no_std_path 0.1.0

use alloc::borrow::{Cow, ToOwned};
use alloc::boxed::Box;
use alloc::collections::TryReserveError;
use alloc::rc::Rc;
use alloc::string::String;
use alloc::sync::Arc;
use alloc::vec::Vec;
use core::borrow::Borrow;
use core::hash::{Hash, Hasher};
use core::iter::FusedIterator;
use core::ops::{self, Deref};
use core::str::FromStr;
use core::{cmp, fmt};

use crate::ffi::{OsStr, OsString};
use crate::sys_path::{HAS_PREFIXES, MAIN_SEP_STR, is_sep_byte, is_verbatim_sep, parse_prefix};

////////////////////////////////////////////////////////////////////////////////
// GENERAL NOTES
////////////////////////////////////////////////////////////////////////////////
//
// Parsing in this module is done by directly transmuting OsStr to [u8] slices,
// taking advantage of the fact that OsStr always encodes ASCII characters
// as-is.  Eventually, this transmutation should be replaced by direct uses of
// OsStr APIs for parsing, but it will take a while for those to become
// available.

////////////////////////////////////////////////////////////////////////////////
// Windows Prefixes
////////////////////////////////////////////////////////////////////////////////

/// Windows path prefixes, e.g., `C:` or `\\server\share`.
///
/// Windows uses a variety of path prefix styles, including references to drive
/// volumes (like `C:`), network shared folders (like `\\server\share`), and
/// others. In addition, some path prefixes are "verbatim" (i.e., prefixed with
/// `\\?\`), in which case `/` is *not* treated as a separator and essentially
/// no normalization is performed.
#[derive(Copy, Clone, Debug, Hash, PartialOrd, Ord, PartialEq, Eq)]
pub enum Prefix<'a> {
    /// Verbatim prefix, e.g., `\\?\cat_pics`.
    Verbatim(&'a OsStr),

    /// Verbatim prefix using Windows' _**U**niform **N**aming **C**onvention_,
    /// e.g., `\\?\UNC\server\share`.
    VerbatimUNC(&'a OsStr, &'a OsStr),

    /// Verbatim disk prefix, e.g., `\\?\C:`.
    VerbatimDisk(u8),

    /// Device namespace prefix, e.g., `\\.\COM42`.
    DeviceNS(&'a OsStr),

    /// Prefix using Windows' _**U**niform **N**aming **C**onvention_, e.g.
    /// `\\server\share`.
    UNC(&'a OsStr, &'a OsStr),

    /// Prefix `C:` for the given disk drive.
    Disk(u8),
}

impl<'a> Prefix<'a> {
    #[inline]
    fn len(&self) -> usize {
        use self::Prefix::*;
        fn os_str_len(s: &OsStr) -> usize {
            s.as_encoded_bytes().len()
        }
        match *self {
            Verbatim(x) => 4 + os_str_len(x),
            VerbatimUNC(x, y) => {
                8 + os_str_len(x) + if os_str_len(y) > 0 { 1 + os_str_len(y) } else { 0 }
            }
            VerbatimDisk(_) => 6,
            UNC(x, y) => {
                2 + os_str_len(x) + if os_str_len(y) > 0 { 1 + os_str_len(y) } else { 0 }
            }
            DeviceNS(x) => 4 + os_str_len(x),
            Disk(_) => 2,
        }
    }

    /// Determines if the prefix is verbatim, i.e., begins with `\\?\`.
    #[inline]
    #[must_use]
    pub fn is_verbatim(&self) -> bool {
        use self::Prefix::*;
        matches!(*self, Verbatim(_) | VerbatimDisk(_) | VerbatimUNC(..))
    }

    #[inline]
    fn is_drive(&self) -> bool {
        matches!(*self, Prefix::Disk(_))
    }

    #[inline]
    fn has_implicit_root(&self) -> bool {
        !self.is_drive()
    }
}

////////////////////////////////////////////////////////////////////////////////
// Exposed parsing helpers
////////////////////////////////////////////////////////////////////////////////

/// Determines whether the character is one of the permitted path
/// separators for the current platform.
#[must_use]
pub fn is_separator(c: char) -> bool {
    c.is_ascii() && is_sep_byte(c as u8)
}

/// The primary separator of path components for the current platform.
///
/// For example, `/` on Unix and `\` on Windows.
pub const MAIN_SEPARATOR: char = crate::sys_path::MAIN_SEP;

/// The primary separator of path components for the current platform.
///
/// For example, `/` on Unix and `\` on Windows.
pub const MAIN_SEPARATOR_STR: &str = crate::sys_path::MAIN_SEP_STR;

////////////////////////////////////////////////////////////////////////////////
// Misc helpers
////////////////////////////////////////////////////////////////////////////////

// Iterate through `iter` while it matches `prefix`; return `None` if `prefix`
// is not a prefix of `iter`, otherwise return `Some(iter_after_prefix)` giving
// `iter` after having exhausted `prefix`.
fn iter_after<'a, 'b, I, J>(mut iter: I, mut prefix: J) -> Option<I>
where
    I: Iterator<Item = Component<'a>> + Clone,
    J: Iterator<Item = Component<'b>>,
{
    loop {
        let mut iter_next = iter.clone();
        match (iter_next.next(), prefix.next()) {
            (Some(ref x), Some(ref y)) if x == y => (),
            (Some(_), Some(_)) => return None,
            (Some(_), None) => return Some(iter),
            (None, None) => return Some(iter),
            (None, Some(_)) => return None,
        }
        iter = iter_next;
    }
}

////////////////////////////////////////////////////////////////////////////////
// Cross-platform, iterator-independent parsing
////////////////////////////////////////////////////////////////////////////////

// See note at the top of this module to understand why these are used:
//
// This is safe because OsStr is guaranteed to represent a contiguous byte
// sequence that includes ASCII.

/// Says whether the first byte after the prefix is a separator.
fn has_physical_root(s: &[u8], prefix: Option<Prefix<'_>>) -> bool {
    let path = if let Some(p) = prefix { &s[p.len()..] } else { s };
    !path.is_empty() && is_sep_byte(path[0])
}

// basic workhorse for splitting stem and extension
fn rsplit_file_at_dot(file: &OsStr) -> (Option<&OsStr>, Option<&OsStr>) {
    if file.as_encoded_bytes() == b".." {
        return (Some(file), None);
    }

    let mut iter = file.as_encoded_bytes().rsplitn(2, |b| *b == b'.');
    let after = iter.next();
    let before = iter.next();
    if before == Some(b"") {
        (Some(file), None)
    } else {
        unsafe {
            (
                before.map(|s| OsStr::from_encoded_bytes_unchecked(s)),
                after.map(|s| OsStr::from_encoded_bytes_unchecked(s)),
            )
        }
    }
}

/// Checks whether the string is valid as a file extension, or panics otherwise.
fn validate_extension(extension: &OsStr) {
    for &b in extension.as_encoded_bytes() {
        if is_sep_byte(b) {
            panic!("extension cannot contain path separators: {extension:?}");
        }
    }
}

////////////////////////////////////////////////////////////////////////////////
// The core iterators
////////////////////////////////////////////////////////////////////////////////

/// Component parsing works by a double-ended state machine; the cursors at the
/// front and back of the path each keep track of what parts of the path have
/// been consumed so far.
///
/// Going front to back, a path is made up of a prefix, a starting
/// directory component, and a body (of normal components)
#[derive(Copy, Clone, PartialEq, PartialOrd, Debug)]
enum State {
    Prefix = 0,   // c:
    StartDir = 1, // / or . or nothing
    Body = 2,     // foo/bar/baz
    Done = 3,
}

/// A structure wrapping a Windows path prefix as well as its unparsed string
/// representation.
///
/// In addition to the parsed [`Prefix`] information returned by [`kind`],
/// `PrefixComponent` also holds the raw and unparsed [`OsStr`] slice,
/// returned by [`as_os_str`].
///
/// Instances of this `struct` can be obtained by matching against the
/// [`Prefix` variant] on [`Component`].
///
/// Does not occur on Unix.
///
/// [`as_os_str`]: PrefixComponent::as_os_str
/// [`kind`]: PrefixComponent::kind
/// [`Prefix` variant]: Component::Prefix
#[derive(Copy, Clone, Eq, Debug)]
pub struct PrefixComponent<'a> {
    /// The prefix as an unparsed `OsStr` slice.
    raw: &'a OsStr,

    /// The parsed prefix data.
    parsed: Prefix<'a>,
}

impl<'a> PrefixComponent<'a> {
    /// Returns the parsed prefix data.
    #[must_use]
    #[inline]
    pub fn kind(&self) -> Prefix<'a> {
        self.parsed
    }

    /// Returns the raw [`OsStr`] slice for this prefix.
    #[must_use]
    #[inline]
    pub fn as_os_str(&self) -> &'a OsStr {
        self.raw
    }
}

impl<'a> PartialEq for PrefixComponent<'a> {
    #[inline]
    fn eq(&self, other: &PrefixComponent<'a>) -> bool {
        self.parsed == other.parsed
    }
}

impl<'a> PartialOrd for PrefixComponent<'a> {
    #[inline]
    fn partial_cmp(&self, other: &PrefixComponent<'a>) -> Option<cmp::Ordering> {
        Some(self.cmp(other))
    }
}

impl Ord for PrefixComponent<'_> {
    #[inline]
    fn cmp(&self, other: &Self) -> cmp::Ordering {
        Ord::cmp(&self.parsed, &other.parsed)
    }
}

impl Hash for PrefixComponent<'_> {
    fn hash<H: Hasher>(&self, h: &mut H) {
        self.parsed.hash(h);
    }
}

/// A single component of a path.
///
/// A `Component` roughly corresponds to a substring between path separators
/// (`/` or `\`).
///
/// This `enum` is created by iterating over [`Components`], which in turn is
/// created by the [`components`](Path::components) method on [`Path`].
#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
pub enum Component<'a> {
    /// A Windows path prefix, e.g., `C:` or `\\server\share`.
    ///
    /// Does not occur on Unix.
    Prefix(PrefixComponent<'a>),

    /// The root directory component, appears after any prefix and before anything else.
    ///
    /// It represents a separator that designates that a path starts from root.
    RootDir,

    /// A reference to the current directory, i.e., `.`.
    CurDir,

    /// A reference to the parent directory, i.e., `..`.
    ParentDir,

    /// A normal component, e.g., `a` and `b` in `a/b`.
    Normal(&'a OsStr),
}

impl<'a> Component<'a> {
    /// Extracts the underlying [`OsStr`] slice.
    #[must_use = "`self` will be dropped if the result is not used"]
    pub fn as_os_str(self) -> &'a OsStr {
        match self {
            Component::Prefix(p) => p.as_os_str(),
            Component::RootDir => OsStr::new(MAIN_SEP_STR),
            Component::CurDir => OsStr::new("."),
            Component::ParentDir => OsStr::new(".."),
            Component::Normal(path) => path,
        }
    }
}

impl AsRef<OsStr> for Component<'_> {
    #[inline]
    fn as_ref(&self) -> &OsStr {
        self.as_os_str()
    }
}

impl AsRef<Path> for Component<'_> {
    #[inline]
    fn as_ref(&self) -> &Path {
        self.as_os_str().as_ref()
    }
}

/// An iterator over the [`Component`]s of a [`Path`].
///
/// This `struct` is created by the [`components`] method on [`Path`].
/// See its documentation for more.
///
/// [`components`]: Path::components
#[derive(Clone)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
pub struct Components<'a> {
    // The path left to parse components from
    path: &'a [u8],

    // The prefix as it was originally parsed, if any
    prefix: Option<Prefix<'a>>,

    // true if path *physically* has a root separator; for most Windows
    // prefixes, it may have a "logical" root separator for the purposes of
    // normalization, e.g., \\server\share == \\server\share\.
    has_physical_root: bool,

    // The iterator is double-ended, and these two states keep track of what has
    // been produced from either end
    front: State,
    back: State,
}

/// An iterator over the [`Component`]s of a [`Path`], as [`OsStr`] slices.
///
/// This `struct` is created by the [`iter`] method on [`Path`].
/// See its documentation for more.
///
/// [`iter`]: Path::iter
#[derive(Clone)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
pub struct Iter<'a> {
    inner: Components<'a>,
}

impl fmt::Debug for Components<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        struct DebugHelper<'a>(&'a Path);

        impl fmt::Debug for DebugHelper<'_> {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                f.debug_list().entries(self.0.components()).finish()
            }
        }

        f.debug_tuple("Components").field(&DebugHelper(self.as_path())).finish()
    }
}

impl<'a> Components<'a> {
    // how long is the prefix, if any?
    #[inline]
    fn prefix_len(&self) -> usize {
        if !HAS_PREFIXES {
            return 0;
        }
        self.prefix.as_ref().map(Prefix::len).unwrap_or(0)
    }

    #[inline]
    fn prefix_verbatim(&self) -> bool {
        if !HAS_PREFIXES {
            return false;
        }
        self.prefix.as_ref().map(Prefix::is_verbatim).unwrap_or(false)
    }

    /// how much of the prefix is left from the point of view of iteration?
    #[inline]
    fn prefix_remaining(&self) -> usize {
        if !HAS_PREFIXES {
            return 0;
        }
        if self.front == State::Prefix { self.prefix_len() } else { 0 }
    }

    // Given the iteration so far, how much of the pre-State::Body path is left?
    #[inline]
    fn len_before_body(&self) -> usize {
        let root = if self.front <= State::StartDir && self.has_physical_root { 1 } else { 0 };
        let cur_dir = if self.front <= State::StartDir && self.include_cur_dir() { 1 } else { 0 };
        self.prefix_remaining() + root + cur_dir
    }

    // is the iteration complete?
    #[inline]
    fn finished(&self) -> bool {
        self.front == State::Done || self.back == State::Done || self.front > self.back
    }

    #[inline]
    fn is_sep_byte(&self, b: u8) -> bool {
        if self.prefix_verbatim() { is_verbatim_sep(b) } else { is_sep_byte(b) }
    }

    /// Extracts a slice corresponding to the portion of the path remaining for iteration.
    #[must_use]
    pub fn as_path(&self) -> &'a Path {
        let mut comps = self.clone();
        if comps.front == State::Body {
            comps.trim_left();
        }
        if comps.back == State::Body {
            comps.trim_right();
        }
        unsafe { Path::from_u8_slice(comps.path) }
    }

    /// Is the *original* path rooted?
    fn has_root(&self) -> bool {
        if self.has_physical_root {
            return true;
        }
        if HAS_PREFIXES && let Some(p) = self.prefix
            && p.has_implicit_root()
        {
            return true;
        }
        false
    }

    /// Should the normalized path include a leading . ?
    fn include_cur_dir(&self) -> bool {
        if self.has_root() {
            return false;
        }
        let slice = &self.path[self.prefix_remaining()..];
        match slice {
            [b'.'] => true,
            [b'.', b, ..] => self.is_sep_byte(*b),
            _ => false,
        }
    }

    // parse a given byte sequence following the OsStr encoding into the
    // corresponding path component
    unsafe fn parse_single_component<'b>(&self, comp: &'b [u8]) -> Option<Component<'b>> {
        match comp {
            b"." if HAS_PREFIXES && self.prefix_verbatim() => Some(Component::CurDir),
            b"." => None, // . components are normalized away, except at
            // the beginning of a path, which is treated
            // separately via `include_cur_dir`
            b".." => Some(Component::ParentDir),
            b"" => None,
            _ => Some(Component::Normal(unsafe { OsStr::from_encoded_bytes_unchecked(comp) })),
        }
    }

    // parse a component from the left, saying how many bytes to consume to
    // remove the component
    fn parse_next_component(&self) -> (usize, Option<Component<'a>>) {
        debug_assert!(self.front == State::Body);
        let (extra, comp) = match self.path.iter().position(|b| self.is_sep_byte(*b)) {
            None => (0, self.path),
            Some(i) => (1, &self.path[..i]),
        };
        // SAFETY: `comp` is a valid substring, since it is split on a separator.
        (comp.len() + extra, unsafe { self.parse_single_component(comp) })
    }

    // parse a component from the right, saying how many bytes to consume to
    // remove the component
    fn parse_next_component_back(&self) -> (usize, Option<Component<'a>>) {
        debug_assert!(self.back == State::Body);
        let start = self.len_before_body();
        let (extra, comp) = match self.path[start..].iter().rposition(|b| self.is_sep_byte(*b)) {
            None => (0, &self.path[start..]),
            Some(i) => (1, &self.path[start + i + 1..]),
        };
        // SAFETY: `comp` is a valid substring, since it is split on a separator.
        (comp.len() + extra, unsafe { self.parse_single_component(comp) })
    }

    // trim away repeated separators (i.e., empty components) on the left
    fn trim_left(&mut self) {
        while !self.path.is_empty() {
            let (size, comp) = self.parse_next_component();
            if comp.is_some() {
                return;
            } else {
                self.path = &self.path[size..];
            }
        }
    }

    // trim away repeated separators (i.e., empty components) on the right
    fn trim_right(&mut self) {
        while self.path.len() > self.len_before_body() {
            let (size, comp) = self.parse_next_component_back();
            if comp.is_some() {
                return;
            } else {
                self.path = &self.path[..self.path.len() - size];
            }
        }
    }
}

impl AsRef<Path> for Components<'_> {
    #[inline]
    fn as_ref(&self) -> &Path {
        self.as_path()
    }
}

impl AsRef<OsStr> for Components<'_> {
    #[inline]
    fn as_ref(&self) -> &OsStr {
        self.as_path().as_os_str()
    }
}

impl fmt::Debug for Iter<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        struct DebugHelper<'a>(&'a Path);

        impl fmt::Debug for DebugHelper<'_> {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                f.debug_list().entries(self.0.iter()).finish()
            }
        }

        f.debug_tuple("Iter").field(&DebugHelper(self.as_path())).finish()
    }
}

impl<'a> Iter<'a> {
    /// Extracts a slice corresponding to the portion of the path remaining for iteration.
    #[must_use]
    #[inline]
    pub fn as_path(&self) -> &'a Path {
        self.inner.as_path()
    }
}

impl AsRef<Path> for Iter<'_> {
    #[inline]
    fn as_ref(&self) -> &Path {
        self.as_path()
    }
}

impl AsRef<OsStr> for Iter<'_> {
    #[inline]
    fn as_ref(&self) -> &OsStr {
        self.as_path().as_os_str()
    }
}

impl<'a> Iterator for Iter<'a> {
    type Item = &'a OsStr;

    #[inline]
    fn next(&mut self) -> Option<&'a OsStr> {
        self.inner.next().map(Component::as_os_str)
    }
}

impl<'a> DoubleEndedIterator for Iter<'a> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a OsStr> {
        self.inner.next_back().map(Component::as_os_str)
    }
}

impl FusedIterator for Iter<'_> {}

impl<'a> Iterator for Components<'a> {
    type Item = Component<'a>;

    fn next(&mut self) -> Option<Component<'a>> {
        while !self.finished() {
            match self.front {
                // most likely case first
                State::Body if !self.path.is_empty() => {
                    let (size, comp) = self.parse_next_component();
                    self.path = &self.path[size..];
                    if comp.is_some() {
                        return comp;
                    }
                }
                State::Body => {
                    self.front = State::Done;
                }
                State::StartDir => {
                    self.front = State::Body;
                    if self.has_physical_root {
                        debug_assert!(!self.path.is_empty());
                        self.path = &self.path[1..];
                        return Some(Component::RootDir);
                    } else if HAS_PREFIXES && let Some(p) = self.prefix {
                        if p.has_implicit_root() && !p.is_verbatim() {
                            return Some(Component::RootDir);
                        }
                    } else if self.include_cur_dir() {
                        debug_assert!(!self.path.is_empty());
                        self.path = &self.path[1..];
                        return Some(Component::CurDir);
                    }
                }
                _ if !HAS_PREFIXES => unreachable!(),
                State::Prefix if self.prefix_len() == 0 => {
                    self.front = State::StartDir;
                }
                State::Prefix => {
                    self.front = State::StartDir;
                    debug_assert!(self.prefix_len() <= self.path.len());
                    let raw = &self.path[..self.prefix_len()];
                    self.path = &self.path[self.prefix_len()..];
                    return Some(Component::Prefix(PrefixComponent {
                        raw: unsafe { OsStr::from_encoded_bytes_unchecked(raw) },
                        parsed: self.prefix.unwrap(),
                    }));
                }
                State::Done => unreachable!(),
            }
        }
        None
    }
}

impl<'a> DoubleEndedIterator for Components<'a> {
    fn next_back(&mut self) -> Option<Component<'a>> {
        while !self.finished() {
            match self.back {
                State::Body if self.path.len() > self.len_before_body() => {
                    let (size, comp) = self.parse_next_component_back();
                    self.path = &self.path[..self.path.len() - size];
                    if comp.is_some() {
                        return comp;
                    }
                }
                State::Body => {
                    self.back = State::StartDir;
                }
                State::StartDir => {
                    self.back = if HAS_PREFIXES { State::Prefix } else { State::Done };
                    if self.has_physical_root {
                        self.path = &self.path[..self.path.len() - 1];
                        return Some(Component::RootDir);
                    } else if HAS_PREFIXES && let Some(p) = self.prefix {
                        if p.has_implicit_root() && !p.is_verbatim() {
                            return Some(Component::RootDir);
                        }
                    } else if self.include_cur_dir() {
                        self.path = &self.path[..self.path.len() - 1];
                        return Some(Component::CurDir);
                    }
                }
                _ if !HAS_PREFIXES => unreachable!(),
                State::Prefix if self.prefix_len() > 0 => {
                    self.back = State::Done;
                    return Some(Component::Prefix(PrefixComponent {
                        raw: unsafe { OsStr::from_encoded_bytes_unchecked(self.path) },
                        parsed: self.prefix.unwrap(),
                    }));
                }
                State::Prefix => {
                    self.back = State::Done;
                    return None;
                }
                State::Done => unreachable!(),
            }
        }
        None
    }
}

impl FusedIterator for Components<'_> {}

impl<'a> PartialEq for Components<'a> {
    #[inline]
    fn eq(&self, other: &Components<'a>) -> bool {
        let Components { path: _, front: _, back: _, has_physical_root: _, prefix: _ } = self;

        // Fast path for exact matches, e.g. for hashmap lookups.
        if self.path.len() == other.path.len()
            && self.front == other.front
            && self.back == State::Body
            && other.back == State::Body
            && self.prefix_verbatim() == other.prefix_verbatim()
            && self.path == other.path
        {
            return true;
        }

        // compare back to front since absolute paths often share long prefixes
        Iterator::eq(self.clone().rev(), other.clone().rev())
    }
}

impl Eq for Components<'_> {}

impl<'a> PartialOrd for Components<'a> {
    #[inline]
    fn partial_cmp(&self, other: &Components<'a>) -> Option<cmp::Ordering> {
        Some(self.cmp(other))
    }
}

impl Ord for Components<'_> {
    #[inline]
    fn cmp(&self, other: &Self) -> cmp::Ordering {
        compare_components(self.clone(), other.clone())
    }
}

fn compare_components(mut left: Components<'_>, mut right: Components<'_>) -> cmp::Ordering {
    // Fast path for long shared prefixes
    if left.prefix.is_none() && right.prefix.is_none() && left.front == right.front {
        let first_difference = match left.path.iter().zip(right.path).position(|(&a, &b)| a != b) {
            None if left.path.len() == right.path.len() => return cmp::Ordering::Equal,
            None => left.path.len().min(right.path.len()),
            Some(diff) => diff,
        };

        if let Some(previous_sep) =
            left.path[..first_difference].iter().rposition(|&b| left.is_sep_byte(b))
        {
            let mismatched_component_start = previous_sep + 1;
            left.path = &left.path[mismatched_component_start..];
            left.front = State::Body;
            right.path = &right.path[mismatched_component_start..];
            right.front = State::Body;
        }
    }

    Iterator::cmp(left, right)
}

/// An iterator over [`Path`] and its ancestors.
///
/// This `struct` is created by the [`ancestors`] method on [`Path`].
/// See its documentation for more.
///
/// [`ancestors`]: Path::ancestors
#[derive(Copy, Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
pub struct Ancestors<'a> {
    next: Option<&'a Path>,
}

impl<'a> Iterator for Ancestors<'a> {
    type Item = &'a Path;

    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        let next = self.next;
        self.next = next.and_then(Path::parent);
        next
    }
}

impl FusedIterator for Ancestors<'_> {}

////////////////////////////////////////////////////////////////////////////////
// Basic types and traits
////////////////////////////////////////////////////////////////////////////////

/// An owned, mutable path (akin to [`String`]).
///
/// This type provides methods like [`push`] and [`set_extension`] that mutate
/// the path in place. It also implements [`Deref`] to [`Path`], meaning that
/// all methods on [`Path`] slices are available on `PathBuf` values as well.
///
/// [`push`]: PathBuf::push
/// [`set_extension`]: PathBuf::set_extension
///
/// More details about the overall approach can be found in
/// the [module documentation](self).
pub struct PathBuf {
    inner: OsString,
}

impl PathBuf {
    /// Allocates an empty `PathBuf`.
    #[must_use]
    #[inline]
    pub const fn new() -> PathBuf {
        PathBuf { inner: OsString::new() }
    }

    /// Creates a new `PathBuf` with a given capacity used to create the
    /// internal [`OsString`]. See [`with_capacity`] defined on [`OsString`].
    ///
    /// [`with_capacity`]: OsString::with_capacity
    #[must_use]
    #[inline]
    pub fn with_capacity(capacity: usize) -> PathBuf {
        PathBuf { inner: OsString::with_capacity(capacity) }
    }

    /// Coerces to a [`Path`] slice.
    #[must_use]
    #[inline]
    pub fn as_path(&self) -> &Path {
        self
    }

    /// Extends `self` with `path`.
    ///
    /// If `path` is absolute, it replaces the current path.
    ///
    /// On Windows:
    ///
    /// * if `path` has a root but no prefix (e.g., `\windows`), it
    ///   replaces everything except for the prefix (if any) of `self`.
    /// * if `path` has a prefix but no root, it replaces `self`.
    /// * if `self` has a verbatim prefix (e.g. `\\?\C:\windows`)
    ///   and `path` is not empty, the new path is normalized: all references
    ///   to `.` and `..` are removed.
    ///
    /// Consider using [`Path::join`] if you need a new `PathBuf` instead of
    /// using this function on a cloned `PathBuf`.
    pub fn push<P: AsRef<Path>>(&mut self, path: P) {
        self._push(path.as_ref())
    }

    fn _push(&mut self, path: &Path) {
        // in general, a separator is needed if the rightmost byte is not a separator
        let buf = self.inner.as_encoded_bytes();
        let mut need_sep = buf.last().map(|c| !is_sep_byte(*c)).unwrap_or(false);

        // in the special case of `C:` on Windows, do *not* add a separator
        let comps = self.components();

        if comps.prefix_len() > 0
            && comps.prefix_len() == comps.path.len()
            && comps.prefix.unwrap().is_drive()
        {
            need_sep = false
        }

        // absolute `path` replaces `self`
        if path.is_absolute() || path.prefix().is_some() {
            self.inner.truncate(0);

        // verbatim paths need . and .. removed
        } else if comps.prefix_verbatim() && !path.inner.is_empty() {
            let mut buf: Vec<_> = comps.collect();
            for c in path.components() {
                match c {
                    Component::RootDir => {
                        buf.truncate(1);
                        buf.push(c);
                    }
                    Component::CurDir => (),
                    Component::ParentDir => {
                        if let Some(Component::Normal(_)) = buf.last() {
                            buf.pop();
                        }
                    }
                    _ => buf.push(c),
                }
            }

            let mut res = OsString::new();
            let mut need_sep = false;

            for c in buf {
                if need_sep && c != Component::RootDir {
                    res.push(MAIN_SEP_STR);
                }
                res.push(c.as_os_str());

                need_sep = match c {
                    Component::RootDir => false,
                    Component::Prefix(prefix) => {
                        !prefix.parsed.is_drive() && prefix.parsed.len() > 0
                    }
                    _ => true,
                }
            }

            self.inner = res;
            return;

        // `path` has a root but no prefix, e.g., `\windows` (Windows only)
        } else if path.has_root() {
            let prefix_len = self.components().prefix_remaining();
            self.inner.truncate(prefix_len);

        // `path` is a pure relative path
        } else if need_sep {
            self.inner.push(MAIN_SEP_STR);
        }

        self.inner.push(path);
    }

    /// Truncates `self` to [`self.parent`].
    ///
    /// Returns `false` and does nothing if [`self.parent`] is [`None`].
    /// Otherwise, returns `true`.
    ///
    /// [`self.parent`]: Path::parent
    pub fn pop(&mut self) -> bool {
        match self.parent().map(|p| p.as_u8_slice().len()) {
            Some(len) => {
                self.inner.truncate(len);
                true
            }
            None => false,
        }
    }

    /// Updates [`self.file_name`] to `file_name`.
    ///
    /// If [`self.file_name`] was [`None`], this is equivalent to pushing
    /// `file_name`.
    ///
    /// Otherwise it is equivalent to calling [`pop`] and then pushing
    /// `file_name`. The new path will be a sibling of the original path.
    /// (That is, it will have the same parent.)
    ///
    /// [`self.file_name`]: Path::file_name
    /// [`pop`]: PathBuf::pop
    pub fn set_file_name<S: AsRef<OsStr>>(&mut self, file_name: S) {
        self._set_file_name(file_name.as_ref())
    }

    fn _set_file_name(&mut self, file_name: &OsStr) {
        if self.file_name().is_some() {
            let popped = self.pop();
            debug_assert!(popped);
        }
        self.push(file_name);
    }

    /// Updates [`self.extension`] to `Some(extension)` or to `None` if
    /// `extension` is empty.
    ///
    /// Returns `false` and does nothing if [`self.file_name`] is [`None`],
    /// returns `true` and updates the extension otherwise.
    ///
    /// If [`self.extension`] is [`None`], the extension is added; otherwise
    /// it is replaced.
    ///
    /// If `extension` is the empty string, [`self.extension`] will be [`None`]
    /// afterwards, not `Some("")`.
    ///
    /// # Panics
    ///
    /// Panics if the passed extension contains a path separator (see
    /// [`is_separator`]).
    ///
    /// [`self.file_name`]: Path::file_name
    /// [`self.extension`]: Path::extension
    pub fn set_extension<S: AsRef<OsStr>>(&mut self, extension: S) -> bool {
        self._set_extension(extension.as_ref())
    }

    fn _set_extension(&mut self, extension: &OsStr) -> bool {
        validate_extension(extension);

        let file_stem = match self.file_stem() {
            None => return false,
            Some(f) => f.as_encoded_bytes(),
        };

        // truncate until right after the file stem
        let end_file_stem = file_stem[file_stem.len()..].as_ptr().addr();
        let start = self.inner.as_encoded_bytes().as_ptr().addr();
        self.inner.truncate(end_file_stem.wrapping_sub(start));

        // add the new extension, if any
        let new = extension.as_encoded_bytes();
        if !new.is_empty() {
            self.inner.reserve_exact(new.len() + 1);
            self.inner.push(".");
            // SAFETY: Since a UTF-8 string was just pushed, it is not possible
            // for the buffer to end with a surrogate half.
            unsafe { self.inner.extend_from_slice_unchecked(new) };
        }

        true
    }

    /// Yields a mutable reference to the underlying [`OsString`] instance.
    #[must_use]
    #[inline]
    pub fn as_mut_os_string(&mut self) -> &mut OsString {
        &mut self.inner
    }

    /// Consumes the `PathBuf`, yielding its internal [`OsString`] storage.
    #[must_use = "`self` will be dropped if the result is not used"]
    #[inline]
    pub fn into_os_string(self) -> OsString {
        self.inner
    }

    /// Create a `PathBuf` from an `OsString` (used internally for From<OsString>).
    #[inline]
    pub(crate) fn from_os_string(s: OsString) -> PathBuf {
        PathBuf { inner: s }
    }

    /// Converts this `PathBuf` into a [boxed](Box) [`Path`].
    #[must_use = "`self` will be dropped if the result is not used"]
    #[inline]
    pub fn into_boxed_path(self) -> Box<Path> {
        let rw = Box::into_raw(self.inner.into_boxed_os_str()) as *mut Path;
        unsafe { Box::from_raw(rw) }
    }

    /// Invokes [`capacity`] on the underlying instance of [`OsString`].
    ///
    /// [`capacity`]: OsString::capacity
    #[must_use]
    #[inline]
    pub fn capacity(&self) -> usize {
        self.inner.capacity()
    }

    /// Invokes [`clear`] on the underlying instance of [`OsString`].
    ///
    /// [`clear`]: OsString::clear
    #[inline]
    pub fn clear(&mut self) {
        self.inner.clear()
    }

    /// Invokes [`reserve`] on the underlying instance of [`OsString`].
    ///
    /// [`reserve`]: OsString::reserve
    #[inline]
    pub fn reserve(&mut self, additional: usize) {
        self.inner.reserve(additional)
    }

    /// Invokes [`try_reserve`] on the underlying instance of [`OsString`].
    ///
    /// [`try_reserve`]: OsString::try_reserve
    #[inline]
    pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
        self.inner.try_reserve(additional)
    }

    /// Invokes [`reserve_exact`] on the underlying instance of [`OsString`].
    ///
    /// [`reserve_exact`]: OsString::reserve_exact
    #[inline]
    pub fn reserve_exact(&mut self, additional: usize) {
        self.inner.reserve_exact(additional)
    }

    /// Invokes [`try_reserve_exact`] on the underlying instance of [`OsString`].
    ///
    /// [`try_reserve_exact`]: OsString::try_reserve_exact
    #[inline]
    pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), TryReserveError> {
        self.inner.try_reserve_exact(additional)
    }

    /// Invokes [`shrink_to_fit`] on the underlying instance of [`OsString`].
    ///
    /// [`shrink_to_fit`]: OsString::shrink_to_fit
    #[inline]
    pub fn shrink_to_fit(&mut self) {
        self.inner.shrink_to_fit()
    }

    /// Invokes [`shrink_to`] on the underlying instance of [`OsString`].
    ///
    /// [`shrink_to`]: OsString::shrink_to
    #[inline]
    pub fn shrink_to(&mut self, min_capacity: usize) {
        self.inner.shrink_to(min_capacity)
    }
}

impl Clone for PathBuf {
    #[inline]
    fn clone(&self) -> Self {
        PathBuf { inner: self.inner.clone() }
    }

    #[inline]
    fn clone_from(&mut self, source: &Self) {
        self.inner.clone_from(&source.inner)
    }
}

impl From<&Path> for Box<Path> {
    /// Creates a boxed [`Path`] from a reference.
    ///
    /// This will allocate and clone `path` to it.
    fn from(path: &Path) -> Box<Path> {
        let boxed: Box<OsStr> = path.inner.to_os_string().into_boxed_os_str();
        let rw = Box::into_raw(boxed) as *mut Path;
        unsafe { Box::from_raw(rw) }
    }
}

impl From<&mut Path> for Box<Path> {
    /// Creates a boxed [`Path`] from a reference.
    ///
    /// This will allocate and clone `path` to it.
    fn from(path: &mut Path) -> Box<Path> {
        Self::from(&*path)
    }
}

impl From<Cow<'_, Path>> for Box<Path> {
    /// Creates a boxed [`Path`] from a clone-on-write pointer.
    ///
    /// Converting from a `Cow::Owned` does not clone or allocate.
    #[inline]
    fn from(cow: Cow<'_, Path>) -> Box<Path> {
        match cow {
            Cow::Borrowed(path) => Box::from(path),
            Cow::Owned(path) => Box::from(path),
        }
    }
}

impl From<Box<Path>> for PathBuf {
    /// Converts a <code>[Box]&lt;[Path]&gt;</code> into a [`PathBuf`].
    ///
    /// This conversion does not allocate or copy memory.
    #[inline]
    fn from(boxed: Box<Path>) -> PathBuf {
        boxed.into_path_buf()
    }
}

impl From<PathBuf> for Box<Path> {
    /// Converts a [`PathBuf`] into a <code>[Box]&lt;[Path]&gt;</code>.
    ///
    /// This conversion currently should not allocate memory,
    /// but this behavior is not guaranteed on all platforms or in all future versions.
    #[inline]
    fn from(p: PathBuf) -> Box<Path> {
        p.into_boxed_path()
    }
}

impl Clone for Box<Path> {
    #[inline]
    fn clone(&self) -> Self {
        self.to_path_buf().into_boxed_path()
    }
}

impl<T: ?Sized + AsRef<OsStr>> From<&T> for PathBuf {
    /// Converts a borrowed [`OsStr`] to a [`PathBuf`].
    ///
    /// Allocates a [`PathBuf`] and copies the data into it.
    #[inline]
    fn from(s: &T) -> PathBuf {
        PathBuf::from(s.as_ref().to_os_string())
    }
}

impl From<String> for PathBuf {
    /// Converts a [`String`] into a [`PathBuf`]
    ///
    /// This conversion does not allocate or copy memory.
    #[inline]
    fn from(s: String) -> PathBuf {
        PathBuf::from(OsString::from(s))
    }
}

impl FromStr for PathBuf {
    type Err = core::convert::Infallible;

    #[inline]
    fn from_str(s: &str) -> Result<Self, Self::Err> {
        Ok(PathBuf::from(s))
    }
}

impl<P: AsRef<Path>> FromIterator<P> for PathBuf {
    fn from_iter<I: IntoIterator<Item = P>>(iter: I) -> PathBuf {
        let mut buf = PathBuf::new();
        buf.extend(iter);
        buf
    }
}

impl<P: AsRef<Path>> Extend<P> for PathBuf {
    fn extend<I: IntoIterator<Item = P>>(&mut self, iter: I) {
        iter.into_iter().for_each(move |p| self.push(p.as_ref()));
    }

}

impl fmt::Debug for PathBuf {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&**self, formatter)
    }
}

impl ops::Deref for PathBuf {
    type Target = Path;
    #[inline]
    fn deref(&self) -> &Path {
        Path::new(&self.inner)
    }
}

impl ops::DerefMut for PathBuf {
    #[inline]
    fn deref_mut(&mut self) -> &mut Path {
        Path::from_inner_mut(&mut self.inner)
    }
}

impl Borrow<Path> for PathBuf {
    #[inline]
    fn borrow(&self) -> &Path {
        self.deref()
    }
}

impl Default for PathBuf {
    #[inline]
    fn default() -> Self {
        PathBuf::new()
    }
}

impl<'a> From<&'a Path> for Cow<'a, Path> {
    #[inline]
    fn from(s: &'a Path) -> Cow<'a, Path> {
        Cow::Borrowed(s)
    }
}

impl<'a> From<PathBuf> for Cow<'a, Path> {
    #[inline]
    fn from(s: PathBuf) -> Cow<'a, Path> {
        Cow::Owned(s)
    }
}

impl<'a> From<&'a PathBuf> for Cow<'a, Path> {
    #[inline]
    fn from(p: &'a PathBuf) -> Cow<'a, Path> {
        Cow::Borrowed(p.as_path())
    }
}

impl<'a> From<Cow<'a, Path>> for PathBuf {
    #[inline]
    fn from(p: Cow<'a, Path>) -> Self {
        p.into_owned()
    }
}

impl From<PathBuf> for Arc<Path> {
    /// Converts a [`PathBuf`] into an <code>[Arc]<[Path]></code> by moving the [`PathBuf`] data
    /// into a new [`Arc`] buffer.
    #[inline]
    fn from(s: PathBuf) -> Arc<Path> {
        let arc: Arc<OsStr> = Arc::from(s.into_os_string());
        unsafe { Arc::from_raw(Arc::into_raw(arc) as *const Path) }
    }
}

impl From<&Path> for Arc<Path> {
    /// Converts a [`Path`] into an [`Arc`] by copying the [`Path`] data into a new [`Arc`] buffer.
    #[inline]
    fn from(s: &Path) -> Arc<Path> {
        let arc: Arc<OsStr> = Arc::from(s.as_os_str());
        unsafe { Arc::from_raw(Arc::into_raw(arc) as *const Path) }
    }
}

impl From<&mut Path> for Arc<Path> {
    /// Converts a [`Path`] into an [`Arc`] by copying the [`Path`] data into a new [`Arc`] buffer.
    #[inline]
    fn from(s: &mut Path) -> Arc<Path> {
        Arc::from(&*s)
    }
}

impl From<PathBuf> for Rc<Path> {
    /// Converts a [`PathBuf`] into an <code>[Rc]<[Path]></code> by moving the [`PathBuf`] data into
    /// a new [`Rc`] buffer.
    #[inline]
    fn from(s: PathBuf) -> Rc<Path> {
        let rc: Rc<OsStr> = Rc::from(s.into_os_string());
        unsafe { Rc::from_raw(Rc::into_raw(rc) as *const Path) }
    }
}

impl From<&Path> for Rc<Path> {
    /// Converts a [`Path`] into an [`Rc`] by copying the [`Path`] data into a new [`Rc`] buffer.
    #[inline]
    fn from(s: &Path) -> Rc<Path> {
        let rc: Rc<OsStr> = Rc::from(s.as_os_str());
        unsafe { Rc::from_raw(Rc::into_raw(rc) as *const Path) }
    }
}

impl From<&mut Path> for Rc<Path> {
    /// Converts a [`Path`] into an [`Rc`] by copying the [`Path`] data into a new [`Rc`] buffer.
    #[inline]
    fn from(s: &mut Path) -> Rc<Path> {
        Rc::from(&*s)
    }
}

impl ToOwned for Path {
    type Owned = PathBuf;
    #[inline]
    fn to_owned(&self) -> PathBuf {
        self.to_path_buf()
    }
    #[inline]
    fn clone_into(&self, target: &mut PathBuf) {
        self.inner.clone_into(&mut target.inner);
    }
}

impl PartialEq for PathBuf {
    #[inline]
    fn eq(&self, other: &PathBuf) -> bool {
        self.components() == other.components()
    }
}

impl Hash for PathBuf {
    fn hash<H: Hasher>(&self, h: &mut H) {
        self.as_path().hash(h)
    }
}

impl Eq for PathBuf {}

impl PartialOrd for PathBuf {
    #[inline]
    fn partial_cmp(&self, other: &PathBuf) -> Option<cmp::Ordering> {
        Some(self.cmp(other))
    }
}

impl Ord for PathBuf {
    #[inline]
    fn cmp(&self, other: &PathBuf) -> cmp::Ordering {
        compare_components(self.components(), other.components())
    }
}

impl AsRef<OsStr> for PathBuf {
    #[inline]
    fn as_ref(&self) -> &OsStr {
        &self.inner[..]
    }
}

/// A slice of a path (akin to [`str`]).
///
/// This type supports a number of operations for inspecting a path, including
/// breaking the path into its components (separated by `/` on Unix and by either
/// `/` or `\` on Windows), extracting the file name, determining whether the path
/// is absolute, and so on.
///
/// This is an *unsized* type, meaning that it must always be used behind a
/// pointer like `&` or [`Box`]. For an owned version of this type,
/// see [`PathBuf`].
///
/// More details about the overall approach can be found in
/// the [module documentation](self).
// `Path::new` current implementation relies on `Path` being layout-compatible with `OsStr`.
// However, `Path` layout is considered an implementation detail and must not be relied upon.
#[repr(transparent)]
pub struct Path {
    inner: OsStr,
}

/// An error returned from [`Path::strip_prefix`] if the prefix was not found.
///
/// This `struct` is created by the [`strip_prefix`] method on [`Path`].
/// See its documentation for more.
///
/// [`strip_prefix`]: Path::strip_prefix
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct StripPrefixError(());

impl Path {
    // The following (private!) function allows construction of a path from a u8
    // slice, which is only safe when it is known to follow the OsStr encoding.
    unsafe fn from_u8_slice(s: &[u8]) -> &Path {
        unsafe { Path::new(OsStr::from_encoded_bytes_unchecked(s)) }
    }
    // The following (private!) function reveals the byte encoding used for OsStr.
    pub(crate) fn as_u8_slice(&self) -> &[u8] {
        self.inner.as_encoded_bytes()
    }

    /// Directly wraps a string slice as a `Path` slice.
    ///
    /// This is a cost-free conversion.
    pub fn new<S: AsRef<OsStr> + ?Sized>(s: &S) -> &Path {
        unsafe { &*(s.as_ref() as *const OsStr as *const Path) }
    }

    fn from_inner_mut(inner: &mut OsStr) -> &mut Path {
        // SAFETY: Path is just a wrapper around OsStr,
        // therefore converting &mut OsStr to &mut Path is safe.
        unsafe { &mut *(inner as *mut OsStr as *mut Path) }
    }

    /// Yields the underlying [`OsStr`] slice.
    #[must_use]
    #[inline]
    pub fn as_os_str(&self) -> &OsStr {
        &self.inner
    }

    /// Yields a mutable reference to the underlying [`OsStr`] slice.
    #[must_use]
    #[inline]
    pub fn as_mut_os_str(&mut self) -> &mut OsStr {
        &mut self.inner
    }

    /// Yields a [`&str`] slice if the `Path` is valid unicode.
    ///
    /// [`&str`]: str
    #[must_use = "this returns the result of the operation, \
                  without modifying the original"]
    #[inline]
    pub fn to_str(&self) -> Option<&str> {
        self.inner.to_str()
    }

    /// Converts a `Path` to a [`Cow<str>`].
    ///
    /// Any non-UTF-8 sequences are replaced with
    /// [`U+FFFD REPLACEMENT CHARACTER`][U+FFFD].
    ///
    /// [U+FFFD]: core::char::REPLACEMENT_CHARACTER
    #[must_use = "this returns the result of the operation, \
                  without modifying the original"]
    #[inline]
    pub fn to_string_lossy(&self) -> Cow<'_, str> {
        self.inner.to_string_lossy()
    }

    /// Converts a `Path` to an owned [`PathBuf`].
    #[must_use = "this returns the result of the operation, \
                  without modifying the original"]
    pub fn to_path_buf(&self) -> PathBuf {
        PathBuf::from(self.inner.to_os_string())
    }

    /// Returns `true` if the `Path` is absolute, i.e., if it is independent of
    /// the current directory.
    ///
    /// * On Unix, a path is absolute if it starts with the root, so
    ///   `is_absolute` and [`has_root`] are equivalent.
    ///
    /// [`has_root`]: Path::has_root
    #[must_use]
    pub fn is_absolute(&self) -> bool {
        // For Unix: absolute == has_root
        self.has_root()
    }

    /// Returns `true` if the `Path` is relative, i.e., not absolute.
    ///
    /// See [`is_absolute`]'s documentation for more details.
    ///
    /// [`is_absolute`]: Path::is_absolute
    #[must_use]
    #[inline]
    pub fn is_relative(&self) -> bool {
        !self.is_absolute()
    }

    pub(crate) fn prefix(&self) -> Option<Prefix<'_>> {
        self.components().prefix
    }

    /// Returns `true` if the `Path` has a root.
    ///
    /// * On Unix, a path has a root if it begins with `/`.
    #[must_use]
    #[inline]
    pub fn has_root(&self) -> bool {
        self.components().has_root()
    }

    /// Returns the `Path` without its final component, if there is one.
    ///
    /// This means it returns `Some("")` for relative paths with one component.
    ///
    /// Returns [`None`] if the path terminates in a root or prefix, or if it's
    /// the empty string.
    #[must_use]
    pub fn parent(&self) -> Option<&Path> {
        let mut comps = self.components();
        let comp = comps.next_back();
        comp.and_then(|p| match p {
            Component::Normal(_) | Component::CurDir | Component::ParentDir => {
                Some(comps.as_path())
            }
            _ => None,
        })
    }

    /// Produces an iterator over `Path` and its ancestors.
    ///
    /// The iterator will yield the `Path` that is returned if the [`parent`] method is used zero
    /// or more times. If the [`parent`] method returns [`None`], the iterator will do likewise.
    /// The iterator will always yield at least one value, namely `Some(&self)`. Next it will yield
    /// `&self.parent()`, `&self.parent().and_then(Path::parent)` and so on.
    ///
    /// [`parent`]: Path::parent
    #[inline]
    pub fn ancestors(&self) -> Ancestors<'_> {
        Ancestors { next: Some(self) }
    }

    /// Returns the final component of the `Path`, if there is one.
    ///
    /// If the path is a normal file, this is the file name. If it's the path of a directory, this
    /// is the directory name.
    ///
    /// Returns [`None`] if the path terminates in `..`.
    #[must_use]
    pub fn file_name(&self) -> Option<&OsStr> {
        self.components().next_back().and_then(|p| match p {
            Component::Normal(p) => Some(p),
            _ => None,
        })
    }

    /// Returns a path that, when joined onto `base`, yields `self`.
    ///
    /// # Errors
    ///
    /// If `base` is not a prefix of `self` (i.e., [`starts_with`]
    /// returns `false`), returns [`Err`].
    ///
    /// [`starts_with`]: Path::starts_with
    pub fn strip_prefix<P>(&self, base: P) -> Result<&Path, StripPrefixError>
    where
        P: AsRef<Path>,
    {
        self._strip_prefix(base.as_ref())
    }

    fn _strip_prefix(&self, base: &Path) -> Result<&Path, StripPrefixError> {
        iter_after(self.components(), base.components())
            .map(|c| c.as_path())
            .ok_or(StripPrefixError(()))
    }

    /// Determines whether `base` is a prefix of `self`.
    ///
    /// Only considers whole path components to match.
    #[must_use]
    pub fn starts_with<P: AsRef<Path>>(&self, base: P) -> bool {
        self._starts_with(base.as_ref())
    }

    fn _starts_with(&self, base: &Path) -> bool {
        iter_after(self.components(), base.components()).is_some()
    }

    /// Determines whether `child` is a suffix of `self`.
    ///
    /// Only considers whole path components to match.
    #[must_use]
    pub fn ends_with<P: AsRef<Path>>(&self, child: P) -> bool {
        self._ends_with(child.as_ref())
    }

    fn _ends_with(&self, child: &Path) -> bool {
        iter_after(self.components().rev(), child.components().rev()).is_some()
    }

    /// Extracts the stem (non-extension) portion of [`self.file_name`].
    ///
    /// [`self.file_name`]: Path::file_name
    ///
    /// The stem is:
    ///
    /// * [`None`], if there is no file name;
    /// * The entire file name if there is no embedded `.`;
    /// * The entire file name if the file name begins with `.` and has no other `.`s within;
    /// * Otherwise, the portion of the file name before the final `.`
    #[must_use]
    pub fn file_stem(&self) -> Option<&OsStr> {
        self.file_name().map(rsplit_file_at_dot).and_then(|(before, after)| before.or(after))
    }

    /// Extracts the extension (without the leading dot) of [`self.file_name`], if possible.
    ///
    /// The extension is:
    ///
    /// * [`None`], if there is no file name;
    /// * [`None`], if there is no embedded `.`;
    /// * [`None`], if the file name begins with `.` and has no other `.`s within;
    /// * Otherwise, the portion of the file name after the final `.`
    ///
    /// [`self.file_name`]: Path::file_name
    #[must_use]
    pub fn extension(&self) -> Option<&OsStr> {
        self.file_name().map(rsplit_file_at_dot).and_then(|(before, after)| before.and(after))
    }

    /// Creates an owned [`PathBuf`] with `path` adjoined to `self`.
    ///
    /// If `path` is absolute, it replaces the current path.
    ///
    /// See [`PathBuf::push`] for more details on what it means to adjoin a path.
    #[must_use]
    pub fn join<P: AsRef<Path>>(&self, path: P) -> PathBuf {
        self._join(path.as_ref())
    }

    fn _join(&self, path: &Path) -> PathBuf {
        let mut buf = self.to_path_buf();
        buf.push(path);
        buf
    }

    /// Creates an owned [`PathBuf`] like `self` but with the given file name.
    ///
    /// See [`PathBuf::set_file_name`] for more details.
    #[must_use]
    pub fn with_file_name<S: AsRef<OsStr>>(&self, file_name: S) -> PathBuf {
        self._with_file_name(file_name.as_ref())
    }

    fn _with_file_name(&self, file_name: &OsStr) -> PathBuf {
        let mut buf = self.to_path_buf();
        buf.set_file_name(file_name);
        buf
    }

    /// Creates an owned [`PathBuf`] like `self` but with the given extension.
    ///
    /// See [`PathBuf::set_extension`] for more details.
    pub fn with_extension<S: AsRef<OsStr>>(&self, extension: S) -> PathBuf {
        self._with_extension(extension.as_ref())
    }

    fn _with_extension(&self, extension: &OsStr) -> PathBuf {
        let self_len = self.as_os_str().len();
        let self_bytes = self.as_os_str().as_encoded_bytes();

        let (new_capacity, slice_to_copy) = match self.extension() {
            None => {
                // Enough capacity for the extension and the dot
                let capacity = self_len + extension.len() + 1;
                let whole_path = self_bytes;
                (capacity, whole_path)
            }
            Some(previous_extension) => {
                let capacity = self_len + extension.len() - previous_extension.len();
                let path_till_dot = &self_bytes[..self_len - previous_extension.len()];
                (capacity, path_till_dot)
            }
        };

        let mut new_path = PathBuf::with_capacity(new_capacity);
        // SAFETY: The path is empty, so cannot have surrogate halves.
        unsafe { new_path.inner.extend_from_slice_unchecked(slice_to_copy) };
        new_path.set_extension(extension);
        new_path
    }

    /// Produces an iterator over the [`Component`]s of the path.
    ///
    /// When parsing the path, there is a small amount of normalization:
    ///
    /// * Repeated separators are ignored, so `a/b` and `a//b` both have
    ///   `a` and `b` as components.
    ///
    /// * Occurrences of `.` are normalized away, except if they are at the
    ///   beginning of the path. For example, `a/./b`, `a/b/`, `a/b/.` and
    ///   `a/b` all have `a` and `b` as components, but `./a/b` starts with
    ///   an additional [`CurDir`] component.
    ///
    /// * Trailing separators are normalized away, so `/a/b` and `/a/b/` are equivalent.
    ///
    /// Note that no other normalization takes place; in particular, `a/c`
    /// and `a/b/../c` are distinct, to account for the possibility that `b`
    /// is a symbolic link (so its parent isn't `a`).
    ///
    /// [`CurDir`]: Component::CurDir
    pub fn components(&self) -> Components<'_> {
        let prefix = parse_prefix(self.as_os_str());
        Components {
            path: self.as_u8_slice(),
            prefix,
            has_physical_root: has_physical_root(self.as_u8_slice(), prefix),
            // Since HAS_PREFIXES is false for our Unix-only path, start at StartDir.
            front: if HAS_PREFIXES { State::Prefix } else { State::StartDir },
            back: State::Body,
        }
    }

    /// Produces an iterator over the path's components viewed as [`OsStr`]
    /// slices.
    ///
    /// For more information about the particulars of how the path is separated
    /// into components, see [`components`].
    ///
    /// [`components`]: Path::components
    #[inline]
    pub fn iter(&self) -> Iter<'_> {
        Iter { inner: self.components() }
    }

    /// Returns an object that implements [`Display`] for safely printing paths
    /// that may contain non-Unicode data. This may perform lossy conversion,
    /// depending on the platform.
    ///
    /// [`Display`]: fmt::Display
    #[must_use = "this does not display the path, \
                  it returns an object that can be displayed"]
    #[inline]
    pub fn display(&self) -> Display<'_> {
        Display { inner: self.inner.display() }
    }

    /// Returns the same path as `&Path`.
    #[inline]
    pub fn as_path(&self) -> &Path {
        self
    }

    /// Converts a [`Box<Path>`](Box) into a [`PathBuf`] without copying or
    /// allocating.
    #[must_use = "`self` will be dropped if the result is not used"]
    pub fn into_path_buf(self: Box<Self>) -> PathBuf {
        let rw = Box::into_raw(self) as *mut OsStr;
        let inner = unsafe { Box::from_raw(rw) };
        PathBuf { inner: OsString::from(inner) }
    }
}

impl AsRef<OsStr> for Path {
    #[inline]
    fn as_ref(&self) -> &OsStr {
        &self.inner
    }
}

impl fmt::Debug for Path {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&self.inner, formatter)
    }
}

/// Helper struct for safely printing paths with [`format!`] and `{}`.
///
/// A [`Path`] might contain non-Unicode data. This `struct` implements the
/// [`Display`] trait in a way that mitigates that. It is created by the
/// [`display`](Path::display) method on [`Path`]. This may perform lossy
/// conversion, depending on the platform.
///
/// [`Display`]: fmt::Display
pub struct Display<'a> {
    inner: crate::ffi::Display<'a>,
}

impl fmt::Debug for Display<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&self.inner, f)
    }
}

impl fmt::Display for Display<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(&self.inner, f)
    }
}

impl PartialEq for Path {
    #[inline]
    fn eq(&self, other: &Path) -> bool {
        self.components() == other.components()
    }
}

impl Hash for Path {
    fn hash<H: Hasher>(&self, h: &mut H) {
        let bytes = self.as_u8_slice();
        let (prefix_len, verbatim) = match parse_prefix(&self.inner) {
            Some(prefix) => {
                prefix.hash(h);
                (prefix.len(), prefix.is_verbatim())
            }
            None => (0, false),
        };
        let bytes = &bytes[prefix_len..];

        let mut component_start = 0;
        let mut chunk_bits: usize = 0;

        for i in 0..bytes.len() {
            let is_sep = if verbatim { is_verbatim_sep(bytes[i]) } else { is_sep_byte(bytes[i]) };
            if is_sep {
                if i > component_start {
                    let to_hash = &bytes[component_start..i];
                    chunk_bits = chunk_bits.wrapping_add(to_hash.len());
                    chunk_bits = chunk_bits.rotate_right(2);
                    h.write(to_hash);
                }

                component_start = i + 1;

                let tail = &bytes[component_start..];

                if !verbatim {
                    component_start += match tail {
                        [b'.'] => 1,
                        [b'.', sep, ..] if is_sep_byte(*sep) => 1,
                        _ => 0,
                    };
                }
            }
        }

        if component_start < bytes.len() {
            let to_hash = &bytes[component_start..];
            chunk_bits = chunk_bits.wrapping_add(to_hash.len());
            chunk_bits = chunk_bits.rotate_right(2);
            h.write(to_hash);
        }

        h.write_usize(chunk_bits);
    }
}

impl Eq for Path {}

impl PartialOrd for Path {
    #[inline]
    fn partial_cmp(&self, other: &Path) -> Option<cmp::Ordering> {
        Some(self.cmp(other))
    }
}

impl Ord for Path {
    #[inline]
    fn cmp(&self, other: &Path) -> cmp::Ordering {
        compare_components(self.components(), other.components())
    }
}

impl AsRef<Path> for Path {
    #[inline]
    fn as_ref(&self) -> &Path {
        self
    }
}

impl AsRef<Path> for Cow<'_, OsStr> {
    #[inline]
    fn as_ref(&self) -> &Path {
        Path::new(self)
    }
}

impl AsRef<Path> for str {
    #[inline]
    fn as_ref(&self) -> &Path {
        Path::new(self)
    }
}

impl AsRef<Path> for String {
    #[inline]
    fn as_ref(&self) -> &Path {
        Path::new(self)
    }
}

impl AsRef<Path> for PathBuf {
    #[inline]
    fn as_ref(&self) -> &Path {
        self
    }
}

impl<'a> IntoIterator for &'a PathBuf {
    type Item = &'a OsStr;
    type IntoIter = Iter<'a>;
    #[inline]
    fn into_iter(self) -> Iter<'a> {
        self.iter()
    }
}

impl<'a> IntoIterator for &'a Path {
    type Item = &'a OsStr;
    type IntoIter = Iter<'a>;
    #[inline]
    fn into_iter(self) -> Iter<'a> {
        self.iter()
    }
}

macro_rules! impl_cmp {
    (<$($life:lifetime),*> $lhs:ty, $rhs: ty) => {
        impl<$($life),*> PartialEq<$rhs> for $lhs {
            #[inline]
            fn eq(&self, other: &$rhs) -> bool {
                <Path as PartialEq>::eq(self, other)
            }
        }

        impl<$($life),*> PartialEq<$lhs> for $rhs {
            #[inline]
            fn eq(&self, other: &$lhs) -> bool {
                <Path as PartialEq>::eq(self, other)
            }
        }

        impl<$($life),*> PartialOrd<$rhs> for $lhs {
            #[inline]
            fn partial_cmp(&self, other: &$rhs) -> Option<cmp::Ordering> {
                <Path as PartialOrd>::partial_cmp(self, other)
            }
        }

        impl<$($life),*> PartialOrd<$lhs> for $rhs {
            #[inline]
            fn partial_cmp(&self, other: &$lhs) -> Option<cmp::Ordering> {
                <Path as PartialOrd>::partial_cmp(self, other)
            }
        }
    };
}

impl_cmp!(<> PathBuf, Path);
impl_cmp!(<'a> PathBuf, &'a Path);
impl_cmp!(<'a> Cow<'a, Path>, Path);
impl_cmp!(<'a, 'b> Cow<'a, Path>, &'b Path);
impl_cmp!(<'a> Cow<'a, Path>, PathBuf);

macro_rules! impl_cmp_os_str {
    (<$($life:lifetime),*> $lhs:ty, $rhs: ty) => {
        impl<$($life),*> PartialEq<$rhs> for $lhs {
            #[inline]
            fn eq(&self, other: &$rhs) -> bool {
                <Path as PartialEq>::eq(self, other.as_ref())
            }
        }

        impl<$($life),*> PartialEq<$lhs> for $rhs {
            #[inline]
            fn eq(&self, other: &$lhs) -> bool {
                <Path as PartialEq>::eq(self.as_ref(), other)
            }
        }

        impl<$($life),*> PartialOrd<$rhs> for $lhs {
            #[inline]
            fn partial_cmp(&self, other: &$rhs) -> Option<cmp::Ordering> {
                <Path as PartialOrd>::partial_cmp(self, other.as_ref())
            }
        }

        impl<$($life),*> PartialOrd<$lhs> for $rhs {
            #[inline]
            fn partial_cmp(&self, other: &$lhs) -> Option<cmp::Ordering> {
                <Path as PartialOrd>::partial_cmp(self.as_ref(), other)
            }
        }
    };
}

impl_cmp_os_str!(<> PathBuf, OsStr);
impl_cmp_os_str!(<'a> PathBuf, &'a OsStr);
impl_cmp_os_str!(<'a> PathBuf, Cow<'a, OsStr>);
impl_cmp_os_str!(<> PathBuf, OsString);
impl_cmp_os_str!(<> Path, OsStr);
impl_cmp_os_str!(<'a> Path, &'a OsStr);
impl_cmp_os_str!(<'a> Path, Cow<'a, OsStr>);
impl_cmp_os_str!(<> Path, OsString);
impl_cmp_os_str!(<'a> &'a Path, OsStr);
impl_cmp_os_str!(<'a, 'b> &'a Path, Cow<'b, OsStr>);
impl_cmp_os_str!(<'a> &'a Path, OsString);
impl_cmp_os_str!(<'a> Cow<'a, Path>, OsStr);
impl_cmp_os_str!(<'a, 'b> Cow<'a, Path>, &'b OsStr);
impl_cmp_os_str!(<'a> Cow<'a, Path>, OsString);

impl fmt::Display for StripPrefixError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        "prefix not found".fmt(f)
    }
}