// Protocol Buffers - Google's data interchange format
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// https://developers.google.com/protocol-buffers/
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//! UPB FFI wrapper code for use by Rust Protobuf.

use std::alloc;
use std::alloc::Layout;
use std::cell::UnsafeCell;
use std::fmt;
use std::marker::PhantomData;
use std::mem::MaybeUninit;
use std::ops::Deref;
use std::ptr::NonNull;
use std::slice;

/// See `upb/port/def.inc`.
const UPB_MALLOC_ALIGN: usize = 8;

/// A UPB-managed pointer to a raw arena.
pub type RawArena = NonNull<RawArenaData>;

/// The data behind a [`RawArena`]. Do not use this type.
#[repr(C)]
pub struct RawArenaData {
    _data: [u8; 0],
}

/// A wrapper over a `upb_Arena`.
///
/// This is not a safe wrapper per se, because the allocation functions still
/// have sharp edges (see their safety docs for more info).
///
/// This is an owning type and will automatically free the arena when
/// dropped.
///
/// Note that this type is neither `Sync` nor `Send`.
pub struct Arena {
    raw: RawArena,
    _not_sync: PhantomData<UnsafeCell<()>>,
}

extern "C" {
    fn upb_Arena_New() -> RawArena;
    fn upb_Arena_Free(arena: RawArena);
    fn upb_Arena_Malloc(arena: RawArena, size: usize) -> *mut u8;
    fn upb_Arena_Realloc(arena: RawArena, ptr: *mut u8, old: usize, new: usize) -> *mut u8;
}

impl Arena {
    /// Allocates a fresh arena.
    #[inline]
    pub fn new() -> Self {
        Self { raw: unsafe { upb_Arena_New() }, _not_sync: PhantomData }
    }

    /// Returns the raw, UPB-managed pointer to the arena.
    #[inline]
    pub fn raw(&self) -> RawArena {
        self.raw
    }

    /// Allocates some memory on the arena.
    ///
    /// # Safety
    ///
    /// `layout`'s alignment must be less than `UPB_MALLOC_ALIGN`.
    #[inline]
    pub unsafe fn alloc(&self, layout: Layout) -> &mut [MaybeUninit<u8>] {
        debug_assert!(layout.align() <= UPB_MALLOC_ALIGN);
        let ptr = upb_Arena_Malloc(self.raw, layout.size());
        if ptr.is_null() {
            alloc::handle_alloc_error(layout);
        }

        slice::from_raw_parts_mut(ptr.cast(), layout.size())
    }

    /// Resizes some memory on the arena.
    ///
    /// # Safety
    ///
    /// After calling this function, `ptr` is essentially zapped. `old` must
    /// be the layout `ptr` was allocated with via [`Arena::alloc()`]. `new`'s
    /// alignment must be less than `UPB_MALLOC_ALIGN`.
    #[inline]
    pub unsafe fn resize(&self, ptr: *mut u8, old: Layout, new: Layout) -> &[MaybeUninit<u8>] {
        debug_assert!(new.align() <= UPB_MALLOC_ALIGN);
        let ptr = upb_Arena_Realloc(self.raw, ptr, old.size(), new.size());
        if ptr.is_null() {
            alloc::handle_alloc_error(new);
        }

        slice::from_raw_parts_mut(ptr.cast(), new.size())
    }
}

impl Drop for Arena {
    #[inline]
    fn drop(&mut self) {
        unsafe {
            upb_Arena_Free(self.raw);
        }
    }
}

/// Represents serialized Protobuf wire format data.
///
/// It's typically produced by `<Message>::serialize()`.
pub struct SerializedData {
    data: NonNull<u8>,
    len: usize,

    // The arena that owns `data`.
    _arena: Arena,
}

impl SerializedData {
    pub unsafe fn from_raw_parts(arena: Arena, data: NonNull<u8>, len: usize) -> Self {
        SerializedData { _arena: arena, data, len }
    }
}

impl Deref for SerializedData {
    type Target = [u8];
    fn deref(&self) -> &Self::Target {
        unsafe { slice::from_raw_parts(self.data.as_ptr() as *const _, self.len) }
    }
}

impl fmt::Debug for SerializedData {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        fmt::Debug::fmt(self.deref(), f)
    }
}

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

    #[test]
    fn test_arena_new_and_free() {
        let arena = Arena::new();
        drop(arena);
    }

    #[test]
    fn test_serialized_data_roundtrip() {
        let arena = Arena::new();
        let original_data = b"Hello world";
        let len = original_data.len();

        let serialized_data = unsafe {
            SerializedData::from_raw_parts(
                arena,
                NonNull::new(original_data as *const _ as *mut _).unwrap(),
                len,
            )
        };
        assert_eq!(&*serialized_data, b"Hello world");
    }
}