#![cfg(any(doc, all(unix, feature = "std")))]
use std::env;
use std::io;
#[cfg(not(doc))]
use std::os::unix::ffi::OsStrExt;
use std::path::Component;
use std::path::{Path, PathBuf};

pub trait PosixPathExt: Sealed {
    /// [Unix only] Make a POSIX path absolute without changing its semantics.
    ///
    /// Unlike canonicalize the path does not need to exist. Symlinks and `..`
    /// components will not be resolved.
    ///
    /// # Example
    ///
    /// ```
    /// #[cfg(unix)]
    /// {
    ///     use omnipath::posix::PosixPathExt;
    ///     use std::path::Path;
    ///     use std::env::current_dir;
    ///
    ///     let path = Path::new(r"path/to/..//./file");
    ///     assert_eq!(
    ///         path.posix_absolute().unwrap(),
    ///         current_dir().unwrap().join("path/to/../file")
    ///     )
    /// }
    /// ```
    fn posix_absolute(&self) -> io::Result<PathBuf>;

    /// [Unix only] Make a POSIX path lexically absolute.
    ///
    /// Unlike `canonicalize` the path does not need to exist. Symlinks will not be resolved.
    /// Unlike [`posix_absolute`][PosixPathExt::posix_absolute] this resolves `..` components by popping the
    /// parent component. This means that it may resolve to a different path
    /// than would be resolved by passing the path directly to the OS.
    ///
    /// Usually this is not the preferred behaviour.
    ///
    /// # Example
    ///
    /// ```
    /// #[cfg(unix)]
    /// {
    ///     use omnipath::posix::PosixPathExt;
    ///     use std::path::Path;
    ///     use std::env::current_dir;
    ///
    ///     let path = Path::new(r"path/to/..//./file");
    ///     assert_eq!(
    ///         path.posix_lexically_absolute().unwrap(),
    ///         current_dir().unwrap().join("path/file")
    ///     )
    /// }
    /// ```
    fn posix_lexically_absolute(&self) -> io::Result<PathBuf>;

    /// [Unix only] Make a POSIX path absolute relative to a provided working
    /// directory without changing its semantics.
    ///
    /// See [`PosixPathExt::posix_absolute`] for a version of this function that
    /// is relative to [`std::env::current_dir()`] instead.
    ///
    /// # Example
    ///
    /// ```
    /// #[cfg(unix)]
    /// {
    ///     use omnipath::posix::PosixPathExt;
    ///     use std::path::Path;
    ///     let working_dir = Path::new("/tmp");
    ///     let path = Path::new("path/to/..//./file");
    ///
    ///     assert_eq!(
    ///         &path.posix_absolute_from(working_dir).unwrap(),
    ///         Path::new("/tmp/path/to/../file"),
    ///     )
    /// }
    /// ```
    ///
    /// ```
    /// #[cfg(unix)]
    /// {
    ///     use omnipath::posix::PosixPathExt;
    ///     use std::path::Path;
    ///     use std::env::current_dir;
    ///     let root = Path::new("/tmp/foo//.././bar");
    ///     let path = Path::new(r"path/to/..//./file");
    ///     assert_eq!(
    ///         &path.posix_absolute_from(root).unwrap(),
    ///         Path::new("/tmp/foo/../bar/path/to/../file"),
    ///     );
    /// }
    /// ```
    fn posix_absolute_from(&self, path: &Path) -> io::Result<PathBuf>;

    /// [Unix only] Make a POSIX path lexically absolute relative to a provided
    /// current working directory.
    ///
    /// Unlike `canonicalize` the path does not need to exist. Symlinks will not be resolved.
    /// Unlike [`posix_absolute`][PosixPathExt::posix_absolute] this resolves `..` components by popping the
    /// parent component. This means that it may resolve to a different path
    /// than would be resolved by passing the path directly to the OS.
    ///
    /// Usually this is not the preferred behaviour.
    ///
    /// See [`PosixPathExt::posix_lexically_absolute`] for a version of this function that
    /// is relative to [`std::env::current_dir()`] instead.
    ///
    /// # Example
    ///
    /// ```
    /// #[cfg(unix)]
    /// {
    ///     use omnipath::posix::PosixPathExt;
    ///     use std::path::Path;
    ///     use std::env::current_dir;
    ///     let root = Path::new("/tmp");
    ///     let path = Path::new(r"path/to/..//./file");
    ///     assert_eq!(
    ///         &path.posix_lexically_absolute_from(root).unwrap(),
    ///         Path::new("/tmp/path/file")
    ///     );
    /// }
    /// ```
    ///
    /// ```
    /// #[cfg(unix)]
    /// {
    ///     use omnipath::posix::PosixPathExt;
    ///     use std::path::Path;
    ///     use std::env::current_dir;
    ///     let root = Path::new("/tmp/foo//.././bar");
    ///     let path = Path::new(r"path/to/..//./file");
    ///     assert_eq!(
    ///         &path.posix_lexically_absolute_from(root).unwrap(),
    ///         Path::new("/tmp/bar/path/file")
    ///     );
    /// }
    /// ```
    fn posix_lexically_absolute_from(&self, cwd: &Path) -> io::Result<PathBuf>;
}

impl PosixPathExt for Path {
    fn posix_absolute(&self) -> io::Result<PathBuf> {
        posix_absolute_from(self, env::current_dir)
    }

    fn posix_lexically_absolute(&self) -> io::Result<PathBuf> {
        posix_lexically_absolute_from(self, env::current_dir)
    }

    fn posix_absolute_from(&self, cwd: &Path) -> io::Result<PathBuf> {
        if !cwd.is_absolute() {
            return Err(cwd_error());
        }
        posix_absolute_from(self, || posix_absolute_from(cwd, || unreachable!()))
    }

    fn posix_lexically_absolute_from(&self, cwd: &Path) -> io::Result<PathBuf> {
        if !cwd.is_absolute() {
            return Err(cwd_error());
        }
        posix_lexically_absolute_from(self, || {
            posix_lexically_absolute_from(cwd, || unreachable!())
        })
    }
}

fn cwd_error() -> io::Error {
    io::Error::new(
        io::ErrorKind::InvalidInput,
        "expected an absolute path as the current working directory",
    )
}

mod private {
    pub trait Sealed {}
    impl Sealed for std::path::Path {}
}
use private::Sealed;

fn posix_lexically_absolute_from<F>(path: &Path, get_cwd: F) -> io::Result<PathBuf>
where
    F: FnOnce() -> io::Result<PathBuf>,
{
    // This is mostly a wrapper around collecting `Path::components`, with
    // exceptions made where this conflicts with the POSIX specification.
    // See 4.13 Pathname Resolution, IEEE Std 1003.1-2017
    // https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_13

    // Get the components, skipping the redundant leading "." component if it exists.
    let mut components = path.strip_prefix(".").unwrap_or(path).components();
    let path_os = path.as_os_str().as_bytes();

    let mut normalized = if path.is_absolute() {
        // "If a pathname begins with two successive <slash> characters, the
        // first component following the leading <slash> characters may be
        // interpreted in an implementation-defined manner, although more than
        // two leading <slash> characters shall be treated as a single <slash>
        // character."
        if path_os.starts_with(b"//") && !path_os.starts_with(b"///") {
            components.next();
            PathBuf::from("//")
        } else {
            PathBuf::new()
        }
    } else {
        get_cwd()?
    };
    components.for_each(|component| {
        if component == Component::ParentDir {
            normalized.pop();
        } else {
            normalized.push(component);
        }
    });

    // "Interfaces using pathname resolution may specify additional constraints
    // when a pathname that does not name an existing directory contains at
    // least one non- <slash> character and contains one or more trailing
    // <slash> characters".
    // A trailing <slash> is also meaningful if "a symbolic link is
    // encountered during pathname resolution".
    if path_os.ends_with(b"/") {
        normalized.push("");
    }

    Ok(normalized)
}

fn posix_absolute_from<F>(path: &Path, get_cwd: F) -> io::Result<PathBuf>
where
    F: FnOnce() -> io::Result<PathBuf>,
{
    // This is mostly a wrapper around collecting `Path::components`, with
    // exceptions made where this conflicts with the POSIX specification.
    // See 4.13 Pathname Resolution, IEEE Std 1003.1-2017
    // https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_13

    // Get the components, skipping the redundant leading "." component if it exists.
    let mut components = path.strip_prefix(".").unwrap_or(path).components();
    let path_os = path.as_os_str().as_bytes();

    let mut normalized = if path.is_absolute() {
        // "If a pathname begins with two successive <slash> characters, the
        // first component following the leading <slash> characters may be
        // interpreted in an implementation-defined manner, although more than
        // two leading <slash> characters shall be treated as a single <slash>
        // character."
        if path_os.starts_with(b"//") && !path_os.starts_with(b"///") {
            components.next();
            PathBuf::from("//")
        } else {
            PathBuf::new()
        }
    } else {
        get_cwd()?
    };
    normalized.extend(components);

    // "Interfaces using pathname resolution may specify additional constraints
    // when a pathname that does not name an existing directory contains at
    // least one non- <slash> character and contains one or more trailing
    // <slash> characters".
    // A trailing <slash> is also meaningful if "a symbolic link is
    // encountered during pathname resolution".
    if path_os.ends_with(b"/") {
        normalized.push("");
    }

    Ok(normalized)
}