2024, day 6

Author: lucianoq

2024/6/Makefile

@@ -0,0 +1,20 @@
+input:
+	http "https://adventofcode.com/2024/day/6/input" "Cookie:session=${AOC_SESSION};" >input
+
+main1:
+	go build -o main1 main1.go
+
+main2:
+	go build -o main2 main2.go
+
+.PHONY: run1 run2 clean
+
+run1: main1 input
+	./main1 <input
+
+run2: main2 input
+	./main2 <input
+
+clean:
+	rm -f main1 main2 input
+

2024/6/common.go

@@ -0,0 +1,35 @@
+package main
+
+import (
+	"bufio"
+	"os"
+)
+
+const Size = 130
+
+const (
+	N uint8 = iota
+	E
+	S
+	W
+)
+
+type P struct{ x, y int }
+
+func parseMap() (map[P]any, P, uint8) {
+	var pos P
+	scanner := bufio.NewScanner(os.Stdin)
+	m := map[P]any{}
+	for i := 0; scanner.Scan(); i++ {
+		line := scanner.Text()
+		for j, c := range line {
+			switch c {
+			case '#':
+				m[P{i, j}] = struct{}{}
+			case '^':
+				pos = P{i, j}
+			}
+		}
+	}
+	return m, pos, N
+}

2024/6/main1.go

@@ -0,0 +1,42 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/lucianoq/container/set"
+)
+
+func main() {
+	m, p, dir := parseMap()
+
+	visited := set.Set[P]{}
+	visited.Add(p)
+
+	for {
+		var next P
+
+		switch dir {
+		case N:
+			next = P{p.x - 1, p.y}
+		case E:
+			next = P{p.x, p.y + 1}
+		case S:
+			next = P{p.x + 1, p.y}
+		case W:
+			next = P{p.x, p.y - 1}
+		}
+
+		if next.x < 0 || next.x >= Size || next.y < 0 || next.y >= Size {
+			break
+		}
+
+		if _, ok := m[next]; ok {
+			dir = (dir + 1) % 4
+		} else {
+			p = next
+			visited.Add(p)
+		}
+	}
+
+	fmt.Println(visited.Len())
+}

2024/6/main2.go

@@ -0,0 +1,79 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/lucianoq/container/set"
+)
+
+func main() {
+	m, p, dir := parseMap()
+
+	var count int
+	for i := 0; i < Size; i++ {
+		for j := 0; j < Size; j++ {
+			if testObstacle(m, P{i, j}, p, dir) {
+				count++
+			}
+		}
+	}
+
+	fmt.Println(count)
+}
+
+type State struct {
+	Pos P
+	Dir uint8
+}
+
+func testObstacle(m map[P]any, obstacle, pos P, dir uint8) bool {
+
+	// impossible if the guard is there
+	if obstacle == pos {
+		return false
+	}
+
+	// impossible if there is already an obstacle there
+	if _, ok := m[obstacle]; ok {
+		return false
+	}
+
+	m[obstacle] = struct{}{}
+	defer func() {
+		delete(m, obstacle)
+	}()
+
+	visited := set.Set[State]{}
+	visited.Add(State{pos, dir})
+
+	for {
+		var next P
+
+		switch dir {
+		case N:
+			next = P{pos.x - 1, pos.y}
+		case E:
+			next = P{pos.x, pos.y + 1}
+		case S:
+			next = P{pos.x + 1, pos.y}
+		case W:
+			next = P{pos.x, pos.y - 1}
+		}
+
+		if next.x < 0 || next.x >= Size || next.y < 0 || next.y >= Size {
+			return false
+		}
+
+		if _, ok := m[next]; ok {
+			dir = (dir + 1) % 4
+		} else {
+			pos = next
+		}
+
+		newState := State{pos, dir}
+		if visited.Contains(newState) {
+			return true
+		}
+		visited.Add(newState)
+	}
+}