1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
|
use std::collections::HashMap;
use std::fs;
// This implementation is more similar to day 23 than day 12
#[derive(Copy, Clone, Debug)]
enum Arg<'a> {
Reg(&'a str),
Lit(isize),
}
impl<'a> From<&'a str> for Arg<'a> {
fn from(s: &'a str) -> Self {
if s.chars().all(|c| c.is_ascii_digit() || c == '-') { // yes I know
Self::Lit(s.parse().unwrap())
} else {
Self::Reg(s)
}
}
}
#[derive(Copy, Clone, Debug)]
enum Oper<'a> {
Inc(Arg<'a>),
Dec(Arg<'a>),
Out(Arg<'a>),
Cpy(Arg<'a>, Arg<'a>),
Jnz(Arg<'a>, Arg<'a>),
}
fn parse(lines: Vec<&str>) -> Vec<Oper> {
let mut insts = Vec::new();
for line in lines {
let words: Vec<&str> = line.split_ascii_whitespace().collect();
if words[0] == "inc" {
insts.push(Oper::Inc(Arg::from(words[1])));
} else if words[0] == "dec" {
insts.push(Oper::Dec(Arg::from(words[1])));
} else if words[0] == "out" {
insts.push(Oper::Out(Arg::from(words[1])));
} else if words[0] == "cpy" {
insts.push(Oper::Cpy(Arg::from(words[1]), Arg::from(words[2])));
} else if words[0] == "jnz" {
insts.push(Oper::Jnz(Arg::from(words[1]), Arg::from(words[2])));
}
}
insts
}
// We pause the program at each `Out' and save the state to resume later
#[derive(Clone, Debug, PartialEq)]
struct State<'a> {
regs: HashMap<&'a str, isize>,
ip: usize,
}
impl<'a> From<Vec<(&'a str, isize)>> for State<'a> {
fn from(init: Vec<(&'a str, isize)>) -> Self {
Self {
regs: init.into_iter().collect(),
ip: 0,
}
}
}
// Execute the program until the next `Out` instruction, return the transmission
fn transmit_next<'a>(insts: &'a Vec<Oper>, state: &mut State<'a>) -> Option<isize> {
let mut transmit = None;
while state.ip < insts.len() && transmit.is_none() {
match insts[state.ip] {
Oper::Inc(arg) => {
if let Arg::Reg(dst) = arg {
*state.regs.entry(dst).or_insert(0) += 1;
}
},
Oper::Dec(arg) => {
if let Arg::Reg(dst) = arg {
*state.regs.entry(dst).or_insert(0) -= 1;
}
},
Oper::Out(arg) => {
let val = match arg {
Arg::Reg(reg) => state.regs.get(®).copied().unwrap_or(0),
Arg::Lit(lit) => lit,
};
transmit = Some(val);
},
Oper::Cpy(arg1, arg2) => {
// This `if let' is actually a leftover from day 23
if let Arg::Reg(dst) = arg2 {
let val = match arg1 {
Arg::Reg(src) => state.regs.get(&src).copied().unwrap_or(0),
Arg::Lit(lit) => lit,
};
*state.regs.entry(dst).or_insert(0) = val;
}
},
Oper::Jnz(arg1, arg2) => {
let val = match arg1 {
Arg::Reg(reg) => state.regs.get(®).copied().unwrap_or(0),
Arg::Lit(lit) => lit,
};
if val != 0 {
let off = match arg2 {
Arg::Reg(reg) => state.regs.get(®).copied().unwrap_or(0),
Arg::Lit(lit) => lit,
};
// Subtract 1 to compensate for `state.ip += 1;' below
state.ip = ((state.ip as isize) + off - 1) as usize;
}
},
};
state.ip += 1;
}
transmit
}
fn solve_part1(insts: &Vec<Oper>) -> isize {
let mut a = 1;
loop {
// History of (transmission, state) for this value of `a'.
// We're looking for a periodic pattern in these tuples.
let mut history: Vec<(isize, State)> = Vec::new();
// Initialize a fresh state with the candidate value of `a'
let mut state: State = State::from(vec![("a", a)]);
// This puzzle is basically Turing's halting problem, so for practicality
// we arbitrarily assume that the periodicity is at most 100 transmissions
for _i in 0..100 {
let ot = transmit_next(insts, &mut state);
// Did the program exit?
if ot.is_none() {
break;
}
// Did we transmit something other than 0 or 1?
let t = ot.unwrap();
if t != 0 && t != 1 {
break;
}
// Didn't we transmit 0 after 1, or 1 after 0?
if history.len() > 0 && history.last().unwrap().0 != 1 - t {
break;
}
let ts = (t, state.clone());
// Have we seen this state before? If yes, we've found a solution!
if history.contains(&ts) {
return a;
} else {
// But the transmission pattern seems promising so far; store it
history.push(ts);
}
}
a += 1;
}
}
fn main() {
// Read assembly from input text file and parse it
let input = fs::read_to_string("input.txt").unwrap();
let lines = input.lines().collect();
let insts = parse(lines);
// Part 1 gives 158 for my input
println!("Part 1 solution: {}", solve_part1(&insts));
// There's no part 2, since this is the last day
}
|
