evan.jarrett.net
no this isn't about alexandria ocasio-cortez
1package main
2
3import (
4 "log/slog"
5 "strconv"
6 "strings"
7
8 "tangled.org/evan.jarrett.net/aoc2025/internal/puzzle"
9)
10
11type ProductId struct {
12 firstId int
13 lastId int
14}
15
16type DayTwo struct {
17 productIds []ProductId
18}
19
20func (d *DayTwo) ParseInput(input string) error {
21 for productId := range strings.SplitSeq(strings.TrimSpace(input), ",") {
22 productId = strings.TrimSpace(productId)
23 if productId == "" {
24 continue
25 }
26 parts := strings.Split(productId, "-")
27 first, _ := strconv.Atoi(parts[0])
28 last, _ := strconv.Atoi(parts[1])
29
30 product := ProductId{
31 firstId: first,
32 lastId: last,
33 }
34
35 d.productIds = append(d.productIds, product)
36 }
37 return nil
38}
39
40func findPattern(productId int, j int) int {
41 s := strconv.Itoa(productId)
42 n := len(s)
43 if n%j == 0 {
44 piece := s[:n/j]
45 if strings.Repeat(piece, j) == s {
46 slog.Debug("found invalid ID", "id", s)
47 return productId
48 }
49 }
50 return 0
51}
52
53// find ids that have a repeating pattern. 55 = 5 twice, 6464 = 64 twice.
54func (d *DayTwo) Part1() (int, error) {
55 sum := 0
56 for _, product := range d.productIds {
57 for i := product.firstId; i <= product.lastId; i++ {
58 sum += findPattern(i, 2)
59 }
60 }
61
62 return sum, nil
63}
64
65func (d *DayTwo) Part2() (int, error) {
66 sum := 0
67 for _, product := range d.productIds {
68 for i := product.firstId; i <= product.lastId; i++ {
69 s := strconv.Itoa(i)
70 n := len(s)
71 for j := 2; j <= n; j++ {
72 if result := findPattern(i, j); result > 0 {
73 sum += result
74 break
75 }
76 }
77 }
78 }
79
80 return sum, nil
81}
82
83func main() {
84 puzzle.Run(2, &DayTwo{})
85}