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use std::collections::HashMap;
use std::fs;
fn sector_if_valid(room: &str) -> Option<u32> {
let mut letters = HashMap::new();
let mut sector = String::new();
let mut checksum = "";
for (i, c) in room.chars().enumerate() {
if c.is_ascii_lowercase() {
// Keep track of each letter's frequency
let count = letters.entry(c).or_insert(0);
*count += 1;
} else if c.is_ascii_digit() {
// Store the sector ID number
sector.push(c);
} else if c == '[' {
// Checksum is 5 characters in square brackets
checksum = &room[i + 1..i + 6];
break;
}
}
// Check if this room's checksum is valid
for c in checksum.chars() {
// Expect the most common letters, in alphabetical order if tied:
// score = 100 * frequency - alphabet index - constant
let &expected = letters
.iter()
.max_by_key(|x| 100 * x.1 - x.0.to_digit(36).unwrap())
.unwrap()
.0;
letters.remove(&c);
if c != expected {
return None;
}
}
Some(sector.parse().unwrap())
}
fn solve_part1(lines: &Vec<&str>) -> u32 {
let mut total = 0;
for room in lines {
let sec = sector_if_valid(room);
if sec.is_some() {
total += sec.unwrap();
}
}
total
}
fn solve_part2(lines: &Vec<&str>) -> Option<u32> {
for line in lines {
let osector = sector_if_valid(line);
if osector.is_none() {
continue;
}
// Extract ciphertext from `line'
let mut ctext = String::new();
for c in line.chars() {
if c.is_ascii_digit() {
break;
}
ctext.push(c);
}
// Ignore last '-' before sector ID
ctext.pop();
// Decrypt ciphertext into plaintext
let mut ptext = String::new();
for c in ctext.chars() {
if c == '-' {
ptext.push(' ');
} else {
// Apply shift cipher (shift by sector ID)
let old = c.to_digit(36).unwrap() - 10;
let new = (old + osector.unwrap()) % 26;
ptext.push(char::from_digit(new + 10, 36).unwrap());
}
}
// Right name found by inspecting decrypted list by eye
if ptext == "northpole object storage" {
return osector;
}
}
None // shouldn't happen
}
fn main() {
// Read room list from input text file
let input = fs::read_to_string("input.txt").unwrap();
let lines = input.lines().collect();
// Part 1 gives 158835 for me
println!("Part 1 solution: {}", solve_part1(&lines));
// Part 2 gives 993 for me
println!("Part 2 solution: {}", solve_part2(&lines).unwrap());
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn part1_example1() {
let lines = vec!["aaaaa-bbb-z-y-x-123[abxyz]"];
assert_eq!(solve_part1(&lines), 123);
}
#[test]
fn part1_example2() {
let lines = vec!["a-b-c-d-e-f-g-h-987[abcde]"];
assert_eq!(solve_part1(&lines), 987);
}
#[test]
fn part1_example3() {
let lines = vec!["not-a-real-room-404[oarel]"];
assert_eq!(solve_part1(&lines), 404);
}
#[test]
fn part1_example4() {
let lines = vec!["totally-real-room-200[decoy]"];
assert_eq!(solve_part1(&lines), 0);
}
}
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