no this isn't about alexandria ocasio-cortez
evan.jarrett.net
1package main
2
3import (
4 "strings"
5
6 "tangled.org/evan.jarrett.net/aoc2025/internal/puzzle"
7)
8
9type PaperRoll struct {
10 row int
11 column int
12 left int
13 right int
14 up int
15 upLeft int
16 upRight int
17 down int
18 downLeft int
19 downRight int
20}
21
22func (p *PaperRoll) CanAccess() bool {
23 return p.left+p.right+p.up+p.down+p.upLeft+p.upRight+p.downLeft+p.downRight < 4
24}
25
26type DayFour struct {
27 grid []string
28 rolls []PaperRoll
29}
30
31const rollchar byte = '@'
32
33func getCell(grid []string, i, j int) int {
34 if i < 0 || i >= len(grid) || j < 0 || j >= len(grid[i]) {
35 return 0
36 }
37 if grid[i][j] == rollchar {
38 return 1
39 }
40 return 0
41}
42
43func (d *DayFour) ParseInputArr(input []string) {
44 for i, line := range d.grid {
45 line = strings.TrimSpace(line)
46 if line == "" {
47 continue
48 }
49 for j, ch := range line {
50 if ch == rune(rollchar) {
51 p := PaperRoll{
52 row: i,
53 column: j,
54 left: getCell(d.grid, i, j-1),
55 right: getCell(d.grid, i, j+1),
56 up: getCell(d.grid, i-1, j),
57 upLeft: getCell(d.grid, i-1, j-1),
58 upRight: getCell(d.grid, i-1, j+1),
59 down: getCell(d.grid, i+1, j),
60 downLeft: getCell(d.grid, i+1, j-1),
61 downRight: getCell(d.grid, i+1, j+1),
62 }
63 d.rolls = append(d.rolls, p)
64 }
65 }
66 }
67}
68
69func (d *DayFour) ParseInput(input string) error {
70 d.grid = strings.Split(strings.TrimSpace(input), "\n")
71 d.ParseInputArr(d.grid)
72 return nil
73}
74
75func (d *DayFour) Part1() (int, error) {
76 count := 0
77 for _, roll := range d.rolls {
78 if roll.CanAccess() {
79 count++
80 }
81 }
82 return count, nil
83}
84
85func (d *DayFour) Part2() (int, error) {
86 count := 0
87 changed := true
88
89 for changed {
90 changed = false
91 d.rolls = nil
92 d.ParseInputArr(d.grid)
93
94 for _, roll := range d.rolls {
95 if roll.CanAccess() {
96 d.grid[roll.row] = d.grid[roll.row][:roll.column] + "." + d.grid[roll.row][roll.column+1:]
97 count++
98 changed = true
99 }
100 }
101 }
102 return count, nil
103}
104
105func main() {
106 puzzle.Run(4, &DayFour{})
107}