From 3cb59b0c67c2edb7b3ace744fc9b0ab7706fb518 Mon Sep 17 00:00:00 2001
From: Nick Krichevsky <nick@ollien.com>
Date: Thu, 21 Dec 2023 19:03:05 -0500
Subject: [PATCH] Add solution to day 21 part 1

---
 day21/main.go | 113 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 113 insertions(+)
 create mode 100644 day21/main.go

diff --git a/day21/main.go b/day21/main.go
new file mode 100644
index 0000000..fee644d
--- /dev/null
+++ b/day21/main.go
@@ -0,0 +1,113 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+)
+
+type Tile rune
+
+const (
+	TileTypeGarden Tile = '.'
+	TileTypeRock        = '#'
+)
+
+type Coordinate struct {
+	Row int
+	Col int
+}
+
+func main() {
+	if len(os.Args) != 2 && len(os.Args) != 3 {
+		fmt.Fprintf(os.Stderr, "Usage: %s inputfile\n", os.Args[0])
+		os.Exit(1)
+	}
+
+	inputFilename := os.Args[1]
+	inputFile, err := os.Open(inputFilename)
+	if err != nil {
+		panic(fmt.Sprintf("could not open input file: %s", err))
+	}
+
+	defer inputFile.Close()
+
+	inputBytes, err := io.ReadAll(inputFile)
+	if err != nil {
+		panic(fmt.Sprintf("could not read input file: %s", err))
+	}
+
+	input := strings.TrimSpace(string(inputBytes))
+	inputLines := strings.Split(input, "\n")
+	grid, start, err := parseGrid(inputLines)
+	if err != nil {
+		panic(fmt.Sprintf("failed to parse input: %s", err))
+	}
+
+	fmt.Printf("Part 1: %d\n", part1(grid, start))
+}
+
+func part1(tiles map[Coordinate]Tile, start Coordinate) int {
+	cursors := []Coordinate{start}
+	lastCount := 0
+	for i := 0; i < 64; i++ {
+		nextCursors := []Coordinate{}
+		visited := map[Coordinate]struct{}{}
+		for _, cursor := range cursors {
+			for _, neighbor := range neighbors(cursor) {
+				if tiles[neighbor] == TileTypeRock {
+					continue
+				} else if _, ok := visited[neighbor]; ok {
+					continue
+				}
+
+				visited[neighbor] = struct{}{}
+				nextCursors = append(nextCursors, neighbor)
+			}
+		}
+
+		cursors = nextCursors
+		lastCount = len(visited)
+	}
+
+	return lastCount
+}
+
+func neighbors(coord Coordinate) []Coordinate {
+	return []Coordinate{
+		{Row: coord.Row + 1, Col: coord.Col},
+		{Row: coord.Row - 1, Col: coord.Col},
+		{Row: coord.Row, Col: coord.Col + 1},
+		{Row: coord.Row, Col: coord.Col - 1},
+	}
+}
+
+func parseGrid(inputLines []string) (map[Coordinate]Tile, Coordinate, error) {
+	tiles := map[Coordinate]Tile{}
+	start := Coordinate{}
+	startFound := false
+
+	for row, line := range inputLines {
+		for col, char := range line {
+			coordinate := Coordinate{Row: row, Col: col}
+			switch char {
+			case 'S':
+				startFound = true
+				start = coordinate
+				tiles[coordinate] = TileTypeGarden
+			case '.', '#':
+				tiles[coordinate] = Tile(char)
+			default:
+				return nil, Coordinate{}, fmt.Errorf("invalid tile char %c", char)
+			}
+		}
+	}
+
+	if !startFound {
+		return nil, Coordinate{}, errors.New("no start tile found")
+	}
+
+	return tiles, start, nil
+}