2023-21 part2 was... not fun, needed help from the megathread for sure

2023/day21/main.go

@@ -0,0 +1,171 @@
+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, 64)
+		util.CopyToClipboard(fmt.Sprintf("%v", ans))
+		fmt.Println("Output:", ans)
+	} else {
+		ans := part2(input, 26501365)
+		util.CopyToClipboard(fmt.Sprintf("%v", ans))
+		fmt.Println("Output:", ans)
+	}
+}
+
+func part1(input string, steps int) int {
+	grid := parseInput(input)
+	var row, col int
+	for r, rowSlice := range grid {
+		for c, val := range rowSlice {
+			if val == "S" {
+				row = r
+				col = c
+				break
+			}
+		}
+	}
+
+	queue := map[[2]int]bool{
+		{row, col}: true,
+	}
+	for i := 0; i < steps; i++ {
+		newQueue := map[[2]int]bool{}
+		for coord := range queue {
+			for _, diff := range [][2]int{
+				{-1, 0},
+				{1, 0},
+				{0, -1},
+				{0, 1},
+			} {
+				nextRow := coord[0] + diff[0]
+				nextCol := coord[1] + diff[1]
+				if nextRow < 0 || nextRow >= len(grid) || nextCol < 0 || nextCol >= len(grid[0]) {
+					continue
+				}
+
+				if grid[nextRow][nextCol] == "." || grid[nextRow][nextCol] == "S" {
+					newQueue[[2]int{nextRow, nextCol}] = true
+				}
+			}
+		}
+
+		queue = newQueue
+	}
+
+	return len(queue)
+}
+
+func part2(input string, steps int) int {
+	grid := parseInput(input)
+	var row, col int
+	for r, rowSlice := range grid {
+		for c, val := range rowSlice {
+			if val == "S" {
+				row = r
+				col = c
+				break
+			}
+		}
+	}
+	grid[row][col] = "."
+
+	// keeps track of the flip-flopping coords separately
+	evenSeenCoords := map[[2]int]bool{}
+	oddSeenCoords := map[[2]int]bool{}
+
+	// need a set of all coords added to the queue so that we're not re-adding the same coords
+	uniqueCoords := map[[2]int]bool{}
+
+	queue := [][2]int{
+		{row, col},
+	}
+
+	// results to calculate quadratic constants with
+	results := []int{}
+
+	// perform two steps at once to always be on an even number of steps
+	for s := 0; s < steps && len(results) < 3; s++ {
+		activeSeenCoords := evenSeenCoords
+		if s%2 == 1 {
+			activeSeenCoords = oddSeenCoords
+		}
+
+		newQueue := [][2]int{}
+		for _, coord := range queue {
+			activeSeenCoords[coord] = true
+
+			for _, diff := range [][2]int{
+				{-1, 0},
+				{1, 0},
+				{0, -1},
+				{0, 1},
+			} {
+				nextRow := coord[0] + diff[0]
+				nextCol := coord[1] + diff[1]
+				nextCoord := [2]int{nextRow, nextCol}
+
+				// handles infinite grid and garden space detection
+				modNextRow := ((nextRow % len(grid)) + len(grid)) % +len(grid)
+				modNextCol := ((nextCol % len(grid[0])) + len(grid[0])) % len(grid[0])
+				if grid[modNextRow][modNextCol] != "." {
+					continue
+				}
+
+				// if already seen, skip
+				if uniqueCoords[nextCoord] {
+					continue
+				}
+				uniqueCoords[nextCoord] = true
+
+				newQueue = append(newQueue, nextCoord)
+			}
+		}
+
+		queue = newQueue
+
+		if s != 0 && s%131 == 65 {
+			results = append(results, len(activeSeenCoords))
+		}
+	}
+
+	// solve quadratic for a b and c constants
+	a := (results[2] + results[0] - 2*results[1]) / 2
+	b := results[1] - results[0] - a
+	c := results[0]
+
+	n := steps / len(grid)
+
+	return a*n*n + b*n + c
+}
+
+func parseInput(input string) (ans [][]string) {
+	for _, line := range strings.Split(input, "\n") {
+		ans = append(ans, strings.Split(line, ""))
+	}
+	return ans
+}

2023/day21/main_test.go

@@ -0,0 +1,93 @@
+package main
+
+import (
+	"testing"
+)
+
+var example = `...........
+.....###.#.
+.###.##..#.
+..#.#...#..
+....#.#....
+.##..S####.
+.##..#...#.
+.......##..
+.##.#.####.
+.##..##.##.
+...........`
+
+func Test_part1(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		steps int
+		want  int
+	}{
+		{
+			name:  "example",
+			input: example,
+			steps: 6,
+			want:  16,
+		},
+		{
+			name:  "actual",
+			input: input,
+			steps: 64,
+			want:  3743,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := part1(tt.input, tt.steps); got != tt.want {
+				t.Errorf("part1() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func Test_part2(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		steps int
+		want  int
+	}{
+		// {
+		// 	name:  "example-10",
+		// 	input: example,
+		// 	steps: 10,
+		// 	want:  50,
+		// },
+		// {
+		// 	name:  "example-50",
+		// 	input: example,
+		// 	steps: 50,
+		// 	want:  1594,
+		// },
+		// {
+		// 	name:  "example-100",
+		// 	input: example,
+		// 	steps: 100,
+		// 	want:  6536,
+		// },
+		// {
+		// 	name:  "example-5k",
+		// 	input: example,
+		// 	steps: 5000,
+		// 	want:  16733044,
+		// },
+		{
+			name:  "actual",
+			input: input,
+			steps: 26501365,
+			want:  618261433219147,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := part2(tt.input, tt.steps); got != tt.want {
+				t.Errorf("part2() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}