2020: day10: part 2

This commit is contained in:
Antoine Martin 2020-12-10 14:51:38 +01:00
parent 2726134d7d
commit 2381f61118

View file

@ -1,3 +1,4 @@
use std::collections::HashMap;
use std::fmt::Write;
use aoc::err;
@ -8,6 +9,7 @@ pub fn run() -> aoc::Result<String> {
let mut res = String::with_capacity(128);
writeln!(res, "part 1: {}", part1(INPUT)?)?;
writeln!(res, "part 2: {}", part2(INPUT)?)?;
Ok(res)
}
@ -45,6 +47,65 @@ fn part1(input: &str) -> aoc::Result<usize> {
Ok(differences[1] * differences[3])
}
fn find_possibilities(jolts: &[u64], possibilities: &mut HashMap<u64, usize>) -> usize {
if let Some(&count) = possibilities.get(&jolts[0]) {
return count;
}
if jolts.len() == 1 {
return 1;
}
let curr = jolts[0];
let possibilities_from_here = jolts
.iter()
.copied()
// we need the index to step in the jolt slice
.enumerate()
// skip the current adapter one
.skip(1)
// its 3 successors can possibly be removed
.take(3)
// remove adapter if the jolt difference is too high
.filter(|(_, jolt)| jolt - curr <= 3)
// count each possible next adapter and all its possibilities
.map(|(idx, _)| find_possibilities(&jolts[idx..], possibilities))
.sum();
possibilities.insert(curr, possibilities_from_here);
possibilities_from_here
}
fn part2(input: &str) -> aoc::Result<usize> {
let mut jolts = input
.lines()
.map(|line| {
line.parse()
.map_err(|e| err!("couldn't parse joltage: {}", e))
})
.collect::<aoc::Result<Vec<u64>>>()?;
if jolts.is_empty() {
return Err(err!("input was empty!"));
}
// charging outlet can be added
jolts.insert(0, 0);
jolts.sort_unstable();
// device is rated for max adapter + 3 jolts
let max_adapter = jolts[jolts.len() - 1];
let device_rating = max_adapter + 3;
jolts.push(device_rating);
let mut possibilities = HashMap::new();
Ok(find_possibilities(&jolts, &mut possibilities))
}
#[cfg(test)]
mod tests {
use super::*;
@ -62,4 +123,15 @@ mod tests {
fn part1_real() {
assert_eq!(part1(INPUT).unwrap(), 2112);
}
#[test]
fn part2_provided() {
assert_eq!(part2(PROVIDED1).unwrap(), 8);
assert_eq!(part2(PROVIDED2).unwrap(), 19208);
}
#[test]
fn part2_real() {
assert_eq!(part2(INPUT).unwrap(), 3022415986688);
}
}