# Advent of Code 2021 Day 4: Play Bingo against a Giant Squid

[This is my first post here - though I have been lurking in the back reading, plussing, and hopefully learning for over two years]

Below is my solution for Advent of Code 2021 Day 4, where you are playing bingo against a giant squid.
You are given a set of bingo cards, and the list of numbers that will be drawn; the target for part 1 is to identify the bingo card that wins first, and for part 2, the card that wins last. For more details, check this link.

Some remarks:

• this is supposed to be developed against the clock. Therefore, reusability is not a very important consideration.
• error checking is not required, and not done. Bad input will blow everything apart; please ignore this.
• I work in a C++20 environment (but didn't necessarily use any features of it). I marked it C++20 because if C++20 offers something I missed, I want to learn about it.

What I am looking for:

• what is bad / poor / smelly?
• what could be improved?
• what could I do better using the C++20 feature set? I know about the capabilities, but didn't see any way to take advantage of it.

Here is the include BingoCard.h:

#pragma once

#include <array>
#include <iostream>
#include <sstream>
#include <vector>

constexpr size_t BingoCardSize = 5;
using Card = std::array<int, BingoCardSize* BingoCardSize>;

class BingoCard
{
public:
static BingoCard Create(std::istream& input);                 // construct by reading the numbers from input
int DrawNumbers(const std::vector<int>& n) noexcept;          // draw numbers till 'bingo'
int GetRemainder() const noexcept;                            // get sum of numbers remaining after 'bingo'

private:
BingoCard(Card& n) noexcept : number(n) {}                    // constructor
bool HasBingo() const noexcept;                               // check for 'bingo'

private:
const Card number;                                            // numbers on the bingo card
std::array<bool, BingoCardSize * BingoCardSize> hit{ false }; // flags if a number was hit
};


Here the class code in BingoCard.cpp:

#include "BingoCard.h"

BingoCard BingoCard::Create(std::istream& input)
{
Card n;
for (int i = 0; i < n.size(); ++i) input >> n[i];
return BingoCard(n);
}

bool BingoCard::HasBingo(void) const noexcept  // check for 'bingo'. note: diagonals are NOT considered 'bingo'
{
for (int i = 0; i < BingoCardSize; ++i)
{
bool row{ true };
bool col{ true };
for (int j = 0; j < BingoCardSize; ++j)
{
row &= hit[BingoCardSize * i + j];   // check along row
col &= hit[i + BingoCardSize * j];   // check along column
}
if (row || col) return true;
}
return false;
}

int BingoCard::DrawNumbers(const std::vector<int>& draw) noexcept
{
for (int n = 0; n < draw.size(); ++n)
{
for (int j = 0; j < BingoCardSize * BingoCardSize; ++j)
{
if (number[j] == draw[n])
{
hit[j] = true;
if (HasBingo()) return n;
break;
}
}
}
return draw.size();
}

int BingoCard::GetRemainder(void) const noexcept
{
int sum{ 0 };
for (int i = 0; i < BingoCardSize * BingoCardSize; ++i)
{
if (!hit[i]) sum += number[i];
}
return sum;
}


and here the main program:

#include "BingoCard.h"

void AoC2021_04(std::istream& input)
{
std::string line{};

// read all numbers drawn during the game
std::vector<int> draw{};
getline(input, line);
std::istringstream str(line);
int z0{ 0 };
while (str >> z0) {
draw.emplace_back(z0);
char comma{};
str >> comma; // read the comma and ignore it
}

std::vector<BingoCard> board{};
while (getline(input, line))     // the success of this blank line read means there is another board coming
{
board.emplace_back(BingoCard::Create(input));
}

// find the BingoCards with the earliest [Part 1] and latest [Part 2] 'bingo' (that is, the minimum / maximum of DrawNumbers return value)
int min{ std::numeric_limits<int>::max() };
int mini{ -1 };
int max{ 0 };
int maxi{ -1 };
for (int i = 0; i < board.size(); ++i)
{
auto n = board[i].DrawNumbers(draw);
if (n < min)
{
min = n;
mini = i;
}
if (n > max)
{
max = n;
maxi = i;
}
}

int z1 = board[mini].GetRemainder() * draw[min];  // AoC solution is min BingoCard's remainder times the winning number
std::cout << "AoC2021 Day 4 Part 1: " << z1 << '\n';

int z2 = board[maxi].GetRemainder() * draw[max];  // AoC solution is max BingoCard's remainder times the winning number
std::cout << "AoC2021 Day 4 Part 2: " << z2 << '\n';
}

int main()
{
AoC2021_04(std::cin);
}


my input data (if you go to AoC and log in, you get a different input - everyone gets a slightly different one. I am quite sure the code above works for any input):

37,60,87,13,34,72,45,49,61,27,97,88,50,30,76,40,63,9,38,67,82,6,59,90,99,54,11,66,98,23,64,14,18,4,10,89,46,32,19,5,1,53,25,96,2,12,86,58,41,68,95,8,7,3,85,70,35,55,77,44,36,51,15,52,56,57,91,16,71,92,84,17,33,29,47,75,80,39,83,74,73,65,78,69,21,42,31,93,22,62,24,48,81,0,26,43,20,28,94,79

66 78  7 45 92
39 38 62 81 77
9 73 25 97 99
87 80 19  1 71
85 35 52 26 68

47 38 84 53 16
66  3 63 92 94
71 41 59  1 87
17 67 62 73 33
69 12 26 82 55

89 94 65 57  6
27 77 60 19 83
72 58  0 29 91
33 75 50 64 87
24 88 32 93 38

35 82  7 49 43
58  8 56 54  6
74 53 98 59 84
4 40 11 67 14
89  1 44 51 45

44 51 57 75 19
33 54 24 96  1
30  0 45 47 38
58 78 17 74 14
91 60 32 67 10

72 91 61 45 49
68 67 31 69 96
92 52  9 34 36
16 77 62 55 41
42 88 53  4 15

68 98 73 81 41
36 43 87 48 95
55 62  8 12 30
23 34 59 72 85
38  7 16 13 79

66 98 23 69 89
71 84 79 14 53
10 62 60 44 28
49 43 29 45 75
72 94  5 64  3

27 99 41 10 91
2 94 16 85 75
25 95 15 34 36
90 72 76 20 29
55 18 93 53 69

84 87 16 80 64
48 45 63 53 25
10 44 65 58 59
94 31 41 35 89
42 29 71 69 61

63 44 74 33  3
65 47 90 52 38
9 13 91  4 45
26 39 94 68 93
56 31 41 59 32

17  6 85 74 19
58 93  9 92 31
90 69  2 87 11
67 94 53 66 30
15 41 59 51 45

7 59 76 14 15
51  3 88 17  2
42 39 98 84 65
54 48  9 56 74
68  5  1 24 13

55 34 94 50 28
8 14 93 64 52
92 62  9 51 38
31 74 73 24 59
33 65  7 75 32

30 93 48 17 33
67  7  5  0 69
54 76 52  1 87
99 73 50 25 16
13 20 41 77 62

54  0 33 23 75
50  4 29 18 94
14  8 38 48 53
84 13 12 91 83
69 78 55 47 26

30 65 70 51  1
22 53 86 46 89
99 79 20 24 64
18 92 82  0 68
57 33 61 12 83

22 47 21 38 57
41 63 61 95 79
7 52 87 71 14
40 45 92 73 48
42  0 26 94  5

90 98 47 85 52
19 44 48 59 88
93 81  7 16 63
1 45 84 11 24
78 27 62 77 37

34  2  6  3 56
48 47 12 58 76
89 78 30 75 20
91 39 97 69 28
96 64 32 61 67

62 25 53 56 97
42 34 24 64 95
17 35  7 79 68
30 74 54 78  3
88 66 18 38 90

63 45 80 89 76
39 57 71 25 79
6  3 28 94 34
82 18 95 42 33
96 36 27 83 66

20 87 59 86 21
14 61  7 23 56
5 33 91  0  1
97 65 82 95 72
18 26 35 47 69

86 65 41 97 40
2 73 68 34  3
88 71 79 70 95
48 62  1 50 77
13 67 20 15 55

98 16 93 30 24
39 11 56 13  3
14 83 76 58 60
73 81 63 88 92
74 96 51 43 77

4 12 43 46 53
34 37 50 60 56
96 98 10 65 38
40 24 80 77 73
23 67 84 64 13

5 19 85  1 58
41 15 44 57 22
2 66 80 94 71
40 55 89 79  0
56 81 23 30 24

0 29 94  4 68
81  2 20 95 43
49 40 37 30 96
19 23 42 26 85
61 17 12 72 88

64 26 61 32 68
80 24 31 67 49
72 15 88 98 50
73 55 86 38 10
87 34 52 29 63

76 61 36 54 10
63 56 75 67 29
11  9 41  4 43
83 88 45 93 74
72 70 94 60 98

44 99 39  5 23
53 69 68 60 83
81 72  1 97 20
43 96 91 11 32
51  7  8 86 25

62 36 39 20  5
52 59  6 74 95
2 83 44 43 66
96 55 79 42 37
34 32 19  0 90

84  8 65 35 78
57 83  4 82 36
3 29 74 60 90
86 47 53 26 10
52 51 40 32  6

69 32 77 72 44
34 54 82 94 23
1 60  2 11 36
17 52  0 73 31
5 90 24 21 53

42 24 18 44  5
74 19 76 62 34
61 66  4 77 47
92 94 78 39 97
25 91 11 22 23

11 87 83 93  8
7 73 85 84  3
65 16 71 12 44
88 61 54 19 97
72 57 80 59 45

30 55  1 62 39
31 45 34  6 43
68 93 38 57 60
70 32  5 95 92
35 83 66 36 91

76 89 54 36 21
99 28 62 78 73
17 72 96 13 91
79 65 37 81 14
33 86 31 11 25

3 93 37 88 47
4  6 19 87 30
76 22 33 69 64
36 52 60 82 53
39  5 61 67 90

28  6 59 61 75
40  4 65 43 93
41 32 60 89 18
22 11 12  9  0
26 15 78 90 30

8 87 74 70 76
45 43 63 84 25
18 68 92  4 31
66 22 29 71 91
15 50 64 52  1

3 93 25 71 43
45 48 52  4 94
36 34 47 74 97
12 27 64 14 80
41 17 21 32 88

76 62 68 24  0
97 43 37  3 79
65 25 52 28 20
29 18 90 89 48
35 66 19 53 82

57 98 86 69 24
55 74 49 12 35
68 50 70 38 75
82 56 73  5 89
90 10 15 14 95

98 86 47 87 81
62 51  3 79 10
60 39 61  9 77
36 50 89 49 88
63 95  1 15 69

72 71 51 33 23
12 35 97 34 99
4 32 36 88 80
5 17  3 56  1
44 19 47 98 26

55 56 75  5 74
4 86 60 67 68
21 45 40 17  2
43 39 20 88 33
24 70 58 13 98

34 28 86 65 80
82 23 98 91  0
61 62 43 51 49
39 10 58  1 11
8 83 17 97 31

7 96  2 18 26
15 86 34 76 61
75 93 68  3 83
63 37 85  8 74
11 51 32 14 23

25 33 24 86 75
63 73 97 37 47
76 72 34 81 27
93 91 70 20 98
11 94 64 28  6

70 86 59 78 95
47 18 64 40 68
49 55 20 12 53
60 76 35 83 50
32 73 91 46 28

31 21 92 97 89
14 82 51  1 62
43 58 90 18 24
56 79 54 35 48
29 39 45 86 66

62 74 81 27 23
28 92 66 86 29
98 95 14 35 11
13 69 70 64 79
51 80 87 96 85

69 15 37 95 29
6 45 20 17 36
62 42  1 77 59
25 44 72 35 31
70 73 86 89 97

81 88 82 99 13
53 87  2 29 47
36 94 48 95 65
96 42 92 61 60
86 33 83 52 12

32  4 52 54 24
28 77 58 80 16
62 82 59  6 66
63 72 91 41 29
36 22 68  0 31

38 58 51 77 26
80  5 90 30 20
33 14 54 53  0
76 74 63 84 15
45 73 41 29 69

28 31 47 97 58
69  7 86 40 57
45  1 11 84 54
29  9 95 93 88
12  5 79 76 77

93  7 60 75 12
49 64 20 46 10
3 23 76 42 47
9 22  6 34 87
41 37 66 45 48

9 53 64 37 83
89 43 91 30 36
39 22 69 79 42
31 28 55 40 54
7 24 44 84 46

81 75 15 89 31
11 13 12  5 10
94 80 22 68  3
27 60 42 65 58
40 82 17 92 73

16 40 98 33 41
79 72  2 77 86
14 32 85 74 50
11 49 62 56  9
69 20 12 19 60

95 72 61 54 92
10  0  1 84 99
88 62 13 18 86
89 59 60 29 78
55 98 27 91 87

58 37 49 43 76
14  3 65 13  2
92 20  6 73  5
96 53 82 95 99
15 83 80 26 11

51 71 56 58 95
59 84 46 54 94
8  7 49 62 83
47 92 32 31 65
48 44 57 25 20

5  3 26 90 14
61 16 44  4  0
49  1 52 82 54
60 77 86 43 87
6 36 73 25 31

32 25 37 18 63
60 65 35 24 34
52 94 12 77 47
99 83  2 30 74
58 38 17 75 56

77 88 70 22 39
4 14 86 99 50
5 79  2 78 16
34 46 83 58 13
71 74 21 76 66

25 15 99 79 68
59 95 82 22 53
0 70 85 73 67
36 10 66  6 32
1 51 52 40  4

29 49 64 26 18
56 28  9  7 99
20  2 74 71 39
81 94 55 75 16
52 45 35 66 11

42 68 76 37 34
0 41 86 19 59
7 46 49 30 88
74 80 24 50 29
96 78 67 61 26

45 96 11 99 44
87 95 32 77 64
8 31 80  4 34
24 91 90 16 60
13 29 63 67 59

15 55  0 52 54
58 45 62 66 85
91 19 32 16 92
93 59  8 22  2
98 33 31 17 84

28 49 63 81 69
30 45 46 90 26
4 76 94 93 24
15 16 18 86 62
2 98 89 44 67

32 35 11  1 38
86 36 70 24 95
59 94 71 99 40
48 64 91 25 57
29 76 68 88 58

45 81 50 20 68
85 91 49 38  9
27  0 58  8 10
59  1 88 19 89
23 29 61 99 12

74 50 41 83  4
33 34 88 23 14
89 39 98 75 37
26 73 80 71 44
52 53 48 84 85

47 35 98 73  9
61  4 71 91 58
65 40 63 36 86
12 30 69 46  2
6 17 23 32 74

84 87 59 58 65
30 53  7 98 48
71 28  1 77 56
36 88 52 64 89
90 37 94 26 39

25 75 32 48  6
49  7 74 50 62
68 51 72 12 10
94 37 97 69 91
73  9 67 18  1

51 96 71 44  0
97 52  1 77 11
90  9 15 76 24
68  6 57 40 98
47 31 94 82 49

48 31 26 77 14
94 93 27  2 82
11 83 85  9  8
90 69  4 16 22
5 62 29 57 30

59 46 82 13 53
71 58 67  2 28
73 52 30 97 12
95 34 51 32 38
62 81  5 15 74

82 10 58 84 67
92  5 66 25 83
90 76 21 26 69
39 63 42 11 53
23 61 80 99 48

8 13 49 63 83
26 48 50 53 43
1 39 95 60 99
96  7 35 94 10
24 44 65 31 46

87 17 72 71 81
35 56  4 99 59
96 90 37 23 86
34 73 18 19 78
41 10 43 92 80

29  7 37 40 70
72 10 85 74 76
36  9 13 73 82
4 65 49 77 42
75 24 84 78 39

29  6 94 59 38
99 55  8 66 85
14 43 25 92 67
49 33 87 39 44
9 61 24 90 75

45 35 78 56 24
53 38 34 92 55
20 44 90 33 83
54 95 71 87 13
40 43 85 88 68

32 80 59 34 22
18  9 12 15 55
85 60 37 64 92
35 39 26 90 50
52 67 58 87  8

90 96 40 14 61
34 33 80 22 38
59 94  3  6  4
0 71 25  1 20
30 57 48 31 10

50 14 40 48 83
85 80 29 60  2
92 59 86 99 57
46  7 89 66 75
98  6 45 97 51

13 17  0 57  5
29 68 52 11 59
78 63 94 88 89
35 54 74 18  2
28 65 86  1 30

57 46 44 86 62
43 89 81 34 13
32 51  9  6 35
29 52 84  8 88
26 21 17 75 85

14 87 26 22 30
65 78 98  6 60
42 99 19 71 11
68 27 48 52 62
51  1 54 37 74

76 69 37 49 61
20 91 14 31 11
54 40 67 71 15
73 64 85 80 62
5 10  3 51  7

63 90 54 46 17
16  5  4 56 33
39 41  8 87 80
14 77 26 47 70
15 48  6 10 76

96 95  0 42 56
3 87 92 18 60
51  7 38  4 91
41 33 80 43 66
61 32 57 84 10

8 64 76 78 17
55 74 68 47 29
46 84 49 58 93
52 26 32 11 33
24 42 98 27 43

14 19 27 93 16
23 32 74 73 67
68 38  2 22 76
6 12 94 15 77
64 62 34  3 37


Some thoughts about design decision I made and why I made them. Let me know if you think I should have made them different and why:

• using the 'factory' method Create: I wanted the list of numbers inside the BingoCard to be const, but that disallows to read it from std::cin in the constructor. Alternatively, I could remove the const, and put the Create-code inside the constructor.
• 'hit' array as a member: originally, I modified the numbers on the bingo card to -1 to declare them 'drawn', but that felt poor. The BingoCard should not lose information by getting its numbers overwritten. It worked with the -1 though.
• min/max calculation: I considered using stuff from <algorithm>, but I need more than the min/max values themselves; I also need their indexes. I don't see how I could use them at all.

• HasBingo works too hard. A hit may create a bingo only along the row and/or column of the square it hits. There is no reason to test all of them.

This observation hints that you may want to have 10 match counts (one per row, and one per column), and declare bingo when one of them reaches 5.

• Representing a card as an std::map<int, square> would let you find the hit much faster than scanning the entire card over and over again.

• DrawNumbers does not belong to BingoCard. Consider iterating over the drawn number first, along the lines of:

  for (auto n: numbers) {
for (auto b: boards) {
auto hit = b[n];
if (hit == b.end() {
continue;
}
if (b.hasBingo(hit) {
....
}
}
}

• The code assumes that a card consists of unique numbers, and the drawn numbers are also unique. However, the problem statement doesn't specify this. A prudent program must take it into an account.