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Abandon setups that are unlikely to win #58

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23 changes: 23 additions & 0 deletions src/year2018/day24.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,13 @@
//! become too weak to destroy any further units. As each fight is independent, we find the
//! minimum boost value with a multithreaded parallel search.
//!
//! Additionally, it is possible to speed up part 2 with a heuristic comparing the ratio of
//! infection hit points to immune effective power, vs. the ratio of immune hit points to
//! infection effective power. A larger ratio requires more turns to defeat the other side. It is
//! possible to win with the larger ratio, but larger boosts bring the ratios closer together;
//! so this code skips boosts where the immune system has more than twice the nominal work to
//! do to wipe out the infection hit points.
//!
//! [`Day 15`]: crate::year2018::day15
use crate::util::hash::*;
use crate::util::parse::*;
Expand All @@ -21,6 +28,7 @@ enum Kind {
Immune,
Infection,
Draw,
Unbalanced,
}

#[derive(Clone, Copy)]
Expand Down Expand Up @@ -118,6 +126,21 @@ fn fight(input: &Input, boost: i32) -> (Kind, i32) {
// Boost reindeer's immune system.
immune.iter_mut().for_each(|group| group.damage += boost);

// Determine if the fight is even worth running.
if boost != 0 {
let infect_hp: usize =
infection.iter().map(|group| (group.units * group.hit_points) as usize).sum();
let immune_hp: usize =
immune.iter().map(|group| (group.units * group.hit_points) as usize).sum();
let infect_eff: usize =
infection.iter().map(|group| (group.units * group.damage) as usize).sum();
let immune_eff: usize =
immune.iter().map(|group| (group.units * group.damage) as usize).sum();
if infect_hp / immune_eff > 2 * immune_hp / infect_eff {
return (Kind::Unbalanced, 0);
}
}

for turn in 1.. {
// Target selection phase.
let mut target_selection = |attacker: &[Group], defender: &mut [Group], kind: Kind| {
Expand Down