Day6

.gitignore

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+target

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 = "anyhow"
+version = "1.0.75"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a4668cab20f66d8d020e1fbc0ebe47217433c1b6c8f2040faf858554e394ace6"
+
+[[package]]
+name = "day6"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+]

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]
+anyhow = "1.0.75"

day6/input

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+Time:        56     97     77     93
+Distance:   499   2210   1097   1440

day6/src/main.rs

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+use std::io::{stdin, BufRead};
+use anyhow::{anyhow, Result};
+
+fn collect_list(prefix: &str, buf: &str) -> Result<Vec<usize>> {
+    Ok(buf.strip_prefix(prefix)
+    .ok_or(anyhow!("Incorrect line: {buf:?}"))?
+    .split_whitespace()
+    .map(|v| v.parse::<usize>())
+    .collect::<Result<_,_>>()?)
+}
+
+fn aggregate(nums: &[usize]) -> usize {
+    nums.iter().fold(0, |acc, d| { acc * (10u64.pow(d.ilog10() + 1) as usize) + d } )
+}
+
+fn ways((t,d) : (usize, usize)) -> usize {
+    if t*t < 4*d { return 0; }
+    let dq = t*t - 4*d;
+
+    let delta = (dq as f64).sqrt();
+    let b1 = ((t as f64 - delta)/2f64).floor() as usize + 1;
+    let b2 = ((t as f64 + delta)/2f64).ceil() as usize - 1;
+
+    if b2 < b1 { return 0; }
+
+    eprintln!("t={t} d={d} b1={b1} b2={b2}");
+
+    b2 - b1 + 1
+}
+
+fn main() -> Result<()> {
+    let mut stdin = stdin().lock();
+
+    let mut buf = String::with_capacity(1024);
+
+    stdin.read_line(&mut buf)?;
+    let times = collect_list("Time:", &buf)?;
+    buf.clear();
+
+    stdin.read_line(&mut buf)?;
+    let distances = collect_list("Distance:", &buf)?;
+    buf.clear();
+
+
+    let ways1: usize = times.iter().copied()
+        .zip(distances.iter().copied())
+        .map(ways)
+        .product();
+
+    println!("Part 1: {ways1} ways.");
+
+    let time2 = aggregate(&times);
+    let dist2 = aggregate(&distances);
+
+    let ways2 = ways((time2, dist2));
+
+    println!("Part 2: {ways2} ways.");
+
+    Ok(())
+}

day6/test

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+Time:      7  15   30
+Distance:  9  40  200