str_queue/range/

chars.rs

//! Chars range in a queue.

use core::cmp::Ordering;
use core::fmt;
use core::num::NonZeroUsize;
use core::ops::{Bound, RangeBounds};

use crate::iter::{Chars, Fragment, Fragments};
use crate::range::BytesRange;
use crate::utf8::{self, REPLACEMENT_CHAR};
use crate::StrQueue;
// `BoundExt` trait is a workaround until Rust 1.55.0.
// Suppress `unused_imports` for Rust 1.55.0 beta or later, for now.
#[allow(unused_imports)]
use crate::BoundExt;

/// An internal error type indicating the character is incomplete.
#[derive(Debug, Clone, Copy)]
struct IncompleteCharError;

/// Subrange of a `StrQueue`.
///
/// This can be created by [`StrQueue::chars_range`].
///
/// The range contains valid UTF-8 sequence with the optional
/// last incomplete character.
#[derive(Debug, Clone, Copy, Eq, Hash)]
pub struct CharsRange<'a> {
    /// The range.
    range: BytesRange<'a>,
}

/// Setup.
impl<'a> CharsRange<'a> {
    /// Creates a new `CharsRange` for the queue.
    ///
    /// # Panics
    ///
    /// Panics if the start bound of the range does not lie on UTF-8 sequence boundary.
    #[must_use]
    pub(crate) fn new<R>(queue: &'a StrQueue, range: R) -> Self
    where
        R: RangeBounds<usize>,
    {
        let (former, latter) = queue.inner.as_slices();
        #[allow(unstable_name_collisions)] // This is intended. See `crate::BoundExt` trait.
        Self::from_slices_and_bounds(
            former,
            latter,
            range.start_bound().cloned(),
            range.end_bound().cloned(),
        )
    }

    /// Creates `CharsRange` from slices and range bounds.
    ///
    /// # Panics
    ///
    /// Panics if the start bound of the range does not lie on UTF-8 sequence boundary.
    /// Panics if `former` and `latter` overlaps.
    /// Panics if the given index is out of range.
    #[must_use]
    fn from_slices_and_bounds(
        former: &'a [u8],
        latter: &'a [u8],
        start: Bound<usize>,
        end: Bound<usize>,
    ) -> Self {
        let range = BytesRange::from_slices_and_bounds(former, latter, start, end);

        // Check if the first byte of the index is the start byte of a character.
        if !matches!(start, Bound::Unbounded | Bound::Included(0))
            && range.first().map(utf8::expected_char_len) == Some(0)
        {
            panic!(
                "[precondition] start bound of the range {:?} \
                 does not lie on UTF-8 sequence boundary",
                (start, end)
            );
        }

        Self { range }
    }
}

/// Subrange access.
impl<'a> CharsRange<'a> {
    /// Returns the subrange.
    ///
    /// The returned range can contain an incomplete character at the end.
    /// If you want to exclude a possible trailing incomplete character in the range,
    /// use [`CharsRange::to_complete`] or [`CharsRange::trim_last_incomplete_char`].
    ///
    /// # Panics
    ///
    /// Panics if the start bound of the range does not lie on UTF-8 sequence boundary.
    /// Panics if the given index is out of range.
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::StrQueue;
    ///
    /// let queue = StrQueue::from(b"Hello \xce\xb1");
    /// let range = queue.chars_range(..);
    /// assert_eq!(range.to_string(), "Hello \u{03B1}");
    ///
    /// assert_eq!(range.range(3..7).to_string(), "lo \u{FFFD}");
    /// ```
    #[must_use]
    pub fn range<R>(&self, range: R) -> Self
    where
        R: RangeBounds<usize>,
    {
        #[allow(unstable_name_collisions)] // This is intended. See `crate::BoundExt` trait.
        Self::from_slices_and_bounds(
            self.range.former,
            self.range.latter,
            range.start_bound().cloned(),
            range.end_bound().cloned(),
        )
    }

    /// Returns the range without the last incomplete character.
    ///
    /// If you want to modify `self` instead of getting a modified copy, use
    /// [`CharsRange::trim_last_incomplete_char`].
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::{PartialHandling, StrQueue};
    ///
    /// let queue = StrQueue::from(b"Hello \xce");
    /// let range = queue.chars_range(..);
    /// assert_eq!(range.to_string(), "Hello \u{FFFD}");
    /// assert_eq!(range.to_complete().to_string(), "Hello ");
    /// ```
    #[inline]
    #[must_use]
    pub fn to_complete(mut self) -> Self {
        self.trim_last_incomplete_char();
        self
    }

    /// Returns the range without the last incomplete character and the trimmed length.
    #[inline]
    #[must_use]
    // TODO: Better name?
    fn to_complete_and_trimmed_len(mut self) -> (Self, usize) {
        let trimmed = self.trim_last_incomplete_char();
        (self, trimmed)
    }
}

/// Content length and existence.
impl<'a> CharsRange<'a> {
    /// Returns the total length.
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::StrQueue;
    ///
    /// let queue = StrQueue::from(b"Hello \xce");
    /// let range = queue.chars_range(..);
    /// assert_eq!(range.len(), b"Hello \xce".len());
    /// ```
    #[inline]
    #[must_use]
    pub fn len(&self) -> usize {
        self.range.len()
    }

    /// Returns true if the range is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::StrQueue;
    ///
    /// let queue = StrQueue::from(b"\xce");
    /// let range = queue.chars_range(..);
    /// assert!(!range.is_empty());
    ///
    /// assert!(queue.chars_range(0..0).is_empty());
    /// ```
    #[inline]
    #[must_use]
    pub fn is_empty(&self) -> bool {
        self.range.is_empty()
    }

    /// Returns the string length in bytes, excluding incomplete bytes.
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::StrQueue;
    ///
    /// let queue = StrQueue::from(b"hello\xce");
    /// assert_eq!(queue.chars_range(..).len_complete(), 5);
    /// ```
    #[inline]
    #[must_use]
    pub fn len_complete(&self) -> usize {
        self.len() - self.len_incomplete()
    }

    /// Returns the length of incomplete bytes.
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::StrQueue;
    ///
    /// let queue = StrQueue::from(b"hello\xce");
    /// assert_eq!(queue.chars_range(..).len_incomplete(), 1);
    /// ```
    #[inline]
    #[must_use]
    pub fn len_incomplete(&self) -> usize {
        let former = self.range.former;
        let latter = self.range.latter;
        if !latter.is_empty() {
            if let Some(partial_len) = utf8::last_char_len_in_last_4bytes(latter) {
                return partial_len.len_incomplete();
            }
        }

        // No characters starts from the latter slice.
        utf8::last_char_len_in_last_4bytes(former)
            .expect("[validity] the former buffer must be null or start with valid UTF-8 sequence")
            .len_incomplete()
    }

    /// Returns true if the chars range contains no complete string.
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::StrQueue;
    ///
    /// let mut queue = StrQueue::new();
    /// assert!(queue.chars_range(..).is_empty_complete());
    ///
    /// queue.push_bytes(b"\xce");
    /// assert!(queue.chars_range(..).is_empty_complete());
    ///
    /// queue.push_bytes(b"\xb1");
    /// assert!(!queue.chars_range(..).is_empty_complete());
    /// ```
    #[inline]
    #[must_use]
    pub fn is_empty_complete(&self) -> bool {
        self.len() == self.len_incomplete()
    }

    /// Returns true if the chars range is a complete string, i.e. has no
    /// trailing incomplete character.
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::StrQueue;
    ///
    /// let mut queue = StrQueue::from(b"abc\xce");
    /// assert!(!queue.chars_range(..).is_complete());
    /// queue.push_bytes(b"\xb1");
    /// // Now the string is "abc\u{03B1}".
    /// assert!(queue.chars_range(..).is_complete());
    /// ```
    pub fn is_complete(&self) -> bool {
        if self.range.latter.is_empty() {
            // Check only the former.
            return utf8::last_char_len_in_last_4bytes(self.range.former)
                .map_or(false, |v| v.is_complete());
        }
        if let Some(partial_len) = utf8::last_char_len_in_last_4bytes(self.range.latter) {
            // Check only the latter.
            return partial_len.is_complete();
        }

        // The latter has no beginning of a character.
        let latter_len = self.range.latter.len();
        debug_assert!(
            latter_len < 4,
            "[consistency] a complete character must start in the last 4 bytes"
        );
        debug_assert!(
            latter_len > 0,
            "[consistency] must be returned earlily when the latter is empty"
        );
        let partial_len = match utf8::last_char_len_in_last_4bytes(self.range.former) {
            Some(v) => v,
            None => unreachable!("[consistency] a character start must exist in the last 4 bytes"),
        };
        match partial_len.len_missing().cmp(&latter_len) {
            Ordering::Greater => false,
            Ordering::Equal => true,
            Ordering::Less => {
                unreachable!("[consistency] a character must not be longer than expected")
            }
        }
    }
}

/// Range and content manipulation.
impl<'a> CharsRange<'a> {
    /// Clears the range, removing all elements.
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::{PartialHandling, StrQueue};
    ///
    /// let queue = StrQueue::from(b"Hello \xce");
    /// let mut range = queue.chars_range(..);
    /// assert!(!range.is_empty());
    ///
    /// range.clear();
    /// assert!(range.is_empty());
    /// // Only the range is cleared. The underlying queue does not change.
    /// assert!(!queue.is_empty());
    /// ```
    #[inline]
    pub fn clear(&mut self) {
        self.range.clear()
    }

    /// Trims the last incomplete character, and returns the length of the trimmed bytes.
    ///
    /// If the string is complete (i.e. no incomplete character follows), does
    /// nothing and returns 0.
    ///
    /// If you want to get a modified copy instead of modifying `self` itself,
    /// use [`CharsRange::to_complete`].
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::{PartialHandling, StrQueue};
    ///
    /// let queue = StrQueue::from(b"Hello \xce");
    /// let mut range = queue.chars_range(..);
    /// assert_eq!(range.to_string(), "Hello \u{FFFD}");
    ///
    /// range.trim_last_incomplete_char();
    /// assert_eq!(range.to_string(), "Hello ");
    /// ```
    pub fn trim_last_incomplete_char(&mut self) -> usize {
        let former = &mut self.range.former;
        let latter = &mut self.range.latter;

        if !latter.is_empty() {
            if let Some(partial_len) = utf8::last_char_len_in_last_4bytes(latter) {
                let len_incomplete = partial_len.len_incomplete();
                let valid_up_to = latter.len() - len_incomplete;
                *latter = &latter[..valid_up_to];
                return len_incomplete;
            }
        }

        // No characters starts from the latter slice.
        *latter = &latter[..0];
        // Check for the former slice.
        let len_incomplete = utf8::last_char_len_in_last_4bytes(former)
            .expect("[validity] the former buffer must be null or start with valid UTF-8 sequence")
            .len_incomplete();
        let valid_up_to = former.len() - len_incomplete;
        *former = &former[..valid_up_to];

        len_incomplete
    }

    /// Pops the first character in the range and returns it.
    ///
    /// Trailing incomplete character is ignored.
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::StrQueue;
    ///
    /// let queue = StrQueue::from("hello");
    /// let mut range = queue.chars_range(..);
    ///
    /// assert_eq!(range.pop_char(), Some('h'));
    /// assert_eq!(range.pop_char(), Some('e'));
    /// assert_eq!(range.pop_char(), Some('l'));
    /// assert_eq!(range.pop_char(), Some('l'));
    /// assert_eq!(range.pop_char(), Some('o'));
    /// assert_eq!(range.pop_char(), None);
    /// assert!(range.is_empty());
    /// ```
    ///
    /// ```
    /// use str_queue::StrQueue;
    ///
    /// let queue = StrQueue::from(b"a\xf0");
    /// let mut range = queue.chars_range(..);
    ///
    /// assert_eq!(range.pop_char(), Some('a'));
    /// assert_eq!(range.pop_char(), None);
    /// assert!(!range.is_empty());
    /// ```
    pub fn pop_char(&mut self) -> Option<char> {
        self.pop_char_raw(false).and_then(Result::ok)
    }

    /// Pops the first character from the range and returns it.
    ///
    /// The trailing incomplete character is replaced with `U+FFFD REPLACEMENT
    /// CHARACTER`, if available.
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::StrQueue;
    ///
    /// let queue = StrQueue::from(b"abc\xce");
    /// let mut range = queue.chars_range(..);
    ///
    /// assert_eq!(range.pop_char_replaced(), Some('a'));
    /// assert_eq!(range.pop_char_replaced(), Some('b'));
    /// assert_eq!(range.pop_char_replaced(), Some('c'));
    /// assert_eq!(range.pop_char_replaced(), Some('\u{FFFD}'));
    /// ```
    pub fn pop_char_replaced(&mut self) -> Option<char> {
        self.pop_char_raw(true)
            .map(|res| res.unwrap_or(REPLACEMENT_CHAR))
    }

    /// Pops the first character from the range and return it.
    ///
    /// If the first character is incomplete, `Some(Err(IncompleteCharError))`
    /// is returned.
    /// The trailing incomplete character is removed from the range if and only
    /// if `pop_incomplete` is true.
    fn pop_char_raw(&mut self, pop_incomplete: bool) -> Option<Result<char, IncompleteCharError>> {
        if self.range.is_empty() {
            return None;
        }
        match utf8::take_char(self.range.bytes()) {
            Some((c, len)) => {
                self.range.trim_start(usize::from(len));
                Some(Ok(c))
            }
            None => {
                debug_assert!(!self.range.is_empty());
                // Incomplete character.
                if pop_incomplete {
                    self.range.clear();
                }
                Some(Err(IncompleteCharError))
            }
        }
    }

    /// Pops the first line in the queue and returns it.
    ///
    /// Incomplete lines are ignored.
    ///
    /// Note that it is unspecified whether the line will be removed from the queue,
    /// if the `PoppedLine` value is not dropped while the borrow it holds expires
    /// (e.g. due to [`core::mem::forget`]).
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::StrQueue;
    ///
    /// // Note that the last "world\r" is not considered as a complete line
    /// // since the line can be finally "world\r" or "world\r\n".
    /// let queue = StrQueue::from("Hello\nworld\r\nGoodbye\rworld\r");
    /// let mut range = queue.chars_range(..);
    ///
    /// assert_eq!(range.pop_line().map(|v| v.to_string()).as_deref(), Some("Hello\n"));
    /// assert_eq!(range.pop_line().map(|v| v.to_string()).as_deref(), Some("world\r\n"));
    /// assert_eq!(range.pop_line().map(|v| v.to_string()).as_deref(), Some("Goodbye\r"));
    /// assert_eq!(range.pop_line().map(|v| v.to_string()).as_deref(), None);
    /// assert_eq!(range, "world\r");
    /// ```
    #[inline]
    pub fn pop_line<'r>(&'r mut self) -> Option<CharsRangePoppedLine<'a, 'r>> {
        let line_len = self.first_line()?.len();
        let line_len = NonZeroUsize::new(line_len)
            .expect("[validity] a complete line must not be empty since it has a line break");
        Some(CharsRangePoppedLine {
            range: self,
            line_len,
        })
    }

    /// Pops the first [`Fragment`] from the range and return it.
    ///
    /// In other words, takes as much content as possible from the range.
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::StrQueue;
    ///
    /// let queue = StrQueue::from(b"Hello \xce");
    /// let mut buf = String::new();
    ///
    /// let mut range = queue.chars_range(..);
    /// while let Some(frag) = range.pop_fragment() {
    ///     buf.push_str(&frag.to_string());
    /// }
    ///
    /// assert_eq!(buf, "Hello \u{FFFD}");
    /// assert!(range.is_empty());
    /// ```
    pub fn pop_fragment(&mut self) -> Option<Fragment<'a>> {
        if self.range.is_empty() {
            return None;
        }

        {
            let (former_valid, former_incomplete) =
                utf8::split_incomplete_suffix(self.range.former);
            if !former_valid.is_empty() {
                // The former slice has valid UTF-8 sequence as a prefix.
                self.range.trim_start(former_valid.len());
                return Some(Fragment::Str(former_valid));
            }
            if former_incomplete.is_some() {
                // The former slice starts with incomplete character.
                let frag = self
                    .pop_char_raw(true)
                    .expect("[consistency] the incomplete bytes are not empty")
                    .map_or(Fragment::Incomplete, Fragment::Char);
                return Some(frag);
            }
        }

        let (latter_valid, latter_incomplete) = utf8::split_incomplete_suffix(self.range.latter);
        if !latter_valid.is_empty() {
            // The latter slice has valid UTF-8 sequence as a prefix since
            // the range starts inside the latter slice.
            self.range.trim_start(latter_valid.len());
            return Some(Fragment::Str(latter_valid));
        }
        assert!(
            latter_incomplete.is_some(),
            "[consistency] this cannot be empty since the range is not empty here"
        );
        // The latter slice starts with (and ends with) the incomplete character.
        Some(Fragment::Incomplete)
    }
}

/// Content access.
impl<'a> CharsRange<'a> {
    /// Returns the first character in the range.
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::StrQueue;
    ///
    /// let queue = StrQueue::from("hello");
    ///
    /// assert_eq!(queue.first_char(), Some('h'));
    /// ```
    #[inline]
    #[must_use]
    pub fn first_char(&self) -> Option<char> {
        let bytes = self
            .range
            .former
            .iter()
            .chain(self.range.latter.iter())
            .take(4)
            .copied();
        utf8::take_char(bytes).map(|(c, _len)| c)
    }

    /// Returns the subrange of the first line, including the line break.
    ///
    /// Returns `Some(range)` if the range contains a complete line, which
    /// won't be changed if more contents are appended.
    /// Returns `None` if the range contains only an incomplete line,
    /// which will be changed if more contents are appended.
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::StrQueue;
    ///
    /// let mut queue = StrQueue::from(b"Hello \xce");
    /// assert_eq!(
    ///     queue.chars_range(..).first_line().map(|r| r.to_string()),
    ///     None
    /// );
    ///
    /// queue.push_bytes(b"\xb1\r");
    /// // The line is still considered incomplete since the line ending can be `\r\n`.
    /// assert_eq!(
    ///     queue.chars_range(..).first_line().map(|r| r.to_string()),
    ///     None
    /// );
    ///
    /// queue.push_bytes(b"\nhello");
    /// assert_eq!(
    ///     queue.chars_range(..).first_line().map(|r| r.to_string()),
    ///     Some("Hello \u{03B1}\r\n".to_owned())
    /// );
    /// ```
    ///
    /// ```
    /// use str_queue::{PartialHandling, StrQueue};
    ///
    /// let queue = StrQueue::from(b"Hello\n");
    /// assert_eq!(
    ///     queue.chars_range(..).first_line().map(|r| r.to_string()),
    ///     Some("Hello\n".to_owned())
    /// );
    /// ```
    pub fn first_line(&self) -> Option<CharsRange<'a>> {
        self.next_line_position().map(|pos| self.range(..pos.get()))
    }

    /// Returns the position where the next line starts.
    #[must_use]
    fn next_line_position(&self) -> Option<NonZeroUsize> {
        let first_newline = self.range.position2(b'\n', b'\r')?;
        debug_assert!(
            first_newline < usize::MAX,
            "the string length won't be `usize::MAX` \
                 since other objects (including self) may exist on memory"
        );
        if self.range.former[first_newline] == b'\n' {
            return NonZeroUsize::new(first_newline + 1);
        }
        assert_eq!(self.range.former[first_newline], b'\r');

        // The string ending with `\r` is considered incomplete line,
        // since it is unsure which is used as the line ending, `\r` or `\r\n`.
        let next = self.range.get_byte(first_newline + 1)?;
        if next == b'\n' {
            debug_assert!(
                first_newline + 1 < usize::MAX,
                "the string length won't be `usize::MAX` \
                 since other objects (including self) may exist on memory"
            );
            NonZeroUsize::new(first_newline + 2)
        } else {
            NonZeroUsize::new(first_newline + 1)
        }
    }
}

/// Iterators.
impl<'a> CharsRange<'a> {
    /// Returns an iterator of `char`s.
    ///
    /// Incomplete characters are emitted as `U+FFFD REPLACEMENT CHARACTER`.
    /// If you want to ignore them, use [`CharsRange::to_complete`] or
    /// [`CharsRange::trim_last_incomplete_char`] before creating the iterator.
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::{PartialHandling, StrQueue};
    ///
    /// let queue = StrQueue::from(b"alpha->\xce");
    /// let range = queue.chars_range(..);
    ///
    /// // The trailing incomplete character is replaced with U+FFFD.
    /// assert_eq!(
    ///     range.chars().collect::<String>(),
    ///     "alpha->\u{FFFD}"
    /// );
    /// ```
    #[inline]
    #[must_use]
    pub fn chars(&self) -> Chars<'_> {
        Chars::from_range(*self)
    }

    /// Returns an iterator of content fragments.
    ///
    /// # Examples
    ///
    /// The code below are redundant since they are intended to show how to use [`Fragment`].
    /// To simply create a string from the queue, use [`StrQueue::display`] method.
    ///
    /// ```
    /// use str_queue::{Fragment, StrQueue};
    ///
    /// // `\xce\xb1` is `\u{03B1}`.
    /// let queue = StrQueue::from(b"hello \xce\xb1");
    /// let range = queue.chars_range(3..7);
    ///
    /// let mut buf = String::new();
    /// for frag in range.fragments() {
    ///     match frag {
    ///         Fragment::Str(s) => buf.push_str(s),
    ///         Fragment::Char(c) => buf.push(c),
    ///         Fragment::Incomplete => buf.push_str("<<incomplete>>"),
    ///     }
    /// }
    /// assert_eq!(buf, "lo <<incomplete>>");
    /// ```
    #[inline]
    #[must_use]
    pub fn fragments(&self) -> Fragments<'_> {
        Fragments::from_range(*self)
    }
}

/// Conversion.
impl<'a> CharsRange<'a> {
    /// Returns the bytes range.
    #[inline]
    #[must_use]
    pub fn to_bytes_range(&self) -> BytesRange<'a> {
        (*self).into()
    }
}

/// Comparison.
impl CharsRange<'_> {
    /// Compares the two chars ranges.
    ///
    /// This comparson is arawe of the existence of the last incomplete
    /// characters, but does not aware of its value.
    /// If you care about the value of the incomplete characters, you should use
    /// bytes comparison.
    ///
    /// If the range has the incomplete character, it is treated as a character
    /// which is smaller than any other characters.
    /// In other words, the comparison result can be thought as
    /// `(lhs.complete(), !lhs.is_complete()).cmp(&(rhs.complete(), !rhs.is_complete()))`.
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::{PartialHandling, StrQueue};
    /// use core::cmp::Ordering;
    ///
    /// let queue_ce = StrQueue::from(b"Hello\xce");
    /// let range_ce = queue_ce.chars_range(..);
    /// let queue_f0 = StrQueue::from(b"Hello\xf0");
    /// let range_f0 = queue_f0.chars_range(..);
    ///
    /// // Bytes for incomplete characters are considered the same.
    /// assert_eq!(range_ce, range_f0);
    /// assert_eq!(range_ce.cmp(&range_f0), Ordering::Equal);
    /// // To distinguish incomplete characters, use bytes range.
    /// assert_ne!(range_ce.to_bytes_range(), range_f0.to_bytes_range());
    /// assert_eq!(
    ///     range_ce.to_bytes_range().cmp(&range_f0.to_bytes_range()),
    ///     Ordering::Less
    /// );
    /// ```
    ///
    /// ```
    /// use str_queue::{PartialHandling, StrQueue};
    /// use core::cmp::Ordering;
    ///
    /// let queue_ce = StrQueue::from(b"Hello\xce");
    /// let range_ce = queue_ce.chars_range(..);
    ///
    /// let queue_none = StrQueue::from(b"Hello");
    /// let range_none = queue_none.chars_range(..);
    /// assert_ne!(range_ce, range_none);
    /// assert_eq!(range_ce.partial_cmp(&range_none), Some(Ordering::Greater));
    ///
    /// let queue_00 = StrQueue::from(b"Hello\x00");
    /// let range_00 = queue_00.chars_range(..);
    /// assert_ne!(range_ce, range_00);
    /// // `\xce` is incomplete, so it is considered smaller than `\x00`.
    /// // Note that `\x00` is a valid ASCII character.
    /// assert_eq!(range_ce.partial_cmp(&range_00), Some(Ordering::Less));
    ///
    /// let queue_ufffd = StrQueue::from("Hello\u{FFFD}");
    /// let range_ufffd = queue_ufffd.chars_range(..);
    /// assert_ne!(range_ce, range_ufffd);
    /// // The incomplete character (which consists of `\xce`) is converted to
    /// // `\u{FFFD}` when displayed, but it is not considered to be the same
    /// // as a valid Unicode character `\u{FFFD}`.
    /// assert_eq!(range_ce.partial_cmp(&range_ufffd), Some(Ordering::Less));
    /// ```
    fn cmp_self(&self, rhs: &Self) -> Ordering {
        let self_has_incomplete = !self.is_complete();
        let rhs_has_incomplete = !rhs.is_complete();
        self.to_complete()
            .range
            .cmp_self(&rhs.to_complete().range)
            .then_with(|| self_has_incomplete.cmp(&rhs_has_incomplete))
    }

    /// Compares the chars range and a string slice.
    ///
    /// If the range has the incomplete character, it is treated as a character
    /// which is smaller than any other characters.
    /// In other words, the comparison result can be thought as
    /// `(range.complete(), !range.is_complete()).cmp(&(rhs, false))`.
    /// The complete part differ from the rhs string slice, that result is
    /// returned regardless of the incomplete character.
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::{PartialHandling, StrQueue};
    /// use core::cmp::Ordering;
    ///
    /// let queue_00 = StrQueue::from(b"Hello\x00");
    /// let range_00 = queue_00.chars_range(..);
    /// let queue_ce = StrQueue::from(b"Hello\xce");
    /// let range_ce = queue_ce.chars_range(..);
    ///
    /// assert_eq!(range_00, "Hello\u{0}");
    /// assert_eq!(range_00.partial_cmp("Hello\u{0}"), Some(Ordering::Equal));
    ///
    /// assert_eq!(range_ce.partial_cmp("Hello"), Some(Ordering::Greater));
    /// // `\xce` is incomplete, so it is considered smaller than `\x00`.
    /// // Note that `\x00` is a valid ASCII character.
    /// assert_eq!(range_ce.partial_cmp("Hello\u{0}"), Some(Ordering::Less));
    /// // The incomplete character (which consists of `\xce`) is converted to
    /// // `\u{FFFD}` when displayed, but it is not considered to be the same
    /// // as a valid Unicode character `\u{FFFD}`.
    /// assert_eq!(range_ce.partial_cmp("Hello\u{FFFD}"), Some(Ordering::Less));
    /// ```
    fn cmp_str(&self, rhs: &str) -> Ordering {
        let (complete, trimmed_len) = self.to_complete_and_trimmed_len();
        let fallback = || {
            if trimmed_len == 0 {
                Ordering::Equal
            } else {
                Ordering::Greater
            }
        };

        let former_len = complete.range.former.len();
        let rhs = rhs.as_bytes();
        let rhs_len = rhs.len();

        if former_len > rhs_len {
            complete.range.former.cmp(rhs).then_with(fallback)
        } else {
            complete
                .range
                .former
                .cmp(&rhs[..former_len])
                .then_with(|| complete.range.latter.cmp(&rhs[former_len..]))
                .then_with(fallback)
        }
    }
}

impl fmt::Display for CharsRange<'_> {
    #[inline]
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        Fragments::from(*self).fmt(f)
    }
}

impl<'a> From<CharsRange<'a>> for BytesRange<'a> {
    #[inline]
    fn from(v: CharsRange<'a>) -> Self {
        v.range
    }
}

impl PartialEq for CharsRange<'_> {
    /// Compares the two chars ranges and returns true if they are equal.
    ///
    /// This comparson is arawe of the existence of the last incomplete
    /// characters, but does not aware of its value.
    /// If you care about the value of the incomplete characters, you should use
    /// bytes comparison.
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::{PartialHandling, StrQueue};
    /// use core::cmp::Ordering;
    ///
    /// let queue1 = StrQueue::from(b"Hello\xce");
    /// let range1 = queue1.chars_range(..);
    /// let queue2 = StrQueue::from(b"Hello\xf0");
    /// let range2 = queue2.chars_range(..);
    ///
    /// assert_eq!(range1, range2);
    /// assert_ne!(range1.to_bytes_range(), range2.to_bytes_range());
    /// ```
    #[inline]
    fn eq(&self, other: &Self) -> bool {
        // Return earlily if lengths are not the same.
        (self.len() == other.len()) && self.cmp_self(other).is_eq()
    }
}

impl PartialOrd for CharsRange<'_> {
    /// Compares the two chars ranges.
    ///
    /// This comparson is arawe of the existence of the last incomplete
    /// characters, but does not aware of its value.
    /// If you care about the value of the incomplete characters, you should use
    /// bytes comparison.
    ///
    /// If the range has the incomplete character, it is treated as a character
    /// which is smaller than any other characters.
    /// In other words, the comparison result can be thought as
    /// `(lhs.complete(), !lhs.is_complete()).cmp(&(rhs.complete(), !rhs.is_complete()))`.
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::{PartialHandling, StrQueue};
    /// use core::cmp::Ordering;
    ///
    /// let queue_ce = StrQueue::from(b"Hello\xce");
    /// let range_ce = queue_ce.chars_range(..);
    /// let queue_f0 = StrQueue::from(b"Hello\xf0");
    /// let range_f0 = queue_f0.chars_range(..);
    ///
    /// // Bytes for incomplete characters are considered the same.
    /// assert_eq!(range_ce, range_f0);
    /// assert_eq!(range_ce.cmp(&range_f0), Ordering::Equal);
    /// // To distinguish incomplete characters, use bytes range.
    /// assert_ne!(range_ce.to_bytes_range(), range_f0.to_bytes_range());
    /// assert_eq!(
    ///     range_ce.to_bytes_range().cmp(&range_f0.to_bytes_range()),
    ///     Ordering::Less
    /// );
    /// ```
    ///
    /// ```
    /// use str_queue::{PartialHandling, StrQueue};
    /// use core::cmp::Ordering;
    ///
    /// let queue_ce = StrQueue::from(b"Hello\xce");
    /// let range_ce = queue_ce.chars_range(..);
    ///
    /// let queue_none = StrQueue::from(b"Hello");
    /// let range_none = queue_none.chars_range(..);
    /// assert_ne!(range_ce, range_none);
    /// assert_eq!(range_ce.partial_cmp(&range_none), Some(Ordering::Greater));
    ///
    /// let queue_00 = StrQueue::from(b"Hello\x00");
    /// let range_00 = queue_00.chars_range(..);
    /// assert_ne!(range_ce, range_00);
    /// // `\xce` is incomplete, so it is considered smaller than `\x00`.
    /// // Note that `\x00` is a valid ASCII character.
    /// assert_eq!(range_ce.partial_cmp(&range_00), Some(Ordering::Less));
    ///
    /// let queue_ufffd = StrQueue::from("Hello\u{FFFD}");
    /// let range_ufffd = queue_ufffd.chars_range(..);
    /// assert_ne!(range_ce, range_ufffd);
    /// // The incomplete character (which consists of `\xce`) is converted to
    /// // `\u{FFFD}` when displayed, but it is not considered to be the same
    /// // as a valid Unicode character `\u{FFFD}`.
    /// assert_eq!(range_ce.partial_cmp(&range_ufffd), Some(Ordering::Less));
    /// ```
    fn partial_cmp(&self, rhs: &Self) -> Option<Ordering> {
        Some(self.cmp_self(rhs))
    }
}

impl Ord for CharsRange<'_> {
    ///
    /// This comparson is arawe of the existence of the last incomplete
    /// characters, but does not aware of its value.
    /// If you care about the value of the incomplete characters, you should use
    /// bytes comparison.
    ///
    /// If the range has the incomplete character, it is treated as a character
    /// which is smaller than any other characters.
    /// In other words, the comparison result can be thought as
    /// `(lhs.complete(), !lhs.is_complete()).cmp(&(rhs.complete(), !rhs.is_complete()))`.
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::{PartialHandling, StrQueue};
    /// use core::cmp::Ordering;
    ///
    /// let queue_ce = StrQueue::from(b"Hello\xce");
    /// let range_ce = queue_ce.chars_range(..);
    /// let queue_f0 = StrQueue::from(b"Hello\xf0");
    /// let range_f0 = queue_f0.chars_range(..);
    ///
    /// // Bytes for incomplete characters are considered the same.
    /// assert_eq!(range_ce, range_f0);
    /// assert_eq!(range_ce.cmp(&range_f0), Ordering::Equal);
    /// // To distinguish incomplete characters, use bytes range.
    /// assert_ne!(range_ce.to_bytes_range(), range_f0.to_bytes_range());
    /// assert_eq!(
    ///     range_ce.to_bytes_range().cmp(&range_f0.to_bytes_range()),
    ///     Ordering::Less
    /// );
    /// ```
    ///
    /// ```
    /// use str_queue::{PartialHandling, StrQueue};
    /// use core::cmp::Ordering;
    ///
    /// let queue_ce = StrQueue::from(b"Hello\xce");
    /// let range_ce = queue_ce.chars_range(..);
    ///
    /// let queue_none = StrQueue::from(b"Hello");
    /// let range_none = queue_none.chars_range(..);
    /// assert_ne!(range_ce, range_none);
    /// assert_eq!(range_ce.partial_cmp(&range_none), Some(Ordering::Greater));
    ///
    /// let queue_00 = StrQueue::from(b"Hello\x00");
    /// let range_00 = queue_00.chars_range(..);
    /// assert_ne!(range_ce, range_00);
    /// // `\xce` is incomplete, so it is considered smaller than `\x00`.
    /// // Note that `\x00` is a valid ASCII character.
    /// assert_eq!(range_ce.partial_cmp(&range_00), Some(Ordering::Less));
    ///
    /// let queue_ufffd = StrQueue::from("Hello\u{FFFD}");
    /// let range_ufffd = queue_ufffd.chars_range(..);
    /// assert_ne!(range_ce, range_ufffd);
    /// // The incomplete character (which consists of `\xce`) is converted to
    /// // `\u{FFFD}` when displayed, but it is not considered to be the same
    /// // as a valid Unicode character `\u{FFFD}`.
    /// assert_eq!(range_ce.partial_cmp(&range_ufffd), Some(Ordering::Less));
    /// ```
    fn cmp(&self, rhs: &Self) -> Ordering {
        self.cmp_self(rhs)
    }
}

impl PartialEq<str> for CharsRange<'_> {
    #[inline]
    fn eq(&self, other: &str) -> bool {
        // Return earlily if lengths are not the same.
        (self.len() == other.len()) && self.cmp_str(other).is_eq()
    }
}

impl PartialOrd<str> for CharsRange<'_> {
    /// Compares the chars range and a string slice.
    ///
    /// If the range has the incomplete character, it is treated as a character
    /// which is smaller than any other characters.
    /// In other words, the comparison result can be thought as
    /// `(range.complete(), !range.is_complete()).cmp(&(rhs, false))`.
    /// The complete part differ from the rhs string slice, that result is
    /// returned regardless of the incomplete character.
    ///
    /// # Examples
    ///
    /// ```
    /// use str_queue::{PartialHandling, StrQueue};
    /// use core::cmp::Ordering;
    ///
    /// let queue_00 = StrQueue::from(b"Hello\x00");
    /// let range_00 = queue_00.chars_range(..);
    /// let queue_ce = StrQueue::from(b"Hello\xce");
    /// let range_ce = queue_ce.chars_range(..);
    ///
    /// assert_eq!(range_00, "Hello\u{0}");
    /// assert_eq!(range_00.partial_cmp("Hello\u{0}"), Some(Ordering::Equal));
    ///
    /// assert_eq!(range_ce.partial_cmp("Hello"), Some(Ordering::Greater));
    /// // `\xce` is incomplete, so it is considered smaller than `\x00`.
    /// // Note that `\x00` is a valid ASCII character.
    /// assert_eq!(range_ce.partial_cmp("Hello\u{0}"), Some(Ordering::Less));
    /// // The incomplete character (which consists of `\xce`) is converted to
    /// // `\u{FFFD}` when displayed, but it is not considered to be the same
    /// // as a valid Unicode character `\u{FFFD}`.
    /// assert_eq!(range_ce.partial_cmp("Hello\u{FFFD}"), Some(Ordering::Less));
    /// ```
    #[inline]
    fn partial_cmp(&self, rhs: &str) -> Option<Ordering> {
        Some(self.cmp_str(rhs))
    }
}

impl PartialEq<CharsRange<'_>> for str {
    #[inline]
    fn eq(&self, other: &CharsRange<'_>) -> bool {
        other.eq(self)
    }
}

impl PartialOrd<CharsRange<'_>> for str {
    #[inline]
    fn partial_cmp(&self, rhs: &CharsRange<'_>) -> Option<Ordering> {
        rhs.partial_cmp(self).map(Ordering::reverse)
    }
}

impl PartialEq<&str> for CharsRange<'_> {
    #[inline]
    fn eq(&self, other: &&str) -> bool {
        self.eq(*other)
    }
}

impl PartialOrd<&str> for CharsRange<'_> {
    #[inline]
    fn partial_cmp(&self, rhs: &&str) -> Option<Ordering> {
        self.partial_cmp(*rhs)
    }
}

impl PartialEq<CharsRange<'_>> for &str {
    #[inline]
    fn eq(&self, other: &CharsRange<'_>) -> bool {
        other.eq(*self)
    }
}

impl PartialOrd<CharsRange<'_>> for &str {
    #[inline]
    fn partial_cmp(&self, rhs: &CharsRange<'_>) -> Option<Ordering> {
        rhs.partial_cmp(*self).map(Ordering::reverse)
    }
}

/// A line popped from [`CharsRange`] using [`pop_line`][`CharsRange::pop_line`] method.
///
/// Note that this may pop a line from `CharsRange`, but `StrQueue` behind the
/// range won't be affected.
///
/// Also note that it is unspecified whether the line will be removed from the
/// queue, if the `CharsRangePoppedLine` value is not dropped while the borrow
/// it holds expires (e.g. due to [`core::mem::forget`]).
#[derive(Debug)]
pub struct CharsRangePoppedLine<'a, 'r> {
    /// Range.
    range: &'r mut CharsRange<'a>,
    /// Line length.
    ///
    /// A complete line cannot be empty since it has a line break.
    line_len: NonZeroUsize,
}

impl<'a, 'r> CharsRangePoppedLine<'a, 'r> {
    /// Returns the chars range for the line.
    #[inline]
    #[must_use]
    pub fn to_chars_range(&self) -> CharsRange<'a> {
        self.range.range(..self.line_len.get())
    }

    /// Returns the bytes range for the line.
    #[inline]
    #[must_use]
    pub fn to_bytes_range(&self) -> BytesRange<'_> {
        self.range.to_bytes_range().range(..self.line_len.get())
    }

    /// Returns the length of the line.
    #[inline]
    #[must_use]
    pub fn len(&self) -> usize {
        self.line_len.get()
    }

    /// Returns whether the line is empty, i.e. **always returns false**.
    ///
    /// A complete line to be removed cannot be empty since it has a line break.
    #[inline]
    #[must_use]
    pub const fn is_empty(&self) -> bool {
        false
    }
}

impl fmt::Display for CharsRangePoppedLine<'_, '_> {
    #[inline]
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        self.to_chars_range().fmt(f)
    }
}

impl core::ops::Drop for CharsRangePoppedLine<'_, '_> {
    #[inline]
    fn drop(&mut self) {
        *self.range = self.range.range(self.line_len.get()..);
    }
}

impl PartialEq for CharsRangePoppedLine<'_, '_> {
    #[inline]
    fn eq(&self, other: &CharsRangePoppedLine<'_, '_>) -> bool {
        self.to_bytes_range().eq(&other.to_bytes_range())
    }
}

impl Eq for CharsRangePoppedLine<'_, '_> {}

impl PartialOrd for CharsRangePoppedLine<'_, '_> {
    #[inline]
    fn partial_cmp(&self, rhs: &CharsRangePoppedLine<'_, '_>) -> Option<Ordering> {
        self.to_bytes_range().partial_cmp(&rhs.to_bytes_range())
    }
}

impl Ord for CharsRangePoppedLine<'_, '_> {
    #[inline]
    fn cmp(&self, rhs: &CharsRangePoppedLine<'_, '_>) -> Ordering {
        self.to_bytes_range().cmp(&rhs.to_bytes_range())
    }
}

impl PartialEq<str> for CharsRangePoppedLine<'_, '_> {
    #[inline]
    fn eq(&self, other: &str) -> bool {
        self.to_bytes_range().eq(other.as_bytes())
    }
}

impl PartialOrd<str> for CharsRangePoppedLine<'_, '_> {
    #[inline]
    fn partial_cmp(&self, rhs: &str) -> Option<Ordering> {
        self.to_bytes_range().partial_cmp(rhs.as_bytes())
    }
}

impl PartialEq<CharsRangePoppedLine<'_, '_>> for str {
    #[inline]
    fn eq(&self, other: &CharsRangePoppedLine<'_, '_>) -> bool {
        other.to_bytes_range().eq(self.as_bytes())
    }
}

impl PartialOrd<CharsRangePoppedLine<'_, '_>> for str {
    #[inline]
    fn partial_cmp(&self, rhs: &CharsRangePoppedLine<'_, '_>) -> Option<Ordering> {
        rhs.to_bytes_range()
            .partial_cmp(self.as_bytes())
            .map(Ordering::reverse)
    }
}

impl PartialEq<&str> for CharsRangePoppedLine<'_, '_> {
    #[inline]
    fn eq(&self, other: &&str) -> bool {
        self.eq(*other)
    }
}

impl PartialOrd<&str> for CharsRangePoppedLine<'_, '_> {
    #[inline]
    fn partial_cmp(&self, rhs: &&str) -> Option<Ordering> {
        self.partial_cmp(*rhs)
    }
}

impl PartialEq<CharsRangePoppedLine<'_, '_>> for &str {
    #[inline]
    fn eq(&self, other: &CharsRangePoppedLine<'_, '_>) -> bool {
        other.eq(*self)
    }
}

impl PartialOrd<CharsRangePoppedLine<'_, '_>> for &str {
    #[inline]
    fn partial_cmp(&self, rhs: &CharsRangePoppedLine<'_, '_>) -> Option<Ordering> {
        rhs.partial_cmp(*self).map(Ordering::reverse)
    }
}