uu_head/

head.rs

// This file is part of the uutils coreutils package.
//
// For the full copyright and license information, please view the LICENSE
// file that was distributed with this source code.

// spell-checker:ignore (vars) seekable

use clap::{crate_version, Arg, ArgAction, ArgMatches, Command};
use std::ffi::OsString;
#[cfg(unix)]
use std::fs::File;
use std::io::{self, BufWriter, Read, Seek, SeekFrom, Write};
use std::num::TryFromIntError;
#[cfg(unix)]
use std::os::fd::{AsRawFd, FromRawFd};
use thiserror::Error;
use uucore::display::Quotable;
use uucore::error::{FromIo, UError, UResult};
use uucore::line_ending::LineEnding;
use uucore::lines::lines;
use uucore::{format_usage, help_about, help_usage, show};

const BUF_SIZE: usize = 65536;

const ABOUT: &str = help_about!("head.md");
const USAGE: &str = help_usage!("head.md");

mod options {
    pub const BYTES_NAME: &str = "BYTES";
    pub const LINES_NAME: &str = "LINES";
    pub const QUIET_NAME: &str = "QUIET";
    pub const VERBOSE_NAME: &str = "VERBOSE";
    pub const ZERO_NAME: &str = "ZERO";
    pub const FILES_NAME: &str = "FILE";
    pub const PRESUME_INPUT_PIPE: &str = "-PRESUME-INPUT-PIPE";
}

mod parse;
mod take;
use take::take_all_but;
use take::take_lines;

#[derive(Error, Debug)]
enum HeadError {
    /// Wrapper around `io::Error`
    #[error("error reading {name}: {err}")]
    Io { name: String, err: io::Error },

    #[error("parse error: {0}")]
    ParseError(String),

    #[error("bad argument encoding")]
    BadEncoding,

    #[error("{0}: number of -bytes or -lines is too large")]
    NumTooLarge(#[from] TryFromIntError),

    #[error("clap error: {0}")]
    Clap(#[from] clap::Error),

    #[error("{0}")]
    MatchOption(String),
}

impl UError for HeadError {
    fn code(&self) -> i32 {
        1
    }
}

type HeadResult<T> = Result<T, HeadError>;

pub fn uu_app() -> Command {
    Command::new(uucore::util_name())
        .version(crate_version!())
        .about(ABOUT)
        .override_usage(format_usage(USAGE))
        .infer_long_args(true)
        .arg(
            Arg::new(options::BYTES_NAME)
                .short('c')
                .long("bytes")
                .value_name("[-]NUM")
                .help(
                    "\
                     print the first NUM bytes of each file;\n\
                     with the leading '-', print all but the last\n\
                     NUM bytes of each file\
                     ",
                )
                .overrides_with_all([options::BYTES_NAME, options::LINES_NAME])
                .allow_hyphen_values(true),
        )
        .arg(
            Arg::new(options::LINES_NAME)
                .short('n')
                .long("lines")
                .value_name("[-]NUM")
                .help(
                    "\
                     print the first NUM lines instead of the first 10;\n\
                     with the leading '-', print all but the last\n\
                     NUM lines of each file\
                     ",
                )
                .overrides_with_all([options::LINES_NAME, options::BYTES_NAME])
                .allow_hyphen_values(true),
        )
        .arg(
            Arg::new(options::QUIET_NAME)
                .short('q')
                .long("quiet")
                .visible_alias("silent")
                .help("never print headers giving file names")
                .overrides_with_all([options::VERBOSE_NAME, options::QUIET_NAME])
                .action(ArgAction::SetTrue),
        )
        .arg(
            Arg::new(options::VERBOSE_NAME)
                .short('v')
                .long("verbose")
                .help("always print headers giving file names")
                .overrides_with_all([options::QUIET_NAME, options::VERBOSE_NAME])
                .action(ArgAction::SetTrue),
        )
        .arg(
            Arg::new(options::PRESUME_INPUT_PIPE)
                .long("presume-input-pipe")
                .alias("-presume-input-pipe")
                .hide(true)
                .action(ArgAction::SetTrue),
        )
        .arg(
            Arg::new(options::ZERO_NAME)
                .short('z')
                .long("zero-terminated")
                .help("line delimiter is NUL, not newline")
                .overrides_with(options::ZERO_NAME)
                .action(ArgAction::SetTrue),
        )
        .arg(
            Arg::new(options::FILES_NAME)
                .action(ArgAction::Append)
                .value_hint(clap::ValueHint::FilePath),
        )
}

#[derive(Debug, PartialEq)]
enum Mode {
    FirstLines(u64),
    AllButLastLines(u64),
    FirstBytes(u64),
    AllButLastBytes(u64),
}

impl Default for Mode {
    fn default() -> Self {
        Self::FirstLines(10)
    }
}

impl Mode {
    fn from(matches: &ArgMatches) -> Result<Self, String> {
        if let Some(v) = matches.get_one::<String>(options::BYTES_NAME) {
            let (n, all_but_last) =
                parse::parse_num(v).map_err(|err| format!("invalid number of bytes: {err}"))?;
            if all_but_last {
                Ok(Self::AllButLastBytes(n))
            } else {
                Ok(Self::FirstBytes(n))
            }
        } else if let Some(v) = matches.get_one::<String>(options::LINES_NAME) {
            let (n, all_but_last) =
                parse::parse_num(v).map_err(|err| format!("invalid number of lines: {err}"))?;
            if all_but_last {
                Ok(Self::AllButLastLines(n))
            } else {
                Ok(Self::FirstLines(n))
            }
        } else {
            Ok(Self::default())
        }
    }
}

fn arg_iterate<'a>(
    mut args: impl uucore::Args + 'a,
) -> HeadResult<Box<dyn Iterator<Item = OsString> + 'a>> {
    // argv[0] is always present
    let first = args.next().unwrap();
    if let Some(second) = args.next() {
        if let Some(s) = second.to_str() {
            match parse::parse_obsolete(s) {
                Some(Ok(iter)) => Ok(Box::new(vec![first].into_iter().chain(iter).chain(args))),
                Some(Err(e)) => match e {
                    parse::ParseError::Syntax => Err(HeadError::ParseError(format!(
                        "bad argument format: {}",
                        s.quote()
                    ))),
                    parse::ParseError::Overflow => Err(HeadError::ParseError(format!(
                        "invalid argument: {} Value too large for defined datatype",
                        s.quote()
                    ))),
                },
                None => Ok(Box::new(vec![first, second].into_iter().chain(args))),
            }
        } else {
            Err(HeadError::BadEncoding)
        }
    } else {
        Ok(Box::new(vec![first].into_iter()))
    }
}

#[derive(Debug, PartialEq, Default)]
struct HeadOptions {
    pub quiet: bool,
    pub verbose: bool,
    pub line_ending: LineEnding,
    pub presume_input_pipe: bool,
    pub mode: Mode,
    pub files: Vec<String>,
}

impl HeadOptions {
    ///Construct options from matches
    pub fn get_from(matches: &clap::ArgMatches) -> Result<Self, String> {
        let mut options = Self::default();

        options.quiet = matches.get_flag(options::QUIET_NAME);
        options.verbose = matches.get_flag(options::VERBOSE_NAME);
        options.line_ending = LineEnding::from_zero_flag(matches.get_flag(options::ZERO_NAME));
        options.presume_input_pipe = matches.get_flag(options::PRESUME_INPUT_PIPE);

        options.mode = Mode::from(matches)?;

        options.files = match matches.get_many::<String>(options::FILES_NAME) {
            Some(v) => v.cloned().collect(),
            None => vec!["-".to_owned()],
        };

        Ok(options)
    }
}

#[inline]
fn wrap_in_stdout_error(err: io::Error) -> io::Error {
    io::Error::new(
        err.kind(),
        format!("error writing 'standard output': {}", err),
    )
}

fn read_n_bytes(input: impl Read, n: u64) -> std::io::Result<u64> {
    // Read the first `n` bytes from the `input` reader.
    let mut reader = input.take(n);

    // Write those bytes to `stdout`.
    let stdout = std::io::stdout();
    let mut stdout = stdout.lock();

    let bytes_written = io::copy(&mut reader, &mut stdout).map_err(wrap_in_stdout_error)?;

    // Make sure we finish writing everything to the target before
    // exiting. Otherwise, when Rust is implicitly flushing, any
    // error will be silently ignored.
    stdout.flush().map_err(wrap_in_stdout_error)?;

    Ok(bytes_written)
}

fn read_n_lines(input: &mut impl std::io::BufRead, n: u64, separator: u8) -> std::io::Result<u64> {
    // Read the first `n` lines from the `input` reader.
    let mut reader = take_lines(input, n, separator);

    // Write those bytes to `stdout`.
    let mut stdout = std::io::stdout();

    let bytes_written = io::copy(&mut reader, &mut stdout).map_err(wrap_in_stdout_error)?;

    // Make sure we finish writing everything to the target before
    // exiting. Otherwise, when Rust is implicitly flushing, any
    // error will be silently ignored.
    stdout.flush().map_err(wrap_in_stdout_error)?;

    Ok(bytes_written)
}

fn catch_too_large_numbers_in_backwards_bytes_or_lines(n: u64) -> Option<usize> {
    match usize::try_from(n) {
        Ok(value) => Some(value),
        Err(e) => {
            show!(HeadError::NumTooLarge(e));
            None
        }
    }
}

fn read_but_last_n_bytes(input: impl std::io::BufRead, n: u64) -> std::io::Result<u64> {
    let mut bytes_written = 0;
    if let Some(n) = catch_too_large_numbers_in_backwards_bytes_or_lines(n) {
        let stdout = std::io::stdout();
        let stdout = stdout.lock();
        // Even though stdout is buffered, it will flush on each newline in the
        // input stream. This can be costly, so add an extra layer of buffering
        // over the top. This gives a significant speedup (approx 4x).
        let mut writer = BufWriter::with_capacity(BUF_SIZE, stdout);
        for byte in take_all_but(input.bytes(), n) {
            writer.write_all(&[byte?]).map_err(wrap_in_stdout_error)?;
            bytes_written += 1;
        }
        // Make sure we finish writing everything to the target before
        // exiting. Otherwise, when Rust is implicitly flushing, any
        // error will be silently ignored.
        writer.flush().map_err(wrap_in_stdout_error)?;
    }
    Ok(bytes_written)
}

fn read_but_last_n_lines(
    input: impl std::io::BufRead,
    n: u64,
    separator: u8,
) -> std::io::Result<u64> {
    let mut bytes_written: u64 = 0;
    if let Some(n) = catch_too_large_numbers_in_backwards_bytes_or_lines(n) {
        let stdout = std::io::stdout();
        let mut stdout = stdout.lock();

        for bytes in take_all_but(lines(input, separator), n) {
            let bytes = bytes?;
            bytes_written += u64::try_from(bytes.len()).unwrap();

            stdout.write_all(&bytes).map_err(wrap_in_stdout_error)?;
        }
        // Make sure we finish writing everything to the target before
        // exiting. Otherwise, when Rust is implicitly flushing, any
        // error will be silently ignored.
        stdout.flush().map_err(wrap_in_stdout_error)?;
    }
    Ok(bytes_written)
}

/// Return the index in `input` just after the `n`th line from the end.
///
/// If `n` exceeds the number of lines in this file, then return 0.
/// This function rewinds the cursor to the
/// beginning of the input just before returning unless there is an
/// I/O error.
///
/// # Errors
///
/// This function returns an error if there is a problem seeking
/// through or reading the input.
///
/// # Examples
///
/// The function returns the index of the byte immediately following
/// the line ending character of the `n`th line from the end of the
/// input:
///
/// ```rust,ignore
/// let mut input = Cursor::new("x\ny\nz\n");
/// assert_eq!(find_nth_line_from_end(&mut input, 0, false).unwrap(), 6);
/// assert_eq!(find_nth_line_from_end(&mut input, 1, false).unwrap(), 4);
/// assert_eq!(find_nth_line_from_end(&mut input, 2, false).unwrap(), 2);
/// ```
///
/// If `n` exceeds the number of lines in the file, always return 0:
///
/// ```rust,ignore
/// let mut input = Cursor::new("x\ny\nz\n");
/// assert_eq!(find_nth_line_from_end(&mut input, 3, false).unwrap(), 0);
/// assert_eq!(find_nth_line_from_end(&mut input, 4, false).unwrap(), 0);
/// assert_eq!(find_nth_line_from_end(&mut input, 1000, false).unwrap(), 0);
/// ```
fn find_nth_line_from_end<R>(input: &mut R, n: u64, separator: u8) -> std::io::Result<u64>
where
    R: Read + Seek,
{
    let file_size = input.seek(SeekFrom::End(0))?;

    let mut buffer = [0u8; BUF_SIZE];

    let mut i = 0u64;
    let mut lines = 0u64;

    loop {
        // the casts here are ok, `buffer.len()` should never be above a few k
        let bytes_remaining_to_search = file_size - i;
        let bytes_to_read_this_loop = bytes_remaining_to_search.min(BUF_SIZE.try_into().unwrap());
        let read_start_offset = bytes_remaining_to_search - bytes_to_read_this_loop;
        let buffer = &mut buffer[..bytes_to_read_this_loop.try_into().unwrap()];

        input.seek(SeekFrom::Start(read_start_offset))?;
        input.read_exact(buffer)?;
        for byte in buffer.iter().rev() {
            if byte == &separator {
                lines += 1;
            }
            // if it were just `n`,
            if lines == n + 1 {
                input.rewind()?;
                return Ok(file_size - i);
            }
            i += 1;
        }
        if file_size - i == 0 {
            input.rewind()?;
            return Ok(0);
        }
    }
}

fn is_seekable(input: &mut std::fs::File) -> bool {
    let current_pos = input.stream_position();
    current_pos.is_ok()
        && input.seek(SeekFrom::End(0)).is_ok()
        && input.seek(SeekFrom::Start(current_pos.unwrap())).is_ok()
}

fn head_backwards_file(input: &mut std::fs::File, options: &HeadOptions) -> std::io::Result<u64> {
    let st = input.metadata()?;
    let seekable = is_seekable(input);
    let blksize_limit = uucore::fs::sane_blksize::sane_blksize_from_metadata(&st);
    if !seekable || st.len() <= blksize_limit {
        head_backwards_without_seek_file(input, options)
    } else {
        head_backwards_on_seekable_file(input, options)
    }
}

fn head_backwards_without_seek_file(
    input: &mut std::fs::File,
    options: &HeadOptions,
) -> std::io::Result<u64> {
    let reader = std::io::BufReader::with_capacity(BUF_SIZE, &*input);
    match options.mode {
        Mode::AllButLastBytes(n) => read_but_last_n_bytes(reader, n),
        Mode::AllButLastLines(n) => read_but_last_n_lines(reader, n, options.line_ending.into()),
        _ => unreachable!(),
    }
}

fn head_backwards_on_seekable_file(
    input: &mut std::fs::File,
    options: &HeadOptions,
) -> std::io::Result<u64> {
    match options.mode {
        Mode::AllButLastBytes(n) => {
            let size = input.metadata()?.len();
            if n >= size {
                Ok(0)
            } else {
                read_n_bytes(
                    &mut std::io::BufReader::with_capacity(BUF_SIZE, input),
                    size - n,
                )
            }
        }
        Mode::AllButLastLines(n) => {
            let found = find_nth_line_from_end(input, n, options.line_ending.into())?;
            read_n_bytes(
                &mut std::io::BufReader::with_capacity(BUF_SIZE, input),
                found,
            )
        }
        _ => unreachable!(),
    }
}

fn head_file(input: &mut std::fs::File, options: &HeadOptions) -> std::io::Result<u64> {
    match options.mode {
        Mode::FirstBytes(n) => {
            read_n_bytes(&mut std::io::BufReader::with_capacity(BUF_SIZE, input), n)
        }
        Mode::FirstLines(n) => read_n_lines(
            &mut std::io::BufReader::with_capacity(BUF_SIZE, input),
            n,
            options.line_ending.into(),
        ),
        Mode::AllButLastBytes(_) | Mode::AllButLastLines(_) => head_backwards_file(input, options),
    }
}

#[allow(clippy::cognitive_complexity)]
fn uu_head(options: &HeadOptions) -> UResult<()> {
    let mut first = true;
    for file in &options.files {
        let res = match (file.as_str(), options.presume_input_pipe) {
            (_, true) | ("-", false) => {
                if (options.files.len() > 1 && !options.quiet) || options.verbose {
                    if !first {
                        println!();
                    }
                    println!("==> standard input <==");
                }
                let stdin = std::io::stdin();

                #[cfg(unix)]
                {
                    let stdin_raw_fd = stdin.as_raw_fd();
                    let mut stdin_file = unsafe { File::from_raw_fd(stdin_raw_fd) };
                    let current_pos = stdin_file.stream_position();
                    if let Ok(current_pos) = current_pos {
                        // We have a seekable file. Ensure we set the input stream to the
                        // last byte read so that any tools that parse the remainder of
                        // the stdin stream read from the correct place.

                        let bytes_read = head_file(&mut stdin_file, options)?;
                        stdin_file.seek(SeekFrom::Start(current_pos + bytes_read))?;
                    } else {
                        let _bytes_read = head_file(&mut stdin_file, options)?;
                    }
                }

                #[cfg(not(unix))]
                {
                    let mut stdin = stdin.lock();

                    match options.mode {
                        Mode::FirstBytes(n) => read_n_bytes(&mut stdin, n),
                        Mode::AllButLastBytes(n) => read_but_last_n_bytes(&mut stdin, n),
                        Mode::FirstLines(n) => {
                            read_n_lines(&mut stdin, n, options.line_ending.into())
                        }
                        Mode::AllButLastLines(n) => {
                            read_but_last_n_lines(&mut stdin, n, options.line_ending.into())
                        }
                    }?;
                }

                Ok(())
            }
            (name, false) => {
                let mut file = match std::fs::File::open(name) {
                    Ok(f) => f,
                    Err(err) => {
                        show!(err.map_err_context(|| format!(
                            "cannot open {} for reading",
                            name.quote()
                        )));
                        continue;
                    }
                };
                if (options.files.len() > 1 && !options.quiet) || options.verbose {
                    if !first {
                        println!();
                    }
                    println!("==> {name} <==");
                }
                head_file(&mut file, options)?;
                Ok(())
            }
        };
        if let Err(e) = res {
            let name = if file.as_str() == "-" {
                "standard input"
            } else {
                file
            };
            return Err(HeadError::Io {
                name: name.to_string(),
                err: e,
            }
            .into());
        }
        first = false;
    }
    // Even though this is returning `Ok`, it is possible that a call
    // to `show!()` and thus a call to `set_exit_code()` has been
    // called above. If that happens, then this process will exit with
    // a non-zero exit code.
    Ok(())
}

#[uucore::main]
pub fn uumain(args: impl uucore::Args) -> UResult<()> {
    let matches = uu_app().try_get_matches_from(arg_iterate(args)?)?;
    let args = match HeadOptions::get_from(&matches) {
        Ok(o) => o,
        Err(s) => {
            return Err(HeadError::MatchOption(s).into());
        }
    };
    uu_head(&args)
}

#[cfg(test)]
mod tests {
    use std::ffi::OsString;
    use std::io::Cursor;

    use super::*;

    fn options(args: &str) -> Result<HeadOptions, String> {
        let combined = "head ".to_owned() + args;
        let args = combined.split_whitespace().map(OsString::from);
        let matches = uu_app()
            .get_matches_from(arg_iterate(args).map_err(|_| String::from("Arg iterate failed"))?);
        HeadOptions::get_from(&matches)
    }

    #[test]
    fn test_args_modes() {
        let args = options("-n -10M -vz").unwrap();
        assert_eq!(args.line_ending, LineEnding::Nul);
        assert!(args.verbose);
        assert_eq!(args.mode, Mode::AllButLastLines(10 * 1024 * 1024));
    }

    #[test]
    fn test_gnu_compatibility() {
        let args = options("-n 1 -c 1 -n 5 -c kiB -vqvqv").unwrap(); // spell-checker:disable-line
        assert!(args.mode == Mode::FirstBytes(1024));
        assert!(args.verbose);
        assert_eq!(options("-5").unwrap().mode, Mode::FirstLines(5));
        assert_eq!(options("-2b").unwrap().mode, Mode::FirstBytes(1024));
        assert_eq!(options("-5 -c 1").unwrap().mode, Mode::FirstBytes(1));
    }

    #[test]
    #[allow(clippy::cognitive_complexity)]
    fn all_args_test() {
        assert!(options("--silent").unwrap().quiet);
        assert!(options("--quiet").unwrap().quiet);
        assert!(options("-q").unwrap().quiet);
        assert!(options("--verbose").unwrap().verbose);
        assert!(options("-v").unwrap().verbose);
        assert_eq!(
            options("--zero-terminated").unwrap().line_ending,
            LineEnding::Nul
        );
        assert_eq!(options("-z").unwrap().line_ending, LineEnding::Nul);
        assert_eq!(options("--lines 15").unwrap().mode, Mode::FirstLines(15));
        assert_eq!(options("-n 15").unwrap().mode, Mode::FirstLines(15));
        assert_eq!(options("--bytes 15").unwrap().mode, Mode::FirstBytes(15));
        assert_eq!(options("-c 15").unwrap().mode, Mode::FirstBytes(15));
    }

    #[test]
    fn test_options_errors() {
        assert!(options("-n IsThisTheRealLife?").is_err());
        assert!(options("-c IsThisJustFantasy").is_err());
    }

    #[test]
    fn test_options_correct_defaults() {
        let opts = HeadOptions::default();

        assert!(!opts.verbose);
        assert!(!opts.quiet);
        assert_eq!(opts.line_ending, LineEnding::Newline);
        assert_eq!(opts.mode, Mode::FirstLines(10));
        assert!(opts.files.is_empty());
    }

    fn arg_outputs(src: &str) -> Result<String, ()> {
        let split = src.split_whitespace().map(OsString::from);
        match arg_iterate(split) {
            Ok(args) => {
                let vec = args
                    .map(|s| s.to_str().unwrap().to_owned())
                    .collect::<Vec<_>>();
                Ok(vec.join(" "))
            }
            Err(_) => Err(()),
        }
    }

    #[test]
    fn test_arg_iterate() {
        // test that normal args remain unchanged
        assert_eq!(
            arg_outputs("head -n -5 -zv"),
            Ok("head -n -5 -zv".to_owned())
        );
        // tests that nonsensical args are unchanged
        assert_eq!(
            arg_outputs("head -to_be_or_not_to_be,..."),
            Ok("head -to_be_or_not_to_be,...".to_owned())
        );
        //test that the obsolete syntax is unrolled
        assert_eq!(
            arg_outputs("head -123qvqvqzc"), // spell-checker:disable-line
            Ok("head -q -z -c 123".to_owned())
        );
        //test that bad obsoletes are an error
        assert!(arg_outputs("head -123FooBar").is_err());
        //test overflow
        assert!(arg_outputs("head -100000000000000000000000000000000000000000").is_err());
        //test that empty args remain unchanged
        assert_eq!(arg_outputs("head"), Ok("head".to_owned()));
    }

    #[test]
    #[cfg(target_os = "linux")]
    fn test_arg_iterate_bad_encoding() {
        use std::os::unix::ffi::OsStringExt;
        let invalid = OsString::from_vec(vec![b'\x80', b'\x81']);
        // this arises from a conversion from OsString to &str
        assert!(arg_iterate(vec![OsString::from("head"), invalid].into_iter()).is_err());
    }

    #[test]
    fn read_early_exit() {
        let mut empty = std::io::BufReader::new(std::io::Cursor::new(Vec::new()));
        assert!(read_n_bytes(&mut empty, 0).is_ok());
        assert!(read_n_lines(&mut empty, 0, b'\n').is_ok());
    }

    #[test]
    fn test_find_nth_line_from_end() {
        // Make sure our input buffer is several multiples of BUF_SIZE in size
        // such that we can be reasonably confident we've exercised all logic paths.
        // Make the contents of the buffer look like...
        // aaaa\n
        // aaaa\n
        // aaaa\n
        // aaaa\n
        // aaaa\n
        // ...
        // This will make it easier to validate the results since each line will have
        // 5 bytes in it.

        let minimum_buffer_size = BUF_SIZE * 4;
        let mut input_buffer = vec![];
        let mut loop_iteration: u64 = 0;
        while input_buffer.len() < minimum_buffer_size {
            for _n in 0..4 {
                input_buffer.push(b'a');
            }
            loop_iteration += 1;
            input_buffer.push(b'\n');
        }

        let lines_in_input_file = loop_iteration;
        let input_length = lines_in_input_file * 5;
        assert_eq!(input_length, input_buffer.len().try_into().unwrap());
        let mut input = Cursor::new(input_buffer);
        // We now have loop_iteration lines in the buffer Now walk backwards through the buffer
        // to confirm everything parses correctly.
        // Use a large step size to prevent the test from taking too long, but don't use a power
        // of 2 in case we miss some corner case.
        let step_size = 511;
        for n in (0..lines_in_input_file).filter(|v| v % step_size == 0) {
            // The 5*n comes from 5-bytes per row.
            assert_eq!(
                find_nth_line_from_end(&mut input, n, b'\n').unwrap(),
                input_length - 5 * n
            );
        }

        // Now confirm that if we query with a value >= lines_in_input_file we get an offset
        // of 0
        assert_eq!(
            find_nth_line_from_end(&mut input, lines_in_input_file, b'\n').unwrap(),
            0
        );
        assert_eq!(
            find_nth_line_from_end(&mut input, lines_in_input_file + 1, b'\n').unwrap(),
            0
        );
        assert_eq!(
            find_nth_line_from_end(&mut input, lines_in_input_file + 1000, b'\n').unwrap(),
            0
        );
    }
}