D9: working Go solution to day 9

day09/go/day09.go

@@ -0,0 +1,147 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+	"log"
+	"os"
+	"strconv"
+	"strings"
+)
+
+var doneErr = errors.New("no more moves left")
+
+const (
+	Up = iota
+	Down
+	Left
+	Right
+)
+
+type Point struct{ x, y int }
+type ropeModel struct {
+	head      Point
+	tail      Point
+	times     int
+	moves     []string
+	grid      map[Point]struct{}
+	moveIndex int
+	currDir   int
+}
+
+func newModel(moves []string) ropeModel {
+	return ropeModel{
+		head:      Point{0, 0},
+		tail:      Point{0, 0},
+		times:     0,
+		moves:     moves,
+		grid:      make(map[Point]struct{}),
+		moveIndex: 0,
+		currDir:   0,
+	}
+}
+
+func (m *ropeModel) step() error {
+	if m.times == 0 {
+		if m.moveIndex+1 > len(m.moves) {
+			return doneErr
+		}
+		m.nextMove()
+	}
+	switch m.currDir {
+	case Up:
+		m.head.y++
+	case Down:
+		m.head.y--
+	case Left:
+		m.head.x--
+	case Right:
+		m.head.x++
+	}
+
+	switch m.head.y - m.tail.y {
+	case 2:
+		m.tail.y++
+		m.tail.x = m.head.x
+	case 1:
+		if m.head.x > m.tail.x+1 {
+			m.tail.y++
+			m.tail.x++
+		} else if m.head.x < m.tail.x-1 {
+			m.tail.y++
+			m.tail.x--
+		}
+	case 0:
+		if m.head.x > m.tail.x+1 {
+			m.tail.x++
+		} else if m.head.x < m.tail.x-1 {
+			m.tail.x--
+		}
+	case -1:
+		if m.head.x > m.tail.x+1 {
+			m.tail.y--
+			m.tail.x++
+		} else if m.head.x < m.tail.x-1 {
+			m.tail.y--
+			m.tail.x--
+		}
+	case -2:
+		m.tail.y--
+		m.tail.x = m.head.x
+	}
+	m.grid[m.tail] = struct{}{}
+	m.times--
+
+	return nil
+}
+
+func (m *ropeModel) nextMove() {
+	newDir, newTimes, ok := strings.Cut(m.moves[m.moveIndex], " ")
+	if !ok {
+		log.Fatal("couldn't parse moves!")
+	}
+	switch newDir {
+	case "U":
+		m.currDir = Up
+	case "D":
+		m.currDir = Down
+	case "L":
+		m.currDir = Left
+	case "R":
+		m.currDir = Right
+	default:
+		log.Fatal("couldn't parse moves! got: ", newDir)
+	}
+	times, err := strconv.Atoi(newTimes)
+	if err != nil {
+		log.Fatal("couldn't parse moves! err: ", err)
+	}
+	m.times = times
+	m.moveIndex++
+}
+
+func (m *ropeModel) solve() int {
+	var res int
+	for {
+		err := m.step()
+		if err == doneErr {
+			for _ = range m.grid {
+				res++
+			}
+			break
+		}
+	}
+	return res
+}
+
+func main() {
+	input, err := os.ReadFile("input9.txt")
+	if err != nil {
+		log.Fatalf("could not read file")
+	}
+	moves := strings.Split(string(input), "\n")
+	moves = moves[:len(moves)-1]
+	p1 := newModel(moves)
+	answer := p1.solve()
+	fmt.Println(answer)
+}

day09/go/go.mod

@@ -0,0 +1,3 @@
+module aoc_day09
+
+go 1.19