use super::{c, fill_utf16_buf, to_u16s};
use crate::ffi::{OsStr, OsString};
use crate::io;
use crate::mem;
use crate::path::{Path, PathBuf, Prefix};
use crate::ptr;

#[cfg(test)]
mod tests;

pub const MAIN_SEP_STR: &str = "\\";
pub const MAIN_SEP: char = '\\';

/// # Safety
///
/// `bytes` must be a valid wtf8 encoded slice
#[inline]
unsafe fn bytes_as_os_str(bytes: &[u8]) -> &OsStr {
    // &OsStr is layout compatible with &Slice, which is compatible with &Wtf8,
    // which is compatible with &[u8].
    mem::transmute(bytes)
}

#[inline]
pub fn is_sep_byte(b: u8) -> bool {
    b == b'/' || b == b'\\'
}

#[inline]
pub fn is_verbatim_sep(b: u8) -> bool {
    b == b'\\'
}

/// Returns true if `path` looks like a lone filename.
pub(crate) fn is_file_name(path: &OsStr) -> bool {
    !path.bytes().iter().copied().any(is_sep_byte)
}
pub(crate) fn has_trailing_slash(path: &OsStr) -> bool {
    let is_verbatim = path.bytes().starts_with(br"\\?\");
    let is_separator = if is_verbatim { is_verbatim_sep } else { is_sep_byte };
    if let Some(&c) = path.bytes().last() { is_separator(c) } else { false }
}

/// Appends a suffix to a path.
///
/// Can be used to append an extension without removing an existing extension.
pub(crate) fn append_suffix(path: PathBuf, suffix: &OsStr) -> PathBuf {
    let mut path = OsString::from(path);
    path.push(suffix);
    path.into()
}

struct PrefixParser<'a, const LEN: usize> {
    path: &'a OsStr,
    prefix: [u8; LEN],
}

impl<'a, const LEN: usize> PrefixParser<'a, LEN> {
    #[inline]
    fn get_prefix(path: &OsStr) -> [u8; LEN] {
        let mut prefix = [0; LEN];
        // SAFETY: Only ASCII characters are modified.
        for (i, &ch) in path.bytes().iter().take(LEN).enumerate() {
            prefix[i] = if ch == b'/' { b'\\' } else { ch };
        }
        prefix
    }

    fn new(path: &'a OsStr) -> Self {
        Self { path, prefix: Self::get_prefix(path) }
    }

    fn as_slice(&self) -> PrefixParserSlice<'a, '_> {
        PrefixParserSlice {
            path: self.path,
            prefix: &self.prefix[..LEN.min(self.path.len())],
            index: 0,
        }
    }
}

struct PrefixParserSlice<'a, 'b> {
    path: &'a OsStr,
    prefix: &'b [u8],
    index: usize,
}

impl<'a> PrefixParserSlice<'a, '_> {
    fn strip_prefix(&self, prefix: &str) -> Option<Self> {
        self.prefix[self.index..]
            .starts_with(prefix.as_bytes())
            .then(|| Self { index: self.index + prefix.len(), ..*self })
    }

    fn prefix_bytes(&self) -> &'a [u8] {
        &self.path.bytes()[..self.index]
    }

    fn finish(self) -> &'a OsStr {
        // SAFETY: The unsafety here stems from converting between &OsStr and
        // &[u8] and back. This is safe to do because (1) we only look at ASCII
        // contents of the encoding and (2) new &OsStr values are produced only
        // from ASCII-bounded slices of existing &OsStr values.
        unsafe { bytes_as_os_str(&self.path.bytes()[self.index..]) }
    }
}

pub fn parse_prefix(path: &OsStr) -> Option<Prefix<'_>> {
    use Prefix::{DeviceNS, Disk, Verbatim, VerbatimDisk, VerbatimUNC, UNC};

    let parser = PrefixParser::<8>::new(path);
    let parser = parser.as_slice();
    if let Some(parser) = parser.strip_prefix(r"\\") {
        // \\

        // The meaning of verbatim paths can change when they use a different
        // separator.
        if let Some(parser) = parser.strip_prefix(r"?\") && !parser.prefix_bytes().iter().any(|&x| x == b'/') {
            // \\?\
            if let Some(parser) = parser.strip_prefix(r"UNC\") {
                // \\?\UNC\server\share

                let path = parser.finish();
                let (server, path) = parse_next_component(path, true);
                let (share, _) = parse_next_component(path, true);

                Some(VerbatimUNC(server, share))
            } else {
                let path = parser.finish();

                // in verbatim paths only recognize an exact drive prefix
                if let Some(drive) = parse_drive_exact(path) {
                    // \\?\C:
                    Some(VerbatimDisk(drive))
                } else {
                    // \\?\prefix
                    let (prefix, _) = parse_next_component(path, true);
                    Some(Verbatim(prefix))
                }
            }
        } else if let Some(parser) = parser.strip_prefix(r".\") {
            // \\.\COM42
            let path = parser.finish();
            let (prefix, _) = parse_next_component(path, false);
            Some(DeviceNS(prefix))
        } else {
            let path = parser.finish();
            let (server, path) = parse_next_component(path, false);
            let (share, _) = parse_next_component(path, false);

            if !server.is_empty() && !share.is_empty() {
                // \\server\share
                Some(UNC(server, share))
            } else {
                // no valid prefix beginning with "\\" recognized
                None
            }
        }
    } else if let Some(drive) = parse_drive(path) {
        // C:
        Some(Disk(drive))
    } else {
        // no prefix
        None
    }
}

// Parses a drive prefix, e.g. "C:" and "C:\whatever"
fn parse_drive(path: &OsStr) -> Option<u8> {
    // In most DOS systems, it is not possible to have more than 26 drive letters.
    // See <https://en.wikipedia.org/wiki/Drive_letter_assignment#Common_assignments>.
    fn is_valid_drive_letter(drive: &u8) -> bool {
        drive.is_ascii_alphabetic()
    }

    match path.bytes() {
        [drive, b':', ..] if is_valid_drive_letter(drive) => Some(drive.to_ascii_uppercase()),
        _ => None,
    }
}

// Parses a drive prefix exactly, e.g. "C:"
fn parse_drive_exact(path: &OsStr) -> Option<u8> {
    // only parse two bytes: the drive letter and the drive separator
    if path.bytes().get(2).map(|&x| is_sep_byte(x)).unwrap_or(true) {
        parse_drive(path)
    } else {
        None
    }
}

// Parse the next path component.
//
// Returns the next component and the rest of the path excluding the component and separator.
// Does not recognize `/` as a separator character if `verbatim` is true.
fn parse_next_component(path: &OsStr, verbatim: bool) -> (&OsStr, &OsStr) {
    let separator = if verbatim { is_verbatim_sep } else { is_sep_byte };

    match path.bytes().iter().position(|&x| separator(x)) {
        Some(separator_start) => {
            let separator_end = separator_start + 1;

            let component = &path.bytes()[..separator_start];

            // Panic safe
            // The max `separator_end` is `bytes.len()` and `bytes[bytes.len()..]` is a valid index.
            let path = &path.bytes()[separator_end..];

            // SAFETY: `path` is a valid wtf8 encoded slice and each of the separators ('/', '\')
            // is encoded in a single byte, therefore `bytes[separator_start]` and
            // `bytes[separator_end]` must be code point boundaries and thus
            // `bytes[..separator_start]` and `bytes[separator_end..]` are valid wtf8 slices.
            unsafe { (bytes_as_os_str(component), bytes_as_os_str(path)) }
        }
        None => (path, OsStr::new("")),
    }
}

/// Returns a UTF-16 encoded path capable of bypassing the legacy `MAX_PATH` limits.
///
/// This path may or may not have a verbatim prefix.
pub(crate) fn maybe_verbatim(path: &Path) -> io::Result<Vec<u16>> {
    // Normally the MAX_PATH is 260 UTF-16 code units (including the NULL).
    // However, for APIs such as CreateDirectory[1], the limit is 248.
    //
    // [1]: https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createdirectorya#parameters
    const LEGACY_MAX_PATH: usize = 248;
    // UTF-16 encoded code points, used in parsing and building UTF-16 paths.
    // All of these are in the ASCII range so they can be cast directly to `u16`.
    const SEP: u16 = b'\\' as _;
    const ALT_SEP: u16 = b'/' as _;
    const QUERY: u16 = b'?' as _;
    const COLON: u16 = b':' as _;
    const DOT: u16 = b'.' as _;
    const U: u16 = b'U' as _;
    const N: u16 = b'N' as _;
    const C: u16 = b'C' as _;

    // \\?\
    const VERBATIM_PREFIX: &[u16] = &[SEP, SEP, QUERY, SEP];
    // \??\
    const NT_PREFIX: &[u16] = &[SEP, QUERY, QUERY, SEP];
    // \\?\UNC\
    const UNC_PREFIX: &[u16] = &[SEP, SEP, QUERY, SEP, U, N, C, SEP];

    let mut path = to_u16s(path)?;
    if path.starts_with(VERBATIM_PREFIX) || path.starts_with(NT_PREFIX) || path == &[0] {
        // Early return for paths that are already verbatim or empty.
        return Ok(path);
    } else if path.len() < LEGACY_MAX_PATH {
        // Early return if an absolute path is less < 260 UTF-16 code units.
        // This is an optimization to avoid calling `GetFullPathNameW` unnecessarily.
        match path.as_slice() {
            // Starts with `D:`, `D:\`, `D:/`, etc.
            // Does not match if the path starts with a `\` or `/`.
            [drive, COLON, 0] | [drive, COLON, SEP | ALT_SEP, ..]
                if *drive != SEP && *drive != ALT_SEP =>
            {
                return Ok(path);
            }
            // Starts with `\\`, `//`, etc
            [SEP | ALT_SEP, SEP | ALT_SEP, ..] => return Ok(path),
            _ => {}
        }
    }

    // Firstly, get the absolute path using `GetFullPathNameW`.
    // https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-getfullpathnamew
    let lpfilename = path.as_ptr();
    fill_utf16_buf(
        // SAFETY: `fill_utf16_buf` ensures the `buffer` and `size` are valid.
        // `lpfilename` is a pointer to a null terminated string that is not
        // invalidated until after `GetFullPathNameW` returns successfully.
        |buffer, size| unsafe { c::GetFullPathNameW(lpfilename, size, buffer, ptr::null_mut()) },
        |mut absolute| {
            path.clear();

            // Secondly, add the verbatim prefix. This is easier here because we know the
            // path is now absolute and fully normalized (e.g. `/` has been changed to `\`).
            let prefix = match absolute {
                // C:\ => \\?\C:\
                [_, COLON, SEP, ..] => VERBATIM_PREFIX,
                // \\.\ => \\?\
                [SEP, SEP, DOT, SEP, ..] => {
                    absolute = &absolute[4..];
                    VERBATIM_PREFIX
                }
                // Leave \\?\ and \??\ as-is.
                [SEP, SEP, QUERY, SEP, ..] | [SEP, QUERY, QUERY, SEP, ..] => &[],
                // \\ => \\?\UNC\
                [SEP, SEP, ..] => {
                    absolute = &absolute[2..];
                    UNC_PREFIX
                }
                // Anything else we leave alone.
                _ => &[],
            };

            path.reserve_exact(prefix.len() + absolute.len() + 1);
            path.extend_from_slice(prefix);
            path.extend_from_slice(absolute);
            path.push(0);
        },
    )?;
    Ok(path)
}

/// Make a Windows path absolute.
pub(crate) fn absolute(path: &Path) -> io::Result<PathBuf> {
    let path = path.as_os_str();
    let prefix = parse_prefix(path);
    // Verbatim paths should not be modified.
    if prefix.map(|x| x.is_verbatim()).unwrap_or(false) {
        // NULs in verbatim paths are rejected for consistency.
        if path.bytes().contains(&0) {
            return Err(io::const_io_error!(
                io::ErrorKind::InvalidInput,
                "strings passed to WinAPI cannot contain NULs",
            ));
        }
        return Ok(path.to_owned().into());
    }

    let path = to_u16s(path)?;
    let lpfilename = path.as_ptr();
    fill_utf16_buf(
        // SAFETY: `fill_utf16_buf` ensures the `buffer` and `size` are valid.
        // `lpfilename` is a pointer to a null terminated string that is not
        // invalidated until after `GetFullPathNameW` returns successfully.
        |buffer, size| unsafe { c::GetFullPathNameW(lpfilename, size, buffer, ptr::null_mut()) },
        super::os2path,
    )
}