package main
import (
	"bufio"
	"log"
	"os"
	"strconv"
)
type coord [2]int
var directions = []coord{
	{-1, 0}, //left
	{1, 0},  //right
	{0, -1}, // up
	{0, 1},  // down
	// diagonal
	{-1, -1},
	{-1, 1},
	{1, -1},
	{1, 1},
}
type matrix [][]int
type seen map[coord]bool
func main() {
	if err := myMain(); err != nil {
		log.Println(err)
	}
}
func myMain() error {
	m, err := parseInput("input.txt")
	if err != nil {
		return err
	}
	log.Println(solveFirst(m))
	return nil
}
func solveFirst(m matrix) int {
	var ret int
	for step := 0; step < 100; step++ {
		for row := range m {
			for col := range m[0] {
				m[row][col]++
			}
		}
		s := seen{}
		for row := range m {
			for col := range m[0] {
				ret += s.flash(m, row, col)
			}
		}
		s = seen{} // reset flashes
		for row := range m {
			for col := range m[0] {
				if m[row][col] > 9 {
					m[row][col] = 0
				}
			}
		}
	}
	return ret
}
func (s seen) flash(m matrix, row, col int) int {
	var flash int
	if m[row][col] <= 9 || s[coord{row, col}] {
		return flash
	}
	s[coord{row, col}] = true
	flash++
	for _, d := range directions {
		newrow := row + d[0]
		newcol := col + d[1]
		if newrow >= 0 && newrow < len(m) &&
			newcol >= 0 && newcol < len(m[0]) {
			m[newrow][newcol]++
			flash += s.flash(m, newrow, newcol)
		}
	}
	return flash
}
func parseInput(fileName string) (matrix, error) {
	var ret = matrix{}
	fd, err := os.Open(fileName)
	if err != nil {
		return ret, err
	}
	defer fd.Close()
	buf := bufio.NewScanner(fd)
	var line []int
	for buf.Scan() {
		for _, ch := range buf.Text() {
			tmp, err := strconv.Atoi(string(ch))
			if err != nil {
				return nil, err
			}
			line = append(line, tmp)
		}
		ret = append(ret, line)
		line = []int{}
	}
	return ret, nil
}

2478668324
4283474125
1663463374
1738271323
4285744861
3551311515
8574335438
7843525826
1366237577
3554687226

5483143223
2745854711
5264556173
6141336146
6357385478
4167524645
2176841721
6882881134
4846848554
5283751526

module main
go 1.17