ns_t_array.rs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at https://mozilla.org/MPL/2.0/. */

//! Rust helpers for Gecko's nsTArray.

use crate::gecko_bindings::bindings;
use crate::gecko_bindings::structs::{nsTArray, nsTArrayHeader};
use std::mem;
use std::ops::{Deref, DerefMut};
use std::slice;

impl<T> Deref for nsTArray<T> {
    type Target = [T];

    fn deref<'a>(&'a self) -> &'a [T] {
        unsafe { slice::from_raw_parts(self.slice_begin(), self.header().mLength as usize) }
    }
}

impl<T> DerefMut for nsTArray<T> {
    fn deref_mut<'a>(&'a mut self) -> &'a mut [T] {
        unsafe { slice::from_raw_parts_mut(self.slice_begin(), self.header().mLength as usize) }
    }
}

impl<T> nsTArray<T> {
    #[inline]
    fn header<'a>(&'a self) -> &'a nsTArrayHeader {
        debug_assert!(!self.mBuffer.is_null());
        unsafe { mem::transmute(self.mBuffer) }
    }

    // unsafe, since header may be in shared static or something
    unsafe fn header_mut<'a>(&'a mut self) -> &'a mut nsTArrayHeader {
        debug_assert!(!self.mBuffer.is_null());

        mem::transmute(self.mBuffer)
    }

    #[inline]
    unsafe fn slice_begin(&self) -> *mut T {
        debug_assert!(!self.mBuffer.is_null());
        (self.mBuffer as *const nsTArrayHeader).offset(1) as *mut _
    }

    /// Ensures the array has enough capacity at least to hold `cap` elements.
    ///
    /// NOTE: This doesn't call the constructor on the values!
    pub fn ensure_capacity(&mut self, cap: usize) {
        if cap >= self.len() {
            unsafe {
                bindings::Gecko_EnsureTArrayCapacity(
                    self as *mut nsTArray<T> as *mut _,
                    cap,
                    mem::size_of::<T>(),
                )
            }
        }
    }

    /// Clears the array storage without calling the destructor on the values.
    #[inline]
    pub unsafe fn clear(&mut self) {
        if self.len() != 0 {
            bindings::Gecko_ClearPODTArray(
                self as *mut nsTArray<T> as *mut _,
                mem::size_of::<T>(),
                mem::align_of::<T>(),
            );
        }
    }

    /// Clears a POD array. This is safe since copy types are memcopyable.
    #[inline]
    pub fn clear_pod(&mut self)
    where
        T: Copy,
    {
        unsafe { self.clear() }
    }

    /// Resize and set the length of the array to `len`.
    ///
    /// unsafe because this may leave the array with uninitialized elements.
    ///
    /// This will not call constructors.  If you need that, either manually add
    /// bindings or run the typed `EnsureCapacity` call on the gecko side.
    pub unsafe fn set_len(&mut self, len: u32) {
        // this can leak
        debug_assert!(len >= self.len() as u32);
        if self.len() == len as usize {
            return;
        }
        self.ensure_capacity(len as usize);
        self.header_mut().mLength = len;
    }

    /// Resizes an array containing only POD elements
    ///
    /// unsafe because this may leave the array with uninitialized elements.
    ///
    /// This will not leak since it only works on POD types (and thus doesn't assert)
    pub unsafe fn set_len_pod(&mut self, len: u32)
    where
        T: Copy,
    {
        if self.len() == len as usize {
            return;
        }
        self.ensure_capacity(len as usize);
        let header = self.header_mut();
        header.mLength = len;
    }

    /// Collects the given iterator into this array.
    ///
    /// Not unsafe because we won't leave uninitialized elements in the array.
    pub fn assign_from_iter_pod<I>(&mut self, iter: I)
    where
        T: Copy,
        I: ExactSizeIterator + Iterator<Item = T>,
    {
        debug_assert!(iter.len() <= 0xFFFFFFFF);
        unsafe {
            self.set_len_pod(iter.len() as u32);
        }
        self.iter_mut().zip(iter).for_each(|(r, v)| *r = v);
    }
}