day 6

day6/Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "day6"
+version = "0.1.0"

day6/Cargo.toml

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+[package]
+name = "day6"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

day6/input.txt

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+Time:        49     87     78     95
+Distance:   356   1378   1502   1882

day6/src/main.rs

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+use std::fs::read_to_string;
+use std::cmp;
+
+fn get_numbers_in_line<T: std::str::FromStr>(str: &str) -> Vec<T> {
+    str.split_whitespace().filter_map(|substr| substr.parse::<T>().ok()).collect()
+}
+
+fn merge_numbers_in_line<T: std::str::FromStr>(str: &str) -> T
+    where <T as std::str::FromStr>::Err: std::fmt::Debug {
+    str.split_whitespace()
+        .filter(|substr| substr.parse::<T>().ok().is_some())
+        .collect::<Vec<_>>()
+        .join("")
+        .parse::<T>().unwrap()
+}
+
+fn race_solutions(time: i64, record_distance: i64) -> i64 {
+    // x - time held
+    // distance = x * (time - x)
+    // solving distance > record_distance:
+    // x^2 - x * time + record_distance < 0
+    // delta = time^2 - 4 * record_distance
+    // solution = (time +- sqrt(delta)) / 2
+    let delta: f64 = (time * time - 4 * record_distance) as f64;
+    if delta < 0.0 {
+        return 0;
+    }
+    let delta_sqrt = f64::sqrt(delta);
+    let min_solution = f64::ceil((time as f64 - delta_sqrt) / 2.0) as i64;
+    let max_solution = f64::floor((time as f64 + delta_sqrt) / 2.0) as i64;
+    cmp::max(max_solution - min_solution + 1, 0 as i64)
+}
+
+fn main() {
+    let lines_str = read_to_string("input.txt").unwrap();
+    let lines = lines_str.lines().collect::<Vec<_>>();
+    let times = get_numbers_in_line::<i64>(lines[0]);
+    let distances = get_numbers_in_line::<i64>(lines[1]);
+    let time_fixed = merge_numbers_in_line::<i64>(lines[0]);
+    let distance_fixed = merge_numbers_in_line::<i64>(lines[1]);
+    let solutions1 = std::iter::zip(times, distances).fold(
+        1, |p, (t, d)| p * race_solutions(t, d));
+    let solutions2 = race_solutions(time_fixed, distance_fixed);
+    println!("Solutions 1: {}", solutions1);
+    println!("Solutions 2: {}", solutions2);
+}