Day06

2024/gareth/day06/day06.go

@@ -0,0 +1,142 @@
+package day06
+
+import (
+	"strings"
+)
+
+type Point struct {
+	x         int
+	y         int
+	direction int
+}
+
+const (
+	Up    = 0
+	Right = 1
+	Down  = 2
+	Left  = 3
+)
+
+var rows, cols int
+
+var moves = []Point{
+	{-1, 0, Up},
+	{0, 1, Up},
+	{1, 0, Up},
+	{0, -1, Up},
+}
+
+func Part1(input string) int {
+	grid, start, direction := ParseInput(input)
+	total := PredictPath(grid, start, direction)
+	return total
+}
+
+func Part2(input string) int {
+	grid, start, direction := ParseInput(input)
+	total := FindLoopingPositions(grid, start, direction)
+	return total
+}
+
+func ParseInput(input string) ([][]rune, Point, int) {
+	lines := strings.Split(strings.TrimSpace(input), "\n")
+	rows = len(lines)
+	cols = len(lines[0])
+	grid := make([][]rune, rows)
+
+	var start Point
+	var direction int
+
+	for i := 0; i < rows; i++ {
+		grid[i] = []rune(lines[i])
+		for j, char := range lines[i] {
+			if char == '^' {
+				start = Point{i, j, Up}
+				direction = Up
+			} else if char == '>' {
+				start = Point{i, j, Right}
+				direction = Right
+			} else if char == 'v' {
+				start = Point{i, j, Down}
+				direction = Down
+			} else if char == '<' {
+				start = Point{i, j, Left}
+				direction = Left
+			}
+		}
+	}
+
+	return grid, start, direction
+}
+
+func PredictPath(grid [][]rune, start Point, direction int) int {
+	visited := make(map[Point]bool)
+	current := start
+
+	visited[current] = true
+
+	// do while
+	for {
+		next := Point{current.x + moves[direction].x, current.y + moves[direction].y, Up}
+
+		if next.x < 0 || next.x >= rows || next.y < 0 || next.y >= cols {
+			break
+		}
+
+		if grid[next.x][next.y] == '#' {
+			direction = (direction + 1) % 4
+		} else {
+			current = next
+			visited[current] = true
+		}
+	}
+
+	return len(visited)
+}
+
+func FindLoopingPositions(grid [][]rune, start Point, direction int) int {
+	possiblePositions := 0
+
+	for i := 0; i < rows; i++ {
+		for j := 0; j < cols; j++ {
+			if grid[i][j] != '.' || (i == start.x && j == start.y) {
+				continue
+			}
+
+			grid[i][j] = '#'
+			if IsLooping(grid, start, direction) {
+				possiblePositions++
+			}
+			grid[i][j] = '.'
+		}
+	}
+
+	return possiblePositions
+}
+
+func IsLooping(grid [][]rune, start Point, direction int) bool {
+	visited := make(map[Point]int)
+	current := start
+
+	step := 0
+	for {
+		step++
+		next := Point{current.x + moves[direction].x, current.y + moves[direction].y, direction}
+
+		if next.x < 0 || next.x >= rows || next.y < 0 || next.y >= cols {
+			return false
+		}
+
+		if grid[next.x][next.y] == '#' {
+			direction = (direction + 1) % 4
+		} else {
+			current = next
+
+			if visitStep, ok := visited[current]; ok && step-visitStep > 4 {
+				return true
+			}
+
+			visited[current] = step
+		}
+	}
+}