2024 day16 - light and mirrors are confusing

2023/day16/main.go

@@ -0,0 +1,236 @@
+package main
+
+import (
+	_ "embed"
+	"flag"
+	"fmt"
+	"strings"
+
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+//go:embed input.txt
+var input string
+
+func init() {
+	// do this in init (not main) so test file has same input
+	input = strings.TrimRight(input, "\n")
+	if len(input) == 0 {
+		panic("empty input.txt file")
+	}
+}
+
+func main() {
+	var part int
+	flag.IntVar(&part, "part", 1, "part 1 or 2")
+	flag.Parse()
+	fmt.Println("Running part", part)
+
+	if part == 1 {
+		ans := part1(input)
+		util.CopyToClipboard(fmt.Sprintf("%v", ans))
+		fmt.Println("Output:", ans)
+	} else {
+		ans := part2(input)
+		util.CopyToClipboard(fmt.Sprintf("%v", ans))
+		fmt.Println("Output:", ans)
+	}
+}
+
+func part1(input string) int {
+	grid := parseInput(input)
+
+	return calcEnergizedTiles(grid, beamHead{
+		velocity: right,
+		row:      0,
+		col:      0,
+	})
+}
+
+func part2(input string) int {
+	grid := parseInput(input)
+	best := 0
+	for r := range len(grid) {
+		// starting from left of grid, headed right
+		startBeam := beamHead{
+			velocity: right,
+			row:      r,
+			col:      0,
+		}
+		// max is a go 1.21 addition: https://pkg.go.dev/builtin#max
+		best = max(best, calcEnergizedTiles(grid, startBeam))
+
+		// starting from right of grid, headed left
+		startBeam = beamHead{
+			velocity: left,
+			row:      r,
+			col:      len(grid[0]) - 1,
+		}
+		best = max(best, calcEnergizedTiles(grid, startBeam))
+	}
+
+	for c := range len(grid[0]) {
+		// starting from top of grid, headed down
+		startBeam := beamHead{
+			velocity: down,
+			row:      0,
+			col:      c,
+		}
+		best = max(best, calcEnergizedTiles(grid, startBeam))
+
+		// starting from bottom of grid, headed up
+		startBeam = beamHead{
+			velocity: up,
+			row:      len(grid) - 1,
+			col:      c,
+		}
+		best = max(best, calcEnergizedTiles(grid, startBeam))
+	}
+
+	return best
+}
+
+func calcEnergizedTiles(grid [][]string, beam beamHead) int {
+	// need to track multiple beams because they can split and generate multiple
+	beams := []beamHead{beam}
+
+	// [4]bool represents being hit from left, right, up and down respectively
+	// need to track direction so that we can terminate beams that are cyclical
+	hitGrid := [][][4]bool{}
+	for range grid {
+		hitGrid = append(hitGrid, make([][4]bool, len(grid[0])))
+	}
+
+	for len(beams) > 0 {
+		b := beams[0]
+		beams = beams[1:]
+
+		skip := false
+		for vel, hitGridIndex := range velToHitGridIndex {
+			if b.velocity == vel && hitGrid[b.row][b.col][hitGridIndex] {
+				skip = true
+			}
+		}
+		if skip {
+			continue
+		}
+
+		// record direction hit in hit grid
+		hitGrid[b.row][b.col][velToHitGridIndex[b.velocity]] = true
+
+		cell := grid[b.row][b.col]
+		nextVelocities, ok := mirrorToNextVelocities[cell][b.velocity]
+		if !ok {
+			panic("no nextVelocities found for cell type: " + cell)
+		}
+		for _, nextVelocity := range nextVelocities {
+			nextRow := b.row + nextVelocity[0]
+			nextCol := b.col + nextVelocity[1]
+
+			if nextRow < 0 || nextRow >= len(grid) ||
+				nextCol < 0 || nextCol >= len(grid[0]) {
+				continue
+			}
+
+			beams = append(beams, beamHead{
+				velocity: nextVelocity,
+				row:      nextRow,
+				col:      nextCol,
+			})
+		}
+	}
+
+	energizedTiles := 0
+	for r := range len(hitGrid) {
+		for c := range len(hitGrid[0]) {
+			for _, dir := range hitGrid[r][c] {
+				if dir {
+					energizedTiles++
+					break
+				}
+			}
+		}
+	}
+	return energizedTiles
+}
+
+var left = [2]int{0, -1}
+var right = [2]int{0, 1}
+var up = [2]int{-1, 0}
+var down = [2]int{1, 0}
+
+var velToHitGridIndex = map[[2]int]int{
+	left:  0,
+	right: 1,
+	up:    2,
+	down:  3,
+}
+
+var mirrorToNextVelocities = map[string]map[[2]int][][2]int{
+	".": {
+		left:  {left},
+		right: {right},
+		up:    {up},
+		down:  {down},
+	},
+	"/": {
+		left:  {down},
+		down:  {left},
+		up:    {right},
+		right: {up},
+	},
+	"\\": {
+		left:  {up},
+		up:    {left},
+		down:  {right},
+		right: {down},
+	},
+	"|": {
+		left:  {up, down},
+		right: {up, down},
+		up:    {up},
+		down:  {down},
+	},
+	"-": {
+		left:  {left},
+		right: {right},
+		up:    {left, right},
+		down:  {left, right},
+	},
+}
+
+type beamHead struct {
+	velocity [2]int
+	row, col int
+}
+
+func (b beamHead) String() string {
+	return fmt.Sprintf("vel: %v, coords: %d, %d", b.velocity, b.row, b.col)
+}
+
+// for debugging
+func condenseHitGrid(grid [][][4]bool) [][]int {
+	ans := [][]int{}
+	for range grid {
+		ans = append(ans, make([]int, len(grid[0])))
+	}
+
+	for r, row := range grid {
+		for c, sli := range row {
+			for _, val := range sli {
+				if val {
+					ans[r][c]++
+				}
+			}
+		}
+	}
+
+	return ans
+}
+
+func parseInput(input string) (ans [][]string) {
+	for _, line := range strings.Split(input, "\n") {
+		ans = append(ans, strings.Split(line, ""))
+	}
+	return ans
+}

2023/day16/main_test.go

@@ -0,0 +1,68 @@
+package main
+
+import (
+	"testing"
+)
+
+var example = `.|...\....
+|.-.\.....
+.....|-...
+........|.
+..........
+.........\
+..../.\\..
+.-.-/..|..
+.|....-|.\
+..//.|....`
+
+func Test_part1(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  int
+	}{
+		{
+			name:  "example",
+			input: example,
+			want:  46,
+		},
+		{
+			name:  "actual",
+			input: input,
+			want:  7046,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := part1(tt.input); got != tt.want {
+				t.Errorf("part1() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func Test_part2(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  int
+	}{
+		{
+			name:  "example",
+			input: example,
+			want:  51,
+		},
+		{
+			name:  "actual",
+			input: input,
+			want:  7313,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := part2(tt.input); got != tt.want {
+				t.Errorf("part2() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}