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Add comments and refactor code for 2024 day 12 in a more functional style

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This commit is contained in:
Chris Alge 2024-11-20 14:55:00 +01:00
parent fe3134d10e
commit 02398d2541
1 changed files with 61 additions and 23 deletions
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80
2024/day12_desert_shower/src/lib.rs
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@ -51,23 +51,35 @@ struct Catapult {
}
impl Catapult {
/// If this catapult is able to hit the `target` coordinates, this returns `Some(p)`, where `p`
/// is the `Phase`, in which it will be hit. If it cannot be hit, `None` is returned.
fn can_hit(&self, target: Coordinates) -> Option<Phase> {
if target.x <= self.coordinates.x {
// we only ever shoot to the right
None
} else if target.y <= self.coordinates.y {
// Special case to avoid underflows and division by zero below.
// If the target is on equal or lower height than the catapult, we can only ever hit it
// in descend phase (or not at all), so we don't need to check the other cases
if (target.x + target.y - (self.coordinates.x + self.coordinates.y)) % 3 == 0 {
Some(Phase::Descend)
} else {
None
}
} else {
match (target.x - self.coordinates.x) / (target.y - self.coordinates.y) {
0 => if target.y - self.coordinates.y == target.x - self.coordinates.x {
match (target.x - self.coordinates.x).div_ceil(target.y - self.coordinates.y) {
// The match formula determines in which phase (if any) we could hit the target:
// * (>0..1): The y difference is greater than the x difference. We can't possibly
// hit. (Exact 0 is already being handled by the special casing above).
// * 1 exactly: The x and y differences are equal. We definately hit in ascend phase.
// * (>1..=2): This marks the glide phase. We definately hit there.
// * (>2..): Descend phase. We hit if the sums of x and y differ by a multiple of 3.
1 => if target.y - self.coordinates.y == target.x - self.coordinates.x {
Some(Phase::Ascend)
} else {
None
},
1 => Some(Phase::Glide),
2 => Some(Phase::Glide),
_ => if (target.x + target.y - (self.coordinates.x + self.coordinates.y)) % 3 == 0 {
Some(Phase::Descend)
} else {
@ -77,6 +89,10 @@ impl Catapult {
}
}
/// Returns the (minimal) shooting power required to hit the `target`, or `None` if it cannot be
/// hit. If the target is being hit in the ascend phase, any power greater than or equal to the
/// returned value will hit the target. In the other two phases, only this exact value will
/// hit.
fn power_to_hit(&self, target: Coordinates) -> Option<usize> {
match self.can_hit(target) {
Some(Phase::Ascend) | Some(Phase::Glide) => Some(target.y - self.coordinates.y),
@ -98,9 +114,12 @@ fn try_parse(input: &str) -> Result<(Vec<Catapult>, Vec<Coordinates>), ParseErro
match c {
'.' | '=' => (),
'T' => targets.push(Coordinates { x, y }),
'H' => targets.append(&mut vec![Coordinates { x, y }; 2]),
c if ['A', 'B', 'C'].contains(&c) => catapults.push(
Catapult { coordinates: Coordinates { x, y }, segment_number: c as usize - b'@' as usize }),
'H' => targets.append(&mut vec![Coordinates { x, y }; 2]), // same as 2 targets in the same spot
c if ['A', 'B', 'C'].contains(&c) =>
catapults.push( Catapult {
coordinates: Coordinates { x, y },
segment_number: c as usize - b'@' as usize,
}),
e => return Err(ParseError::ParseCharError(e)),
}
}
@ -112,11 +131,30 @@ pub fn run(input: &str, part: usize) -> Result<usize, ParseError> {
match part {
1 | 2 => {
let (catapults, targets) = try_parse(input)?;
let score = (0..targets.len())
.map(|shot| {
let target = targets[shot];
let catapult = catapults.iter().find(|c| c.can_hit(target).is_some()).unwrap();
catapult.segment_number * catapult.power_to_hit(target).unwrap()
// Despite the challenge suggesting it, the order in which we attack the targets
// doesn't actually matter. Since all targets are being hit in the descend phase, and
// during that phase, any point can be reached by exactly one of our catapults (see
// sketch below) with exactly one value of shooting power, the ranking of each target
// can only ever have one value. Hence, we only need to make sure to visit every target
// exactly once.
//
// The following sketch shows, which points are reachable from which catapult in
// descend phase with shooting powers 1..=3 (lowercase letters indicate the catapults,
// uppercase the reachable points; dots are not reachable in descend phase):
//
// ........C
// ......C.BC
// .c..C.BCABC
// .b..BCABCABC
// .a..ABCABCABC
// =============
let score = targets
.iter()
.map(|&target| {
catapults
.iter()
.find_map(|c| c.power_to_hit(target).map(|p| p * c.segment_number))
.expect("target unreachable")
}).sum();
Ok(score)
},
@ -127,17 +165,17 @@ pub fn run(input: &str, part: usize) -> Result<usize, ParseError> {
Catapult { coordinates: Coordinates { x: 0, y: 2 }, segment_number: 3 },
];
let meteors = input.lines().map(Coordinates::try_from).collect::<Result<Vec<_>, _>>()?;
let score = meteors.iter().map(|meteor| {
for time in meteor.x.div_ceil(2)..(meteor.x).max(meteor.y) {
let score = meteors
.iter()
.map(|meteor| {
(meteor.x.div_ceil(2)..=(meteor.x).max(meteor.y))
.find_map(|time| {
let target = Coordinates { x: meteor.x - time, y: meteor.y - time };
let shooters: Vec<_> = catapults.iter().filter(|c| c.can_hit(target).is_some()).collect();
if !shooters.is_empty() {
return shooters.iter().map(|c|
c.segment_number * c.power_to_hit(target).unwrap()
).min().unwrap();
}
}
panic!("A meteor could not be hit by any catapult");
catapults
.iter()
.filter_map(|c| c.power_to_hit(target).map(|p| p * c.segment_number))
.min()
}).expect("target unreachable")
}).sum();
Ok(score)