// Copyright 2018-2022 the Deno authors. All rights reserved. MIT license.

use std::ops::Deref;
use std::ops::DerefMut;
use std::ops::Range;

use super::transl8::FromV8;

/// A V8Slice encapsulates a slice that's been borrowed from a JavaScript
/// ArrayBuffer object. JavaScript objects can normally be garbage collected,
/// but the existence of a V8Slice inhibits this until it is dropped. It
/// behaves much like an Arc<[u8]>.
///
/// # Cloning
/// Cloning a V8Slice does not clone the contents of the buffer,
/// it creates a new reference to that buffer.
///
/// To actually clone the contents of the buffer do
/// `let copy = Vec::from(&*zero_copy_buf);`
#[derive(Clone)]
pub struct V8Slice {
  pub(crate) store: v8::SharedRef<v8::BackingStore>,
  pub(crate) range: Range<usize>,
}

unsafe impl Send for V8Slice {}

impl V8Slice {
  pub fn from_buffer(
    buffer: v8::Local<v8::ArrayBuffer>,
    range: Range<usize>,
  ) -> Result<Self, v8::DataError> {
    let store = buffer.get_backing_store();
    if store.is_shared() {
      return Err(v8::DataError::BadType {
        actual: "shared ArrayBufferView",
        expected: "non-shared ArrayBufferView",
      });
    }
    Ok(Self { store, range })
  }

  fn as_slice(&self) -> &[u8] {
    // SAFETY: v8::SharedRef<v8::BackingStore> is similar to Arc<[u8]>,
    // it points to a fixed continuous slice of bytes on the heap.
    // We assume it's initialized and thus safe to read (though may not contain meaningful data)
    unsafe { &*(&self.store[self.range.clone()] as *const _ as *const [u8]) }
  }

  fn as_slice_mut(&mut self) -> &mut [u8] {
    // SAFETY: v8::SharedRef<v8::BackingStore> is similar to Arc<[u8]>,
    // it points to a fixed continuous slice of bytes on the heap.
    // It's safe-ish to mutate concurrently because it can not be
    // shrunk/grown/moved/reallocated, thus avoiding dangling refs (unlike a Vec).
    // Concurrent writes can't lead to meaningful structural invalidation
    // since we treat them as opaque buffers / "bags of bytes",
    // concurrent mutation is simply an accepted fact of life.
    // And in practice V8Slices also do not have overallping read/write phases.
    // TLDR: permissive interior mutability on slices of bytes is "fine"
    #[allow(clippy::cast_ref_to_mut)]
    unsafe {
      &mut *(&self.store[self.range.clone()] as *const _ as *mut [u8])
    }
  }
}

pub(crate) fn to_ranged_buffer<'s>(
  scope: &mut v8::HandleScope<'s>,
  value: v8::Local<v8::Value>,
) -> Result<(v8::Local<'s, v8::ArrayBuffer>, Range<usize>), v8::DataError> {
  if value.is_array_buffer_view() {
    let view: v8::Local<v8::ArrayBufferView> = value.try_into()?;
    let (offset, len) = (view.byte_offset(), view.byte_length());
    let buffer = view.buffer(scope).ok_or(v8::DataError::NoData {
      expected: "view to have a buffer",
    })?;
    let buffer = v8::Local::new(scope, buffer); // recreate handle to avoid lifetime issues
    return Ok((buffer, offset..offset + len));
  }
  let b: v8::Local<v8::ArrayBuffer> = value.try_into()?;
  let b = v8::Local::new(scope, b); // recreate handle to avoid lifetime issues
  Ok((b, 0..b.byte_length()))
}

impl FromV8 for V8Slice {
  fn from_v8(
    scope: &mut v8::HandleScope,
    value: v8::Local<v8::Value>,
  ) -> Result<Self, crate::Error> {
    to_ranged_buffer(scope, value)
      .and_then(|(b, r)| Self::from_buffer(b, r))
      .map_err(|_| crate::Error::ExpectedBuffer)
  }
}

impl Deref for V8Slice {
  type Target = [u8];
  fn deref(&self) -> &[u8] {
    self.as_slice()
  }
}

impl DerefMut for V8Slice {
  fn deref_mut(&mut self) -> &mut [u8] {
    self.as_slice_mut()
  }
}

impl AsRef<[u8]> for V8Slice {
  fn as_ref(&self) -> &[u8] {
    self.as_slice()
  }
}

impl AsMut<[u8]> for V8Slice {
  fn as_mut(&mut self) -> &mut [u8] {
    self.as_slice_mut()
  }
}