akshitgaur2005.tngl.sh
Musings from the mountains
himwant.org
Advent of Code Day 1
+317
-26
+174
-25
src/content/aoc/2025/01.md
+174
-25
src/content/aoc/2025/01.md
···
1
1
---
2
2
title: "Secret Entrance"
3
-
published: 2025-12-15
3
+
published: 2025-12-16
4
4
draft: false
5
5
---
6
6
···
10
10
11
11
We are given line seperated operations that happen on a 100 pin circular dial. The dial starts at 50 and we are to output, how many times the dial stops at 0!
12
12
13
-
An example-
13
+
We first, as expected, just take in the input and initialise variables to store current dial position and total number of times the dial has stopped at 0.
14
+
15
+
```rust title="setup"
16
+
use std::fs::read_to_string;
17
+
fn main() {
18
+
let contents = read_to_string("./input.txt").unwrap();
19
+
let mut curr: i32 = 50;
20
+
let mut tot = 0;
21
+
22
+
<<logic_loop>>
23
+
24
+
println!("Password: {tot}");
25
+
}
26
+
```
27
+
28
+
We go through each line of input, and parse it. Our input is in the form -"L50" where the first character is the direction in which the dial is to be rotated and the rest is the amplitude.
29
+
30
+
```rust title="logic_loop"
31
+
for line in contents.lines() {
32
+
let words: Vec<char> = line.chars().collect();
33
+
let dir = words[0];
34
+
let amp: String = words[1..].iter().collect();
35
+
let amp: i32 = amp.parse().unwrap();
36
+
37
+
<<rotate_dial>>
38
+
}
39
+
```
40
+
41
+
Now that we have both the dir and the amp, we can modify the dial position.
42
+
43
+
```rust title="rotate_dial"
44
+
if dir == 'L' {
45
+
curr -= amp;
46
+
} else {
47
+
curr += amp;
48
+
}
49
+
50
+
<<loopback>>
51
+
```
52
+
53
+
The value of curr could have gone above 99 or below 0, which we need to rectify.
54
+
55
+
```rust title="loopback"
56
+
if curr < 0 {
57
+
curr = 100 + curr;
58
+
}
59
+
60
+
curr = curr % 100;
61
+
62
+
<<count_zeros>>
63
+
```
64
+
65
+
Now that we have the current position of the dial, we only need to check if it currently points at 0, and increment `tot`, if it does.
66
+
67
+
```rust title="count_zeros"
68
+
if curr == 0 {
69
+
tot += 1;
70
+
}
71
+
```
72
+
73
+
We have the final program-
74
+
75
+
```rust title="Full Program"
76
+
use std::fs::read_to_string;
77
+
fn main() {
78
+
let contents = read_to_string("./input.txt").unwrap();
79
+
let mut curr: i32 = 50;
80
+
let mut tot = 0;
81
+
82
+
for line in contents.lines() {
83
+
let words: Vec<char> = line.chars().collect();
84
+
let dir = words[0];
85
+
let amp: String = words[1..].iter().collect();
86
+
let amp: i32 = amp.parse().unwrap();
87
+
88
+
if dir == 'L' {
89
+
curr -= amp;
90
+
} else {
91
+
curr += amp;
92
+
}
93
+
94
+
if curr < 0 {
95
+
curr = 100 + curr;
96
+
}
97
+
98
+
curr = curr % 100;
99
+
100
+
if curr == 0 {
101
+
tot += 1;
102
+
}
103
+
}
104
+
105
+
println!("Password: {tot}");
106
+
}
107
+
```
108
+
109
+
110
+
# Part 2
111
+
112
+
Another complication has been added to our little problem! Now we not only need to say how many times the dial stopped at 0, but also how many times it crossed it!
113
+
114
+
We add two new variables that are going to help us! `prev` and `cross`. We are going to keep `tot` and its related logic and add onto it!
115
+
116
+
```rust
117
+
// In setup
118
+
let mut prev: i32 = 50;
119
+
let mut cross = 0;
120
+
```
121
+
122
+
If the amplitude is >100, that means atleast one full rotation, therefore, atleast oe crossing of 0! We can figure out the number of crossing here using division. We also bring the value of amp to the range [0, 99].
123
+
124
+
```rust
125
+
cross += amp / 100;
126
+
let amp = amp % 100;
127
+
```
128
+
129
+
And easy way to detect a crossing is to check if the difference in `curr` and `prev` is opposite to what we expected. For example, let's say dial position is at `5` and we get input `L10`, as the `dir` is `L`, the dial position should decrease, that is, `curr` < `prev`, but as the `amp` was 10, the new dial position becomes `95`. Now, `curr` (95) > `prev` (5). Detecting for this, and similarly `curr` < `prev` for `R`, we can get if there was a crossing on this move! One thing we need to keep in mind is to ignore those where either `curr` or `prev` were 0, as they would have been already counted by `tot`!
130
+
131
+
:::duck
132
+
Also, do remember to set `prev = curr` at the end, you don't want to know how much time I spent debugging this!
133
+
:::
134
+
135
+
```rust
136
+
if ((dir == 'L' && prev < curr) || (dir == 'R' && prev > curr)) && curr != 0 && prev != 0 {
137
+
cross += 1;
138
+
}
139
+
140
+
prev = curr;
141
+
```
142
+
143
+
Our final program-
144
+
145
+
```rust title="main.rs"
146
+
use std::fs::read_to_string;
147
+
148
+
fn main() {
149
+
let contents = read_to_string("./input.txt").unwrap();
150
+
151
+
let mut curr: i32 = 50;
152
+
let mut prev: i32 = 50;
153
+
let mut tot = 0;
154
+
let mut cross = 0;
155
+
156
+
for line in contents.lines() {
157
+
let words: Vec<char> = line.chars().collect();
158
+
let dir = words[0];
159
+
let amp: String = words[1..].iter().collect();
160
+
let amp: i32 = amp.parse().unwrap();
161
+
162
+
cross += amp / 100;
163
+
let amp = amp % 100;
164
+
165
+
if dir == 'L' {
166
+
curr -= amp;
167
+
} else {
168
+
curr += amp;
169
+
}
170
+
171
+
if curr < 0 {
172
+
curr = 100 + curr;
173
+
}
174
+
curr = curr % 100;
14
175
15
-
> L68
16
-
> L30
17
-
> R48
18
-
> L5
19
-
> R60
20
-
> L55
21
-
> L1
22
-
> L99
23
-
> R14
24
-
> L82
176
+
if curr == 0 {
177
+
tot += 1;
178
+
}
179
+
if ((dir == 'L' && prev < curr) || (dir == 'R' && prev > curr)) && curr != 0 && prev != 0 {
180
+
cross += 1;
181
+
}
25
182
26
-
- Dial @ 50
27
-
- L68 -> 82
28
-
- L30 -> 52
29
-
- R48 -> **0**!
30
-
- L5 -> 95
31
-
- R60 -> 55
32
-
- L55 -> **0**!
33
-
- L1 -> 99
34
-
- L99 -> **0**!
35
-
- R14 -> 14
36
-
- L82 -> 32
37
-
38
-
So, in total `3` times! `3` is our answer!
183
+
prev = curr;
184
+
}
185
+
println!("Password: {}", cross + tot);
186
+
}
187
+
```
39
188
+143
-1
src/content/aoc/2025/01.org
+143
-1
src/content/aoc/2025/01.org
···
1
1
#+title: Secret Entrance
2
2
#+OPTIONS: toc:nil
3
+
#+PROPERTY: header-args:rust :noweb no-export
3
4
4
5
* Part 1
5
6
We are given line seperated operations that happen on a 100 pin circular dial. The dial starts at 50 and we are to output, how many times the dial stops at 0!
6
7
8
+
9
+
We first, as expected, just take in the input and initialise variables to store current dial position and total number of times the dial has stopped at 0.
10
+
#+name: setup
11
+
#+begin_src rust :title "setup"
12
+
use std::fs::read_to_string;
13
+
fn main() {
14
+
let contents = read_to_string("./input.txt").unwrap();
15
+
let mut curr: i32 = 50;
16
+
let mut tot = 0;
17
+
18
+
<<logic_loop>>
19
+
20
+
println!("Password: {tot}");
21
+
}
22
+
#+end_src
23
+
24
+
We go through each line of input, and parse it. Our input is in the form -"L50" where the first character is the direction in which the dial is to be rotated and the rest is the amplitude.
25
+
26
+
#+name: logic_loop
27
+
#+begin_src rust :title "logic_loop"
28
+
for line in contents.lines() {
29
+
let words: Vec<char> = line.chars().collect();
30
+
let dir = words[0];
31
+
let amp: String = words[1..].iter().collect();
32
+
let amp: i32 = amp.parse().unwrap();
33
+
34
+
<<rotate_dial>>
35
+
}
36
+
#+end_src
37
+
38
+
Now that we have both the dir and the amp, we can modify the dial position.
39
+
#+name: rotate_dial
40
+
#+begin_src rust :title "rotate_dial"
41
+
if dir == 'L' {
42
+
curr -= amp;
43
+
} else {
44
+
curr += amp;
45
+
}
46
+
47
+
<<loopback>>
48
+
#+end_src
49
+
50
+
The value of curr could have gone above 99 or below 0, which we need to rectify.
51
+
#+name: loopback
52
+
#+begin_src rust :title "loopback"
53
+
if curr < 0 {
54
+
curr = 100 + curr;
55
+
}
56
+
57
+
curr = curr % 100;
58
+
59
+
<<count_zeros>>
60
+
#+end_src
61
+
62
+
Now that we have the current position of the dial, we only need to check if it currently points at 0, and increment =tot=, if it does.
63
+
#+name: count_zeros
64
+
#+begin_src rust :title "count_zeros"
65
+
if curr == 0 {
66
+
tot += 1;
67
+
}
68
+
#+end_src
69
+
70
+
We have the final program-
71
+
72
+
#+begin_src rust :noweb yes :title "Full Program"
73
+
<<setup>>
74
+
#+end_src
75
+
76
+
* Part 2
77
+
78
+
Another complication has been added to our little problem! Now we not only need to say how many times the dial stopped at 0, but also how many times it crossed it!
79
+
80
+
We add two new variables that are going to help us! =prev= and =cross=. We are going to keep =tot= and its related logic and add onto it!
81
+
7
82
#+begin_src rust
83
+
// In setup
84
+
let mut prev: i32 = 50;
85
+
let mut cross = 0;
8
86
#+end_src
9
87
10
-
#+RESULTS:
88
+
If the amplitude is >100, that means atleast one full rotation, therefore, atleast oe crossing of 0! We can figure out the number of crossing here using division. We also bring the value of amp to the range [0, 99].
89
+
#+begin_src rust
90
+
cross += amp / 100;
91
+
let amp = amp % 100;
92
+
#+end_src
93
+
94
+
And easy way to detect a crossing is to check if the difference in =curr= and =prev= is opposite to what we expected. For example, let's say dial position is at =5= and we get input =L10=, as the =dir= is =L=, the dial position should decrease, that is, =curr= < =prev=, but as the =amp= was 10, the new dial position becomes =95=. Now, =curr= (95) > =prev= (5). Detecting for this, and similarly =curr= < =prev= for =R=, we can get if there was a crossing on this move! One thing we need to keep in mind is to ignore those where either =curr= or =prev= were 0, as they would have been already counted by =tot=!
95
+
96
+
:::duck
97
+
Also, do remember to set =prev = curr= at the end, you don't want to know how much time I spent debugging this!
98
+
:::
99
+
100
+
#+begin_src rust
101
+
if ((dir == 'L' && prev < curr) || (dir == 'R' && prev > curr)) && curr != 0 && prev != 0 {
102
+
cross += 1;
103
+
}
104
+
105
+
prev = curr;
106
+
#+end_src
107
+
108
+
109
+
Our final program-
110
+
#+begin_src rust :title "main.rs"
111
+
use std::fs::read_to_string;
112
+
113
+
fn main() {
114
+
let contents = read_to_string("./input.txt").unwrap();
115
+
116
+
let mut curr: i32 = 50;
117
+
let mut prev: i32 = 50;
118
+
let mut tot = 0;
119
+
let mut cross = 0;
120
+
121
+
for line in contents.lines() {
122
+
let words: Vec<char> = line.chars().collect();
123
+
let dir = words[0];
124
+
let amp: String = words[1..].iter().collect();
125
+
let amp: i32 = amp.parse().unwrap();
126
+
127
+
cross += amp / 100;
128
+
let amp = amp % 100;
129
+
130
+
if dir == 'L' {
131
+
curr -= amp;
132
+
} else {
133
+
curr += amp;
134
+
}
135
+
136
+
if curr < 0 {
137
+
curr = 100 + curr;
138
+
}
139
+
curr = curr % 100;
140
+
141
+
if curr == 0 {
142
+
tot += 1;
143
+
}
144
+
if ((dir == 'L' && prev < curr) || (dir == 'R' && prev > curr)) && curr != 0 && prev != 0 {
145
+
cross += 1;
146
+
}
147
+
148
+
prev = curr;
149
+
}
150
+
println!("Password: {}", cross + tot);
151
+
}
152
+
#+end_src