feat(2024): add day 6 part 2

Author: believer

go/2024/README.md

@@ -19,20 +19,20 @@ Collect stars by solving puzzles. Two puzzles will be made available on each day
 | [Day 3: Mull It Over](https://github.com/believer/advent-of-code/blob/master/go/2024/puzzles/day03/main.go)       | 🌟  | 161085926 | 🌟  |  82045421 |
 | [Day 4: Ceres Search](https://github.com/believer/advent-of-code/blob/master/go/2024/puzzles/day04/main.go)       | 🌟  |      2562 | 🌟  |      1902 |
 | [Day 5: Print Queue](https://github.com/believer/advent-of-code/blob/master/go/2024/puzzles/day05/main.go)        | 🌟  |      3608 | 🌟  |      4922 |
-| [Day 6: Guard Gallivant](https://github.com/believer/advent-of-code/blob/master/go/2024/puzzles/day06/main.go)    | 🌟  |      4778 |     |           |
+| [Day 6: Guard Gallivant](https://github.com/believer/advent-of-code/blob/master/go/2024/puzzles/day06/main.go)    | 🌟  |      4778 | 🌟  |      1618 |
 
 ## Benchmarks
 
 Using Go's built-in benchmarking with the [testing](https://pkg.go.dev/testing#hdr-Benchmarks) package. Computer is a 2021 MacBook Pro M1 Pro, 32 GB RAM.
 
-| Day |           #1 |            #2 | Improvement\*       |
-| --- | -----------: | ------------: | ------------------- |
-| 1   | 116264 ns/op |  131233 ns/op | `3.53%` / `68.43%`  |
-| 2   | 310935 ns/op |  723512 ns/op |                     |
-| 3   | 336448 ns/op |  785320 ns/op | - / `36.98%`        |
-| 4   | 523315 ns/op |  217584 ns/op | `81.73%` / `26.09%` |
-| 5   | 778880 ns/op | 3129873 ns/op | `53.34%` / `81.91%` |
-| 5   | 349256 ns/op |               |                     |
+| Day |           #1 |               #2 | Improvement\*       |
+| --- | -----------: | ---------------: | ------------------- |
+| 1   | 116264 ns/op |     131233 ns/op | `3.53%` / `68.43%`  |
+| 2   | 310935 ns/op |     723512 ns/op |                     |
+| 3   | 336448 ns/op |     785320 ns/op | - / `36.98%`        |
+| 4   | 523315 ns/op |     217584 ns/op | `81.73%` / `26.09%` |
+| 5   | 778880 ns/op |    3129873 ns/op | `53.34%` / `81.91%` |
+| 6   | 312461 ns/op | 1153391125 ns/op |                     |
 
 \* compared to first solution
 

go/2024/puzzles/day06/main.go

@@ -6,6 +6,10 @@ import (
 	"github.com/believer/aoc-2024/utils/files"
 )
 
+// Brute forced part 2 first by checking every position on the grid.
+// It worked, but was slow. We can instead check only the path that
+// the guard took during the first run which was more that 78% faster.
+// Still over a second, but a lot better.
 func main() {
 	fmt.Println("Part 1: ", part1("input.txt"))
 	fmt.Println("Part 2: ", part2("input.txt"))
@@ -15,31 +19,76 @@ type Position struct {
 	r, c int
 }
 
+type PositionWithDirection struct {
+	r, c, dr, dc int
+}
+
 func part1(name string) int {
 	lines := files.ReadLines(name)
-	visitedLocations := make(map[Position]struct{})
+	guardLocation := initialGuardLocation(lines)
+
+	return len(getPath(lines, guardLocation))
+}
+
+func part2(name string) int {
+	lines := files.ReadLines(name)
+	possibleLoops := 0
+	guardLocation := initialGuardLocation(lines)
+
+	// Create a grid to modify
+	var grid [][]byte
+
+	for _, l := range lines {
+		grid = append(grid, []byte(l))
+	}
+
+	guardPath := getPath(lines, guardLocation)
 
+	for p := range guardPath {
+		if lines[p.r][p.c] != '.' {
+			continue
+		}
+
+		grid[p.r][p.c] = '#'
+
+		if isLoop(grid, guardLocation.r, guardLocation.c) {
+			possibleLoops++
+		}
+
+		grid[p.r][p.c] = '.'
+	}
+
+	return possibleLoops
+}
+
+func initialGuardLocation(lines []string) Position {
+	guardLocation := Position{0, 0}
 	rows := len(lines)
 	cols := len(lines[0])
-	guardLocation := Position{0, 0}
 
-	// Find initial guard location
 outer:
 	for r := range rows {
 		for c := range cols {
-			if string(lines[r][c]) == "^" {
+			if lines[r][c] == '^' {
 				guardLocation = Position{r, c}
 				break outer
 			}
 		}
 	}
 
+	return guardLocation
+}
+
+func getPath(lines []string, guard Position) map[Position]struct{} {
+	visitedLocations := make(map[Position]struct{})
+	rows := len(lines)
+	cols := len(lines[0])
 	dr := -1
 	dc := 0
 
 	for {
-		r, c := guardLocation.r, guardLocation.c
-		visitedLocations[guardLocation] = struct{}{}
+		r, c := guard.r, guard.c
+		visitedLocations[guard] = struct{}{}
 
 		// Check bounds
 		if r+dr < 0 || r+dr >= rows || c+dc < 0 || c+dc >= cols {
@@ -48,16 +97,43 @@ outer:
 
 		// We always rotate to the right on obstacles
 		// (-1,0) becomes (0,1) becomes (1,0) becomes (0,-1)
-		if string(lines[r+dr][c+dc]) == "#" {
+		if lines[r+dr][c+dc] == '#' {
 			dc, dr = -dr, dc
 		} else {
-			guardLocation = Position{r: r + dr, c: c + dc}
+			guard = Position{r: r + dr, c: c + dc}
 		}
 	}
 
-	return len(visitedLocations)
+	return visitedLocations
 }
 
-func part2(name string) int {
-	return 0
+func isLoop(grid [][]byte, r, c int) bool {
+	visitedLocations := make(map[PositionWithDirection]struct{})
+	dr := -1
+	dc := 0
+	rows := len(grid)
+	cols := len(grid[0])
+
+	for {
+		// Save with direction as well to find when we're looping
+		visitedLocations[PositionWithDirection{r, c, dr, dc}] = struct{}{}
+
+		// Check bounds
+		if r+dr < 0 || r+dr >= rows || c+dc < 0 || c+dc >= cols {
+			return false
+		}
+
+		// We always rotate to the right on obstacles
+		// (-1,0) becomes (0,1) becomes (1,0) becomes (0,-1)
+		if grid[r+dr][c+dc] == '#' {
+			dc, dr = -dr, dc
+		} else {
+			r += dr
+			c += dc
+		}
+
+		if _, ok := visitedLocations[PositionWithDirection{r, c, dr, dc}]; ok {
+			return true
+		}
+	}
 }

go/2024/puzzles/day06/main_test.go

@@ -16,7 +16,7 @@ func TestPart1(t *testing.T) {
 
 func TestPart2(t *testing.T) {
 	t.Run("Part 2", func(t *testing.T) {
-		expected := 0
+		expected := 6
 		actual := part2("test-input.txt")
 		assert.Equal(t, expected, actual)
 	})