advent_of_code_2025/4/solve_2.go

package main

import (
	"bufio"
	"fmt"
	"os"
)

type coord struct {
	x,y,nb_neighbors int
	
}

func update_neighbors_insertion(positions* []*coord, i,j int) int{
	nbn := 0
	for _, paper_roll := range *positions {
		switch(paper_roll.x){
			case i:
			switch(paper_roll.y){
				case j-1, j+1:
				nbn++
				paper_roll.nb_neighbors++
			}
			case i-1, i+1:
			switch(paper_roll.y){
				case j-1,j,j+1:
				nbn++
				paper_roll.nb_neighbors++
			}
		}
	}
	return nbn
}

func continue_removing(positions* []*coord){
	for i:=0; i < len(*positions); {
		paper_roll := (*positions)[i]
		if paper_roll.nb_neighbors < 4 {
			(*positions)[i] = (*positions)[len(*positions)-1]
			(*positions)[len(*positions)-1] = nil
			*positions = (*positions)[:len(*positions)-1]
		} else {
			i++
		}
	}
}

func main() {
	fmt.Println("Advent of Code 2025 - Day 4 - Part 2")
	f,_ := os.Open("4/input")
	defer f.Close()

	var positions []*coord
	scanner := bufio.NewScanner(f)
	line_num := 0
	for scanner.Scan() {
		line := scanner.Text()
		for i:= 0; i < len(line); i++{
			if line[i] == '@'{
				nbn := update_neighbors_insertion(&positions,line_num,i)
				positions = append(positions, &coord{line_num,i,nbn})
			}
		}
		line_num++
	}
	
	paper_rolls_pre := len(positions)
	
	for {
		loop_entry_paper_rolls := len(positions)
		// remove paper_rolls with less than 4 neighbors
		continue_removing(&positions)
		update_neighbors_after_removal(&positions)
		// if nothing was removed break from loop
		if loop_entry_paper_rolls == len(positions) {
			break
		}
	}
	fmt.Println(paper_rolls_pre - len(positions))
}

func update_neighbors_after_removal(positions* []*coord){
	// reset nb_neighbors
	for _, paper_roll := range *positions{
		paper_roll.nb_neighbors = 0
	}
	
	// update nb_neighbors
	for _, paper_roll := range *positions{
		x,y := paper_roll.x, paper_roll.y
		for _, npar := range *positions {
			if &npar != &paper_roll{
				switch(npar.x){
					case x+1, x-1:
					switch(npar.y){
						case y, y+1, y-1:
						paper_roll.nb_neighbors++
					}
					case x:
					switch(npar.y){
						case y+1, y-1:
						paper_roll.nb_neighbors++
					}
				}
			}
		}
	}
}