utils.rs

//! Some useful types.

use super::write::Write;
use crate::Error;

/// A `Cursor` contains a slice of a buffer.
#[derive(Debug, Clone)]
pub(crate) struct Cursor<'a> {
    /// Slice representing the remaining data to be read.
    remaining: &'a [u8],
}

impl<'a> Cursor<'a> {
    /// Construct a new `Cursor` from remaining data.
    pub(crate) fn new(remaining: &'a [u8]) -> Self {
        Self { remaining }
    }

    /// Returns remaining data.
    pub(crate) fn remaining(&self) -> &'a [u8] {
        self.remaining
    }

    /// Returns the next byte.
    pub(crate) fn next(&mut self) -> Option<u8> {
        let (&first, tail) = self.remaining.split_first()?;
        self.remaining = tail;
        Some(first)
    }

    /// Read bytes if the remaining data is prefixed by the provided tag.
    pub(crate) fn read_optional_tag(&mut self, tag: &[u8]) -> bool {
        if self.remaining.starts_with(tag) {
            self.read_exact(tag.len());
            true
        } else {
            false
        }
    }

    /// Read bytes as long as the provided predicate is true.
    pub(crate) fn read_while<F: Fn(&u8) -> bool>(&mut self, f: F) -> &'a [u8] {
        match self.remaining.iter().position(|x| !f(x)) {
            None => self.read_exact(self.remaining.len()),
            Some(position) => self.read_exact(position),
        }
    }

    /// Read bytes until the provided predicate is true.
    pub(crate) fn read_until<F: Fn(&u8) -> bool>(&mut self, f: F) -> &'a [u8] {
        match self.remaining.iter().position(f) {
            None => self.read_exact(self.remaining.len()),
            Some(position) => self.read_exact(position),
        }
    }

    /// Read exactly `count` bytes.
    fn read_exact(&mut self, count: usize) -> &'a [u8] {
        let (result, remaining) = self.remaining.split_at(count);
        self.remaining = remaining;
        result
    }
}

/// A `SizeLimiter` limits the maximum amount a writer can write.
pub(crate) struct SizeLimiter<'a> {
    /// Inner writer.
    inner: &'a mut dyn Write,
    /// Size limit.
    size_limit: usize,
    /// Current write count.
    count: usize,
}

impl<'a> SizeLimiter<'a> {
    /// Construct a new `SizeLimiter`.
    pub(crate) fn new(inner: &'a mut dyn Write, size_limit: usize) -> Self {
        Self {
            inner,
            size_limit,
            count: 0,
        }
    }

    /// Pad with the specified character.
    pub(crate) fn pad(&mut self, c: u8, n: usize) -> Result<(), Error> {
        if self.count.saturating_add(n) > self.size_limit {
            return Err(Error::FormattedStringTooLarge);
        }

        let buffer = [c; 1024];
        let mut remaining = n;

        while remaining > 0 {
            let size = remaining.min(1024);
            self.inner.write_all(&buffer[..size])?;
            remaining -= size;
        }

        self.count += n;

        Ok(())
    }
}

impl Write for SizeLimiter<'_> {
    fn write(&mut self, buf: &[u8]) -> Result<usize, Error> {
        if self.count.saturating_add(buf.len()) > self.size_limit {
            return Err(Error::FormattedStringTooLarge);
        }

        let written = self.inner.write(buf)?;
        self.count += written;
        Ok(written)
    }
}

#[cfg(test)]
mod tests {
    #[cfg(feature = "alloc")]
    #[test]
    fn test_cursor_debug_is_non_empty() {
        use alloc::format;

        use super::Cursor;

        assert!(!format!("{:?}", Cursor::new(&[])).is_empty());
    }
}