use std::{ffi::OsStr, path::Path};

pub trait PathExtras {
    /// See [std::path::Path::file_prefix]
    #[must_use]
    fn ext_file_prefix(&self) -> Option<&OsStr>;
}

impl PathExtras for Path {
    fn ext_file_prefix(&self) -> Option<&OsStr> {
        self.file_name()
            .map(split_file_at_dot)
            .map(|(before, _after)| before)
    }
}

/// Note: Copied from private fn `std::path::split_file_at_dot`
fn split_file_at_dot(file: &OsStr) -> (&OsStr, Option<&OsStr>) {
    let slice = file.as_encoded_bytes();
    if slice == b".." {
        return (file, None);
    }

    // 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.
    let i = match slice[1..].iter().position(|b| *b == b'.') {
        Some(i) => i + 1,
        None => return (file, None),
    };
    let before = &slice[..i];
    let after = &slice[i + 1..];
    unsafe {
        (
            OsStr::from_encoded_bytes_unchecked(before),
            Some(OsStr::from_encoded_bytes_unchecked(after)),
        )
    }
}