//! Search for a byte in a byte array using libc.

//!

//! When nothing pulls in libc, then just use a trivial implementation. Note

//! that we only depend on libc on unix.

#[cfg(not(all(unix, feature = "libc")))]

fn memchr_inner(needle: u8, haystack: &[u8]) -> Option<usize> {

    haystack.iter().position(|val| needle == *val)

}

#[cfg(all(unix, feature = "libc"))]

fn memchr_inner(needle: u8, haystack: &[u8]) -> Option<usize> {

    let start = haystack.as_ptr();

    // SAFETY: `start` is valid for `haystack.len()` bytes.

    let ptr = (unsafe { libc::memchr(start.cast(), needle as _, haystack.len()) })

        .cast::<u8>()

        .cast_const();

    if ptr.is_null() {

        None

    } else {

        // SAFETY: `ptr` will always be in bounds, since libc guarantees that the ptr will either

        //          be to an element inside the array or the ptr will be null

        //          since the ptr is in bounds the offset must also always be non null

        //          and there can't be more than isize::MAX elements inside an array

        //          as rust guarantees that the maximum number of bytes a allocation

        //          may occupy is isize::MAX

        unsafe {

            // TODO(MSRV 1.87): When bumping MSRV, switch to `ptr.byte_offset_from_unsigned(start)`.

            Some(usize::try_from(ptr.offset_from(start)).unwrap_unchecked())

        }

    }

}

pub(crate) fn memchr(needle: u8, haystack: &[u8]) -> Option<usize> {

    let index = memchr_inner(needle, haystack)?;

    // SAFETY: `memchr_inner` returns Some(index) and in that case index must point to an element in haystack

    //         or `memchr_inner` None which is guarded by the `?` operator above

    //         therefore the index must **always** point to an element in the array

    //         and so this indexing operation is safe

    // TODO(MSRV 1.81): When bumping MSRV, switch to `std::hint::assert_unchecked(haystack.get(..=index).is_some());`

    unsafe {

        if haystack.get(..=index).is_none() {

            std::hint::unreachable_unchecked()

        }

    }

    Some(index)

}

#[cfg(test)]

mod tests {

    use super::memchr;

    #[test]

    fn memchr_test() {

        let haystack = b"123abc456\0\xffabc\n";

        assert_eq!(memchr(b'1', haystack), Some(0));

        assert_eq!(memchr(b'2', haystack), Some(1));

        assert_eq!(memchr(b'3', haystack), Some(2));

        assert_eq!(memchr(b'4', haystack), Some(6));

        assert_eq!(memchr(b'5', haystack), Some(7));

        assert_eq!(memchr(b'6', haystack), Some(8));

        assert_eq!(memchr(b'7', haystack), None);

        assert_eq!(memchr(b'a', haystack), Some(3));

        assert_eq!(memchr(b'b', haystack), Some(4));

        assert_eq!(memchr(b'c', haystack), Some(5));

        assert_eq!(memchr(b'd', haystack), None);

        assert_eq!(memchr(b'A', haystack), None);

        assert_eq!(memchr(0, haystack), Some(9));

        assert_eq!(memchr(0xff, haystack), Some(10));

        assert_eq!(memchr(0xfe, haystack), None);

        assert_eq!(memchr(1, haystack), None);

        assert_eq!(memchr(b'\n', haystack), Some(14));

        assert_eq!(memchr(b'\r', haystack), None);

    }

    #[test]

    fn memchr_all() {

        let mut arr = Vec::new();

        for b in 0..=255 {

            arr.push(b);

        }

        for b in 0..=255 {

            assert_eq!(memchr(b, &arr), Some(b as usize));

        }

        arr.reverse();

        for b in 0..=255 {

            assert_eq!(memchr(b, &arr), Some(255 - b as usize));

        }

    }

    #[test]

    fn memchr_empty() {

        for b in 0..=255 {

            assert_eq!(memchr(b, b""), None);

        }

    }

}