Snippets

Published: 2023-09-02
Updated: 2024-12-28

Library Candidates 🔗

Read number from stdin 🔗

Source: https://doc.rust-lang.org/book/ch02-00-guessing-game-tutorial.html#handling-invalid-input

use std::io;

fn main() {
    println!("Please input a number");

    let mut user_input = String::new();

    io::stdin()
        .read_line(&mut user_input)
        .expect("Failed to read line");

    if let Ok(num) = user_input.trim().parse::<u32>() {
        println!("The number entered is {num}");
    }
}

Read file (line by line) 🔗

Source: https://doc.rust-lang.org/rust-by-example/std_misc/file/read_lines.html

use std::error::Error;
use std::fs::File;
use std::io::{self, BufRead};
use std::path::Path;

fn main() -> Result<(), Box<dyn Error>> {
    for line in read_lines("filename")? {
        let line = line?;
        println!("{line}");
    }
    Ok(())
}

// The output is wrapped in a Result to allow matching on errors
// Returns an Iterator to the Reader of the lines of the file.
fn read_lines<P: AsRef<Path>>(path: P) -> io::Result<io::Lines<io::BufReader<File>>>
{
    let file = File::open(path)?;
    Ok(io::BufReader::new(file).lines())
}

Read file to string 🔗

fn main() -> Result<(), Box<dyn Error>> {
    let _ = fs::read_to_string("filename")?;
    Ok(())
}

Write string to a file 🔗

use std::{error::Error, io::Write, path::Path};

fn main() -> Result<(), Box<dyn Error>> {
    write_to_path("filename.txt", "text for file")?;
    Ok(())
}

fn write_to_path<P: AsRef<Path>>(path: P, s: &str) -> Result<(), Box<dyn Error>> {
    let mut file = std::fs::OpenOptions::new()
            .write(true)
            .create(true)
            .truncate(true)
            .open(path)?;
    file.write_all(s.as_bytes())?;
    Ok(())
}

OR with a trait (may not be great for a library as is because it’s very opinionated with regard to overwrite)

use anyhow::Context as _;

pub trait SaveToFile {
    /// Writes the file replacing if it already exists
    fn write_to_path<P: AsRef<Path>>(&self, path: P) -> anyhow::Result<()>;
}

impl<T: AsRef<str>> SaveToFile for T {
    fn write_to_path<P: AsRef<Path>>(&self, path: P) -> anyhow::Result<()> {
        let mut file = std::fs::OpenOptions::new()
            .truncate(true)
            .write(true)
            .create(true)
            .open(&path)
            .with_context(|| {
                format!(
                    "failed to open file for writing. Filename: {:?}",
                    path.as_ref()
                )
            })?;
        file.write_all(self.as_ref().as_bytes())
            .with_context(|| format!("failed to write contents to: {:?}", path.as_ref()))?;
        Ok(())
    }
}

Print without new line (and flush) 🔗

Note: flush is required as print!() does not automatically flush like println!() does

print!("This may not print when we want unless we flush");
io::stdout().flush()?;

Add an extension to a path 🔗

Source: https://users.rust-lang.org/t/append-an-additional-extension/23586/13

pub trait PathExtension {
    /// Returns the result of set_extension with the appended extension
    fn add_extension<P: AsRef<Path>>(&mut self, new_extension: P) -> bool;
}

impl PathExtension for PathBuf {
    fn add_extension<P: AsRef<Path>>(&mut self, extension: P) -> bool {
        match self.extension() {
            Some(ext) => {
                let mut ext = ext.to_os_string();
                ext.push(".");
                ext.push(extension.as_ref());
                self.set_extension(ext)
            }
            None => self.set_extension(extension.as_ref()),
        }
    }
}

Add commas to number for display 🔗

NB: To the best of my recollection here is also a crate that provides a more robust implementation

fn as_string_with_separators(value: usize) -> String {
    value
        .to_string()
        .as_bytes()
        .rchunks(3)
        .rev()
        .map(std::str::from_utf8)
        .collect::<Result<Vec<&str>, _>>()
        .unwrap()
        .join(",")
}

Replace escaped special characters with the actual special characters 🔗

This one feels like there should be a better way but I haven’t needed it so not sure but the aho_corasick crate could do multiple find and replace simultaneously instead of sequentially if I recall correctly.

fn clean_msg<S: AsRef<str>>(msg: S) -> String {
    msg.as_ref().replace(r"\n", "\n").replace(r#"\""#, "\"")
}

Tests 🔗

Unit tests 🔗

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn it_works() {
        let result = 2 + 2;
        assert_eq!(result, 4);
    }
}

Panic expectation 🔗

#[should_panic(expected = "message seems to need to be on first line of text")]
fn it_should_panic() {}

Run CLI App to test it 🔗

NOTE: Only works from integration tests

# Cargo.toml
[dev-dependencies]
assert_cmd = "2.0.7"
predicates = "2.1.5"

// main.rs
use std::io::BufRead;

fn main() {
    for line in std::io::stdin().lock().lines() {
        println!("{}", line.expect("Unable to read input line"));
    }
}

// tests/integration_test.rs
use assert_cmd::prelude::*; // Add methods on commands
use predicates::prelude::*; // Used for writing assertions
use std::{
    fs::{self, File},
    process::Command,
};

#[test]
fn run_executable() -> Result<(), Box<dyn std::error::Error>> {
    // You may want to include the calls to trim but depending on your use case you might not.
    // They are optional but included as they are easier to remove than figure out where to add
    // Note that input it not trimmed, only expected_output and output

    let input_file = File::open("input.txt")?;
    let expected_output_filename = "expected_output.txt";
    let expected_output = fs::read_to_string(expected_output_filename)
        .map_err(|e| format!("Failed to load output: {expected_output_filename} Error:{e}"))?
        .trim() // Trim expected output
        .to_owned();

    Command::cargo_bin("myapp")?
        .stdin(input_file)
        .assert()
        .stdout(
            predicate::str::diff(expected_output).trim(), // Trims actual output
        );
    Ok(())
}

Get input arguments to a program 🔗

Source: https://doc.rust-lang.org/book/ch12-01-accepting-command-line-arguments.html#saving-the-argument-values-in-variables

use std::env;

fn main() {
    let args: Vec<String> = env::args().collect();

    let query = &args[1];
    let file_path = &args[2];

    println!("Searching for {}", query);
    println!("In file {}", file_path);
}

Loop nesting and labels 🔗

Source: https://doc.rust-lang.org/rust-by-example/flow_control/loop/nested.html

fn main() {
    'outer: loop {
        println!("Entered the outer loop");

        'inner: loop {
            println!("Entered the inner loop");

            // This would break only the inner loop
            //break;

            // This breaks the outer loop
            break 'outer;
        }

        println!("This point will never be reached");
    }

    println!("Exited the outer loop");
}

Float Eq and Ord Wrapper 🔗

This may be better served by using the crate float_ord. See useful crate

#[derive(Debug, Clone, Copy, PartialEq)]
/// Wraps a float and implements Ord but panics if value is NaN
struct FloatNonNAN(f64);
impl FloatNonNAN {
    /// Creates a new [Self] with constraint that value cannot be NAN or function panics
    pub fn new(value: f64) -> Self {
        assert!(
            !value.is_nan(),
            "NAN is not allowed as it is not equal to itself and thus invalid for an impl of Ord"
        );
        Self(value)
    }
}
impl Eq for FloatNonNAN {}
impl Ord for FloatNonNAN {
    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
        self.0
            .partial_cmp(&other.0)
            .expect("This should work unless one of them is NAN")
    }
}
impl PartialOrd for FloatNonNAN {
    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
        self.0.partial_cmp(&other.0)
    }
}
impl AsRef<f64> for FloatNonNAN {
    fn as_ref(&self) -> &f64 {
        &self.0
    }
}
impl PartialEq<f64> for FloatNonNAN {
    fn eq(&self, other: &f64) -> bool {
        &self.0 == other
    }
}
impl From<FloatNonNAN> for f64 {
    fn from(value: FloatNonNAN) -> Self {
        value.0
    }
}
impl std::ops::Mul for FloatNonNAN {
    type Output = Self;

    fn mul(self, rhs: Self) -> Self::Output {
        FloatNonNAN::new(self.0 * rhs.0)
    }
}

Collect into a result 🔗

One fail means the first error is returned (Remember this from the rust book but couldn’t find it). Mostly needed when I’m using iterators and need to do a map step that may fail. Usual issue is not remembering to wrap the good option in Ok.

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let numbers: Vec<u8> = (0..10).step_by(2).collect();
    let all_even: Vec<u8> = numbers
        .iter()
        .cloned()
        .map(|x| {
            if x % 2 == 0 {
                Ok(x)
            } else {
                Err("Oops we founds a odd number")
            }
        })
        .collect::<Result<Vec<u8>, _>>()?;
    println!("{all_even:?}");
    Ok(())
}

Get number of seconds until midnight 🔗

Source: https://stackoverflow.com/questions/47708305/calculate-the-duration-between-now-and-the-next-midnight-using-chrono

Requires the chrono crate.

fn main() {
    let duration = seconds_to(1, 0, 0, 0).unwrap();

    println!("Duration until midnight {duration:?}");
}

fn seconds_to(days: i64, hour: u32, min: u32, sec: u32) -> Option<std::time::Duration> {
    let now = chrono::Local::now();

    let new_date_time = (now + chrono::Duration::days(days))
        .date_naive()
        .and_hms_opt(hour, min, sec)?;

    match new_date_time
        .signed_duration_since(now.naive_local())
        .to_std()
    {
        Ok(result) => Some(result),
        Err(e) => {
            dbg!(e); // Might want to log this error or return a result as suitable to your application
            None
        }
    }
}