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// Copyright 2024 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

use core::marker::PhantomData;
use core::ptr::{null_mut, NonNull};

use munge::munge;

use crate::{u64_le, Chunk, DecodeError, Owned, Slot};

/// A raw FIDL pointer
#[repr(C, align(8))]
pub union WirePointer<'buf, T> {
    encoded: u64_le,
    decoded: *mut T,
    _phantom: PhantomData<&'buf mut [Chunk]>,
}

impl<'buf, T> WirePointer<'buf, T> {
    /// Returns the null wire pointer.
    pub const fn null() -> Self {
        Self { decoded: null_mut() }
    }

    /// Returns the dangling wire pointer.
    pub const fn dangling() -> Self {
        Self { decoded: NonNull::dangling().as_ptr() }
    }

    /// Returns whether the wire pointer was encoded present.
    pub fn is_encoded_present(slot: Slot<'_, Self>) -> Result<bool, DecodeError> {
        munge!(let Self { encoded } = slot);
        match encoded.to_native() {
            0 => Ok(false),
            u64::MAX => Ok(true),
            x => Err(DecodeError::InvalidPointerPresence(x)),
        }
    }

    /// Encodes that a pointer is present in a slot.
    pub fn encode_present(slot: Slot<'_, Self>) {
        munge!(let Self { mut encoded } = slot);
        *encoded = u64_le::from_native(u64::MAX);
    }

    /// Encodes that a pointer is absent in a slot.
    pub fn encode_absent(slot: Slot<'_, Self>) {
        munge!(let Self { mut encoded } = slot);
        *encoded = u64_le::from_native(0);
    }

    /// Sets the decoded value of the pointer.
    pub fn set_decoded(slot: Slot<'_, Self>, ptr: Owned<'buf, T>) {
        munge!(let Self { mut decoded } = slot);
        // SAFETY: Identical to `decoded.write(ptr.into_raw())`, but raw
        // pointers don't currently implement `IntoBytes`.
        unsafe { decoded.as_mut_ptr().write(ptr.into_raw()) };
    }

    /// Returns the underlying pointer.
    pub fn as_ptr(&self) -> *mut T {
        unsafe { self.decoded }
    }
}