/*

 * Copyright 2018 Google Inc. All rights reserved.

 *

 * Licensed under the Apache License, Version 2.0 (the "License");

 * you may not use this file except in compliance with the License.

 * You may obtain a copy of the License at

 *

 *     http://www.apache.org/licenses/LICENSE-2.0

 *

 * Unless required by applicable law or agreed to in writing, software

 * distributed under the License is distributed on an "AS IS" BASIS,

 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

 * See the License for the specific language governing permissions and

 * limitations under the License.

 */

use core::ptr::write_bytes;

use crate::endian_scalar::emplace_scalar;

use crate::primitives::*;

/// VTableWriter compartmentalizes actions needed to create a vtable.

#[derive(Debug)]

pub struct VTableWriter<'a> {

    buf: &'a mut [u8],

}

impl<'a> VTableWriter<'a> {

    #[inline(always)]

    pub fn init(buf: &'a mut [u8]) -> Self {

        VTableWriter { buf }

    }

    /// Writes the vtable length (in bytes) into the vtable.

    ///

    /// Note that callers already need to have computed this to initialize

    /// a VTableWriter.

    ///

    /// In debug mode, asserts that the length of the underlying data is equal

    /// to the provided value.

    #[inline(always)]

    pub fn write_vtable_byte_length(&mut self, n: VOffsetT) {

        let buf = &mut self.buf[..SIZE_VOFFSET];

        // Safety:

        // Validated range above

        unsafe {

            emplace_scalar::<VOffsetT>(buf, n);

        }

        debug_assert_eq!(n as usize, self.buf.len());

    }

    /// Writes an object length (in bytes) into the vtable.

    #[inline(always)]

    pub fn write_object_inline_size(&mut self, n: VOffsetT) {

        let buf = &mut self.buf[SIZE_VOFFSET..2 * SIZE_VOFFSET];

        // Safety:

        // Validated range above

        unsafe {

            emplace_scalar::<VOffsetT>(buf, n);

        }

    }

    /// Writes an object field offset into the vtable.

    ///

    /// Note that this expects field offsets (which are like pointers), not

    /// field ids (which are like array indices).

    #[inline(always)]

    pub fn write_field_offset(&mut self, vtable_offset: VOffsetT, object_data_offset: VOffsetT) {

        let idx = vtable_offset as usize;

        let buf = &mut self.buf[idx..idx + SIZE_VOFFSET];

        // Safety:

        // Validated range above

        unsafe {

            emplace_scalar::<VOffsetT>(buf, object_data_offset);

        }

    }

    /// Clears all data in this VTableWriter. Used to cleanly undo a

    /// vtable write.

    #[inline(always)]

    pub fn clear(&mut self) {

        // This is the closest thing to memset in Rust right now.

        let len = self.buf.len();

        let p = self.buf.as_mut_ptr() as *mut u8;

        // Safety:

        // p is byte aligned and of length `len`

        unsafe {

            write_bytes(p, 0, len);

        }

    }

}