Solve day 6

input/06-test

@@ -0,0 +1,2 @@
+Time:      7  15   30
+Distance:  9  40  200

src/bin/06.rs

@@ -0,0 +1,71 @@
+use aoc2023::{concat, parse_ws_separated};
+use itertools::Itertools;
+use std::ops::RangeInclusive;
+
+const INPUT: &str = include_str!("../../input/06");
+const TEST_INPUT: &str = include_str!("../../input/06-test");
+
+fn main() {
+    assert_eq!(part1(TEST_INPUT), 288);
+    println!("Part 1: {}", part1(INPUT));
+    assert_eq!(part2(TEST_INPUT), 71503);
+    println!("Part 2: {}", part2(INPUT));
+}
+
+fn part1(input: &str) -> u64 {
+    parse(input).map(how_to_beat).map(len).product()
+}
+
+fn part2(input: &str) -> u64 {
+    let race = parse(input).reduce(Race::concat).unwrap();
+    let range = how_to_beat(race);
+    len(range)
+}
+
+/// Range of time the button might be pressed to beat a record.
+fn how_to_beat(race: Race) -> RangeInclusive<u64> {
+    let time = race.time as f64;
+    let record = race.record as f64;
+
+    // The distance travelled d is dependant on the time t the button is pressed: d = time*t - t^2.
+    // Solve d = record for min and max t:
+    let offset = time * 0.5;
+    let add = ((-time * 0.5).powf(2.0) - record).sqrt();
+    let min = offset - add;
+    let max = offset + add;
+
+    // Shrink the interval to only return times to beat the record.
+    let min = min.floor() as u64 + 1;
+    let max = max.ceil() as u64 - 1;
+
+    min..=max
+}
+
+fn len(range: RangeInclusive<u64>) -> u64 {
+    range.try_len().unwrap() as u64
+}
+
+struct Race {
+    time: u64,
+    record: u64,
+}
+
+impl Race {
+    fn concat(self, other: Self) -> Self {
+        Self {
+            time: concat(self.time, other.time),
+            record: concat(self.record, other.record),
+        }
+    }
+}
+
+fn parse(input: &str) -> impl Iterator<Item = Race> + '_ {
+    let mut lines = input.lines();
+    let mut parse_next_line =
+        |prefix: &str| parse_ws_separated(lines.next().unwrap().trim_start_matches(prefix));
+    let times = parse_next_line("Time:      ");
+    let records = parse_next_line("Distance:  ");
+    times
+        .zip(records)
+        .map(|(time, record)| Race { time, record })
+}

src/lib.rs

@@ -1 +1,29 @@
+use std::fmt::{Debug, Display};
+use std::str::FromStr;
 
+/// Parse a whitespace separated list of things.
+///
+/// Panics on parse error.
+pub fn parse_ws_separated<T>(s: &str) -> impl Iterator<Item = T> + '_
+where
+    T: FromStr,
+    <T as FromStr>::Err: Debug,
+{
+    s.split_ascii_whitespace().map(|s| s.parse().unwrap())
+}
+
+/// Concatenate two things.
+///
+/// Panics if the concatenation is not be parseable.
+///
+/// ```rust
+/// # use aoc2023::concat;
+/// assert_eq!(concat(123, 456), 123456);
+/// ```
+pub fn concat<T>(a: T, b: T) -> T
+where
+    T: Display + FromStr,
+    <T as FromStr>::Err: Debug,
+{
+    format!("{a}{b}").parse().unwrap()
+}