//! This module contains a few non-required helpers to deal with some of the
//! more annoying memory-twiddling in the PAM API.
//!
//! Using these is optional but you might find it useful.
use std::error::Error;
use std::marker::{PhantomData, PhantomPinned};
use std::mem::ManuallyDrop;
use std::ptr::NonNull;
use std::{fmt, slice};

/// Error returned when attempting to allocate a buffer that is too big.
///
/// This is specifically used in [`OwnedBinaryPayload`] when you try to allocate
/// a message larger than 2<sup>32</sup> bytes.
#[derive(Debug, PartialEq)]
pub struct TooBigError {
    pub size: usize,
    pub max: usize,
}

impl Error for TooBigError {}

impl fmt::Display for TooBigError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(
            f,
            "can't allocate a message of {size} bytes (max {max})",
            size = self.size,
            max = self.max
        )
    }
}

/// A trait wrapping memory management.
///
/// This is intended to allow you to bring your own allocator for
/// [`OwnedBinaryPayload`]s.
///
/// For an implementation example, see the implementation of this trait
/// for [`Vec`].
pub trait Buffer<T: Default> {
    /// Allocates a buffer of `len` elements, filled with the default.
    fn allocate(len: usize) -> Self;

    fn as_ptr(&self) -> *const T;

    /// Returns a slice view of `size` elements of the given memory.
    ///
    /// # Safety
    ///
    /// The caller must not request more elements than are allocated.
    unsafe fn as_mut_slice(&mut self, len: usize) -> &mut [T];

    /// Consumes this ownership and returns a pointer to the start of the arena.
    fn into_ptr(self) -> NonNull<T>;

    /// "Adopts" the memory at the given pointer, taking it under management.
    ///
    /// Running the operation:
    ///
    /// ```
    /// # use libpam_sys::helpers::Buffer;
    /// # fn test<T: Default, OwnerType: Buffer<T>>(bytes: usize) {
    /// let owner = OwnerType::allocate(bytes);
    /// let ptr = owner.into_ptr();
    /// let owner = unsafe { OwnerType::from_ptr(ptr, bytes) };
    /// # }
    /// ```
    ///
    /// must be a no-op.
    ///
    /// # Safety
    ///
    /// The pointer must be valid, and the caller must provide the exact size
    /// of the given arena.
    unsafe fn from_ptr(ptr: NonNull<T>, bytes: usize) -> Self;
}

impl<T: Default> Buffer<T> for Vec<T> {
    fn allocate(bytes: usize) -> Self {
        (0..bytes).map(|_| Default::default()).collect()
    }

    fn as_ptr(&self) -> *const T {
        Vec::as_ptr(self)
    }

    unsafe fn as_mut_slice(&mut self, bytes: usize) -> &mut [T] {
        debug_assert!(bytes <= self.len());
        Vec::as_mut(self)
    }

    fn into_ptr(self) -> NonNull<T> {
        let mut me = ManuallyDrop::new(self);
        // SAFETY: a Vec is guaranteed to have a nonzero pointer.
        unsafe { NonNull::new_unchecked(me.as_mut_ptr()) }
    }

    unsafe fn from_ptr(ptr: NonNull<T>, bytes: usize) -> Self {
        Vec::from_raw_parts(ptr.as_ptr(), bytes, bytes)
    }
}

/// A binary message owned by some storage.
///
/// This is an owned, memory-managed version of [`BinaryPayload`].
/// The `O` type manages the memory where the payload lives.
/// [`Vec<u8>`] is one such manager and can be used when ownership
/// of the data does not need to transit through PAM.
#[derive(Debug)]
pub struct OwnedBinaryPayload<Owner: Buffer<u8>>(Owner);

impl<O: Buffer<u8>> OwnedBinaryPayload<O> {
    /// Allocates a new OwnedBinaryPayload.
    ///
    /// This will return a [`TooBigError`] if you try to allocate too much
    /// (more than [`BinaryPayload::MAX_SIZE`]).
    pub fn new(data_type: u8, data: &[u8]) -> Result<Self, TooBigError> {
        let total_len: u32 = (data.len() + 5).try_into().map_err(|_| TooBigError {
            size: data.len(),
            max: BinaryPayload::MAX_SIZE,
        })?;
        let total_len = total_len as usize;
        let mut buf = O::allocate(total_len);
        // SAFETY: We just allocated this exact size.
        BinaryPayload::fill(unsafe { &mut buf.as_mut_slice(total_len) }, data_type, data);
        Ok(Self(buf))
    }

    /// The contents of the buffer.
    pub fn contents(&self) -> (u8, &[u8]) {
        unsafe { BinaryPayload::contents(self.as_ptr()) }
    }

    /// The total bytes needed to store this, including the header.
    pub fn total_bytes(&self) -> usize {
        unsafe {
            self.0
                .as_ptr()
                .cast::<BinaryPayload>()
                .as_ref()
                .unwrap_unchecked()
                .total_bytes()
        }
    }

    /// Unwraps this into the raw storage backing it.
    pub fn into_inner(self) -> O {
        self.0
    }

    /// Gets a const pointer to the start of the message's buffer.
    pub fn as_ptr(&self) -> *const BinaryPayload {
        self.0.as_ptr().cast()
    }

    /// Consumes ownership of this message and converts it to a raw pointer
    /// to the start of the message.
    ///
    /// To clean this up, you should eventually pass it into [`Self::from_ptr`]
    /// with the same `O` ownership type.
    pub fn into_ptr(self) -> NonNull<BinaryPayload> {
        self.0.into_ptr().cast()
    }

    /// Takes ownership of the given pointer.
    ///
    /// # Safety
    ///
    /// You must provide a valid pointer, allocated by (or equivalent to one
    /// allocated by) [`Self::new`]. For instance, passing a pointer allocated
    /// by `malloc` to `OwnedBinaryPayload::<Vec<u8>>::from_ptr` is not allowed.
    pub unsafe fn from_ptr(ptr: NonNull<BinaryPayload>) -> Self {
        Self(O::from_ptr(ptr.cast(), ptr.as_ref().total_bytes() as usize))
    }
}

/// The structure of the "binary message" payload for the `PAM_BINARY_PROMPT`
/// extension from Linux-PAM.
pub struct BinaryPayload {
    /// The total length of the message, including this header,
    /// as a u32 in network byte order (big endian).
    total_length: [u8; 4],
    /// A tag used to provide some kind of hint as to what the data is.
    /// This is not defined by PAM.
    data_type: u8,
    /// Where the data itself would start, used as a marker to make this
    /// not [`Unpin`] (since it is effectively an intrusive data structure
    /// pointing to immediately after itself).
    _marker: PhantomData<PhantomPinned>,
}

impl BinaryPayload {
    /// The most data it's possible to put into a [`BinaryPayload`].
    pub const MAX_SIZE: usize = (u32::MAX - 5) as usize;

    /// Fills in the provided buffer with the given data.
    ///
    /// This uses [`copy_from_slice`](slice::copy_from_slice) internally,
    /// so `buf` must be exactly 5 bytes longer than `data`, or this function
    /// will panic.
    pub fn fill(buf: &mut [u8], data_type: u8, data: &[u8]) {
        let ptr: *mut Self = buf.as_mut_ptr().cast();
        // SAFETY: We're given a slice, which always has a nonzero pointer.
        let me = unsafe { ptr.as_mut().unwrap_unchecked() };
        me.total_length = u32::to_be_bytes(buf.len() as u32);
        me.data_type = data_type;
        buf[5..].copy_from_slice(data)
    }

    /// The size of the message contained in the buffer.
    fn len(&self) -> usize {
        self.total_bytes().saturating_sub(5)
    }

    /// The total storage needed for the message, including header.
    pub fn total_bytes(&self) -> usize {
        u32::from_be_bytes(self.total_length) as usize
    }

    /// Gets the contents of the BinaryMessage starting at the given location.
    ///
    /// # Safety
    ///
    /// You must pass in a valid pointer, and ensure that the pointer passed in
    /// outlives your borrow lifetime `'a`.
    unsafe fn contents<'a>(ptr: *const Self) -> (u8, &'a [u8]) {
        let header: &Self = ptr.as_ref().unwrap_unchecked();
        let typ = header.data_type;
        (
            typ,
            slice::from_raw_parts(ptr.cast::<u8>().offset(5), header.len()),
        )
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    type VecPayload = OwnedBinaryPayload<Vec<u8>>;

    #[test]
    fn test_binary_payload() {
        let simple_message = &[0u8, 0, 0, 16, 0xff, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
        let empty = &[0u8; 5];

        assert_eq!((0xff, &[0u8, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10][..]), unsafe {
            BinaryPayload::contents(simple_message.as_ptr().cast())
        });
        assert_eq!((0x00, &[][..]), unsafe {
            BinaryPayload::contents(empty.as_ptr().cast())
        });
    }

    #[test]
    fn test_owned_binary_payload() {
        let (typ, data) = (
            112,
            &[0, 1, 1, 8, 9, 9, 9, 8, 8, 1, 9, 9, 9, 1, 1, 9, 7, 2, 5, 3][..],
        );
        let payload = VecPayload::new(typ, data).unwrap();
        assert_eq!((typ, data), payload.contents());
        let ptr = payload.into_ptr();
        let payload = unsafe { VecPayload::from_ptr(ptr) };
        assert_eq!((typ, data), payload.contents());
    }

    #[test]
    #[ignore]
    fn test_owned_too_big() {
        let data = vec![0xFFu8; 0x1_0000_0001];
        assert_eq!(
            TooBigError {
                max: 0xffff_fffa,
                size: 0x1_0000_0001
            },
            VecPayload::new(5, &data).unwrap_err()
        )
    }
}