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day03.rs
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day03.rs
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//! # Crossed Wires
//!
//! The input follow some implicit rules that can be used to simplify our approach:
//!
//! * Wires cross only at right angles to each other, so we only need to consider horizontal lines
//! when moving vertically and vice-versa.
//! * There is only a single vertical line at a given x coordinates and vice-versa.
//!
//! This makes [`BTreeMap`] a great choice to store horizontal or vertical line segments as there
//! are no collisions. The [`range`] method can lookup all line segments contained between two
//! coordinates to check for intersections.
//!
//! First we build two maps, one vertical and one horizontal, of each line segment for the first
//! wire. Then we trace the steps of the second wire, looking for any intersections. We calculate
//! both part one and part two at the same time, by also including the distance so far
//! from the starting point of each lines.
//!
//! [`range`]: BTreeMap::range
use crate::util::parse::*;
use crate::util::point::*;
use std::collections::BTreeMap;
type Input = (i32, i32);
struct Line {
start: Point,
end: Point,
distance: i32,
}
pub fn parse(input: &str) -> Input {
// Map a line into an iterator of direction and distance pairs.
let lines: Vec<_> = input.lines().collect();
let steps = |i: usize| {
let first = lines[i].bytes().filter(u8::is_ascii_alphabetic);
let second = lines[i].iter_signed::<i32>();
first.zip(second)
};
// Build two maps, one for vertical segments and one for horizontal.
let mut start = ORIGIN;
let mut distance = 0;
let mut vertical = BTreeMap::new();
let mut horizontal = BTreeMap::new();
for (direction, amount) in steps(0) {
let delta = Point::from(direction);
let end = start + delta * amount;
let line = Line { start, end, distance };
if start.x == end.x {
vertical.insert(start.x, line);
} else {
horizontal.insert(start.y, line);
}
start = end;
distance += amount;
}
// Trace the steps of the second wire, checking for intersections.
let mut start = ORIGIN;
let mut distance = 0;
let mut manhattan = i32::MAX;
let mut delay = i32::MAX;
for (direction, amount) in steps(1) {
let delta = Point::from(direction);
let end = start + delta * amount;
// Use a block to scope the `update` lamdbas mutable borrow of `distance`.
{
// Checks for intersections, ignoring the initial intersection at the origin.
let mut update = |line: &Line, candidate: Point| {
if candidate.manhattan(line.start) < line.end.manhattan(line.start)
&& candidate.signum(line.start) == line.end.signum(line.start)
&& candidate.manhattan(ORIGIN) > 0
{
manhattan = manhattan.min(candidate.manhattan(ORIGIN));
delay = delay.min(
distance
+ candidate.manhattan(start)
+ line.distance
+ candidate.manhattan(line.start),
);
}
};
// BTreeMaps are sorted and can return all key/value pairs in a range.
match direction {
b'U' => {
for (&y, line) in horizontal.range(end.y..=start.y) {
update(line, Point::new(start.x, y));
}
}
b'D' => {
for (&y, line) in horizontal.range(start.y..=end.y) {
update(line, Point::new(start.x, y));
}
}
b'L' => {
for (&x, line) in vertical.range(end.x..=start.x) {
update(line, Point::new(x, start.y));
}
}
b'R' => {
for (&x, line) in vertical.range(start.x..=end.x) {
update(line, Point::new(x, start.y));
}
}
_ => unreachable!(),
}
}
start = end;
distance += amount;
}
(manhattan, delay)
}
pub fn part1(input: &Input) -> i32 {
input.0
}
pub fn part2(input: &Input) -> i32 {
input.1
}