Day 06. part 2

day_06/solution.go

@@ -16,25 +16,27 @@ const (
 const (
 	VISITED  = 'X'
 	OCCUPIED = '#'
+	FREE     = '.'
 )
 
-var directions = map[rune]Direction{
+var toDirections = map[rune]Direction{
 	'^': UP,
 	'>': RIGHT,
 	'v': DOWN,
 	'<': LEFT,
 }
 
+var fromDirections = map[Direction]rune{
+	UP:    '^',
+	RIGHT: '>',
+	DOWN:  'v',
+	LEFT:  '<',
+}
+
 // task:https://adventofcode.com/2024/day/5
 // short summary - count visited steps
 func SolveBasic(input string) int {
-	grid := loadGrid(input)
-	x, y, dir := findPlayer(grid)
-
-	for inGrid(grid, x, y) {
-		grid[y][x] = VISITED
-		x, y, dir = nextSpot(grid, x, y, dir)
-	}
+	grid := visitationGrid(input)
 
 	sum := 0
 	for _, row := range grid {
@@ -47,6 +49,55 @@ func SolveBasic(input string) int {
 	return sum
 }
 
+// task:https://adventofcode.com/2024/day/5#part2
+// short summary - find all position that will create an infinite loop
+func SolveComplex(input string) int {
+	grid := visitationGrid(input)
+
+	dirGrid := loadGrid(input)
+	startX, startY, startDir := findPlayer(dirGrid)
+	dirGrid[startY][startX] = FREE
+
+	positions := 0
+
+	for y, row := range grid {
+		for x, value := range row {
+			if value == VISITED && dirGrid[y][x] == FREE {
+				dirGrid[y][x] = OCCUPIED
+
+				curX := startX
+				curY := startY
+				curDir := startDir
+
+				for inGrid(dirGrid, curX, curY) {
+					if dirGrid[curY][curX] == fromDirections[curDir] {
+						// we visited a same point second time in the same direction
+						// which means we are in an infinite list
+						positions++
+						break
+					}
+					dirGrid[curY][curX] = fromDirections[curDir]
+					curX, curY, curDir = nextSpot(dirGrid, curX, curY, curDir)
+
+				}
+				dirGrid[y][x] = FREE
+				clearDirGrid(dirGrid)
+			}
+		}
+	}
+	return positions
+}
+
+func clearDirGrid(grid [][]rune) {
+	for y, row := range grid {
+		for x, value := range row {
+			if value != OCCUPIED {
+				grid[y][x] = FREE
+			}
+		}
+	}
+}
+
 func nextSpot(grid [][]rune, x int, y int, dir Direction) (int, int, Direction) {
 	for {
 		nx, ny := calculateNext(x, y, dir)
@@ -81,7 +132,7 @@ func inGrid(grid [][]rune, x int, y int) bool {
 func findPlayer(grid [][]rune) (int, int, Direction) {
 	for y, row := range grid {
 		for x, value := range row {
-			if dir, present := directions[value]; present {
+			if dir, present := toDirections[value]; present {
 				return x, y, dir
 			}
 		}
@@ -98,8 +149,13 @@ func loadGrid(input string) [][]rune {
 	return grid
 }
 
-// task:https://adventofcode.com/2024/day/5#part2
-// short summary - fix and count invalid columns
-func SolveComplex(input string) int {
-	return 0
+func visitationGrid(input string) [][]rune {
+	grid := loadGrid(input)
+	x, y, dir := findPlayer(grid)
+
+	for inGrid(grid, x, y) {
+		grid[y][x] = VISITED
+		x, y, dir = nextSpot(grid, x, y, dir)
+	}
+	return grid
 }

day_06/solution_test.go

@@ -18,7 +18,7 @@ func TestBasicSolutionExample(t *testing.T) {
 
 func TestComplexSolutionExample(t *testing.T) {
 	result := day_06.SolveComplex(ExampleData)
-	if result != 123 {
-		t.Fatalf("Expected 123 received %d", result)
+	if result != 6 {
+		t.Fatalf("Expected 6 received %d", result)
 	}
 }