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// Find the next Elf still in the game, to the left of `i'
fn next_elf(elves: &[bool], i: usize) -> usize {
let mut n = (i + 1) % elves.len();
while !elves[n] {
n = (n + 1) % elves.len();
}
n
}
fn solve_puzzle(
num_elves: usize,
victim1: usize,
next_victim: fn(&[bool], usize, usize, usize) -> usize,
) -> usize {
// We only need to store whether an Elf is still in the game;
// we don't care how many presents each one has.
let mut elves = [true].repeat(num_elves);
// Thief and victim indices, respectively
let mut t = 0;
let mut v = victim1;
let mut rem = num_elves;
while rem > 1 {
// Perform the act of stealing
elves[v] = false;
rem -= 1;
// Who does the stealing next?
t = next_elf(&elves, t);
// Who gets stolen from next?
v = next_victim(&elves, rem, t, v);
}
// Elves should be numbered from 1 instead of 0
t + 1
}
// Part 1: victim is always to the left of thief.
fn next_victim_part1(elves: &[bool], _rem: usize, nthief: usize, _victim: usize) -> usize {
next_elf(elves, nthief)
}
// Part 2: victim is always across from thief.
// It's less clear that this function is correct, but it is!
fn next_victim_part2(elves: &[bool], rem: usize, _nthief: usize, victim: usize) -> usize {
let mut nv = next_elf(elves, victim);
if rem % 2 == 0 {
nv = next_elf(elves, nv);
}
nv
}
fn main() {
// My personal puzzle input
let input = 3014603;
// Part 1 gives 1834903 for me
println!(
"Part 1 solution: {}",
solve_puzzle(input, 1, next_victim_part1)
);
// Part 2 gives 1420280 for me
println!(
"Part 2 solution: {}",
solve_puzzle(input, input / 2, next_victim_part2)
);
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn part1_example1() {
assert_eq!(solve_puzzle(5, 1, next_victim_part1), 3);
}
#[test]
fn part2_example1() {
assert_eq!(solve_puzzle(5, 5 / 2, next_victim_part2), 2);
}
}
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