use crate::ffi::OsStr;
use crate::mem;
use crate::path::Prefix;

#[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'\\'
}

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

    if let Some(path) = strip_prefix(path, r"\\") {
        // \\
        if let Some(path) = strip_prefix(path, r"?\") {
            // \\?\
            if let Some(path) = strip_prefix(path, r"UNC\") {
                // \\?\UNC\server\share

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

                Some(VerbatimUNC(server, share))
            } else {
                let (prefix, _) = parse_next_component(path, true);

                // in verbatim paths only recognize an exact drive prefix
                if let Some(drive) = parse_drive_exact(prefix) {
                    // \\?\C:
                    Some(VerbatimDisk(drive))
                } else {
                    // \\?\prefix
                    Some(Verbatim(prefix))
                }
            }
        } else if let Some(path) = strip_prefix(path, r".\") {
            // \\.\COM42
            let (prefix, _) = parse_next_component(path, false);
            Some(DeviceNS(prefix))
        } else {
            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(prefix: &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 prefix.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(prefix: &OsStr) -> Option<u8> {
    // only parse two bytes: the drive letter and the drive separator
    if prefix.len() == 2 { parse_drive(prefix) } else { None }
}

fn strip_prefix<'a>(path: &'a OsStr, prefix: &str) -> Option<&'a OsStr> {
    // `path` and `prefix` are valid wtf8 and utf8 encoded slices respectively, `path[prefix.len()]`
    // is thus a code point boundary and `path[prefix.len()..]` is a valid wtf8 encoded slice.
    match path.bytes().strip_prefix(prefix.as_bytes()) {
        Some(path) => unsafe { Some(bytes_as_os_str(path)) },
        None => 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 mut separator_end = separator_start + 1;

            // a series of multiple separator characters is treated as a single separator,
            // except in verbatim paths
            while !verbatim && separator_end < path.len() && separator(path.bytes()[separator_end])
            {
                separator_end += 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("")),
    }
}