "%c %d", &m.dir, &m.steps)
+		moves = append(moves, m)
+	}
+	return moves
+}
+
+func newKnot() *knot {
+	k := &knot{visited: make(map[pos]bool)}
+	k.move(pos{})
+	return k
+}
+
+var dirs = map[byte]pos{'U': {0, -1}, 'D': {0, 1}, 'L': {-1, 0}, 'R': {1, 0}}