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/// An amount of linear memory pages.
#[derive(Debug, Default, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
#[repr(transparent)]
pub struct Pages(u32);
impl Pages {
/// The maximum number of pages on the `wasm32` target.
///
/// # Note
///
/// This is the maximum since WebAssembly is a 32-bit platform
/// and a page is 2^16 bytes in size. Therefore, there can be at
/// most 2^16 pages of a single linear memory so that all bytes
/// are still accessible.
pub const fn max() -> Self {
Self(65536) // 2^16
}
pub fn into_inner(self) -> u32 {
self.0
}
}
impl From<u16> for Pages {
/// Creates an `amount` of [`Pages`].
///
/// # Note
///
/// This is infallible since `u16` cannot represent invalid amounts
/// of [`Pages`]. However, `u16` can also not represent [`Pages::max()`].
///
/// [`Pages::max()`]: struct.Pages.html#method.max
fn from(amount: u16) -> Self {
Self(u32::from(amount))
}
}
impl Pages {
/// Creates a new amount of [`Pages`] if the amount is within bounds.
///
/// Returns `None` if the given `amount` of [`Pages`] exceeds [`Pages::max()`].
///
/// [`Pages::max()`]: struct.Pages.html#method.max
pub fn new(amount: u32) -> Option<Self> {
if amount > u32::from(Self::max()) {
return None;
}
Some(Self(amount))
}
pub const fn new_unchecked(amount: u32) -> Self {
Self(amount)
}
/// Adds the given amount of pages to `self`.
///
/// Returns `Some` if the result is within bounds and `None` otherwise.
pub fn checked_add<T>(self, rhs: T) -> Option<Self>
where
T: Into<u32>,
{
let lhs: u32 = self.into();
let rhs: u32 = rhs.into();
lhs.checked_add(rhs).and_then(Self::new)
}
/// Substracts the given amount of pages from `self`.
///
/// Returns `None` if the subtraction underflows or the result is out of bounds.
pub fn checked_sub<T>(self, rhs: T) -> Option<Self>
where
T: Into<u32>,
{
let lhs: u32 = self.into();
let rhs: u32 = rhs.into();
lhs.checked_sub(rhs).and_then(Self::new)
}
/// Returns the amount of bytes required for the amount of [`Pages`].
///
/// Returns `None` if the amount of pages represented by `self` cannot
/// be represented as bytes on the executing platform.
pub fn to_bytes(self) -> Option<usize> {
Bytes::new(self).map(Into::into)
}
}
impl From<Pages> for u32 {
fn from(pages: Pages) -> Self {
pages.0
}
}
/// An amount of bytes of a linear memory.
#[derive(Debug, Default, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
#[repr(transparent)]
pub struct Bytes(usize);
impl Bytes {
/// A 16-bit platform cannot represent the size of a single Wasm page.
const fn max16() -> u64 {
i16::MAX as u64 + 1
}
/// A 32-bit platform can represent at most i32::MAX + 1 Wasm pages.
const fn max32() -> u64 {
i32::MAX as u64 + 1
}
/// A 64-bit platform can represent all possible u32::MAX + 1 Wasm pages.
const fn max64() -> u64 {
u32::MAX as u64 + 1
}
/// The bytes per WebAssembly linear memory page.
///
/// # Note
///
/// As mandated by the WebAssembly specification every linear memory page
/// has exactly 2^16 (65536) bytes.
pub const fn per_page() -> Self {
Self(65536) // 2^16
}
/// Creates [`Bytes`] from the given amount of [`Pages`] if possible.
///
/// Returns `None` if the amount of bytes is out of bounds. This may
/// happen for example when trying to allocate bytes for more than
/// `i16::MAX + 1` pages on a 32-bit platform since that amount would
/// not be representable by a pointer sized `usize`.
fn new(pages: Pages) -> Option<Bytes> {
if cfg!(target_pointer_width = "16") {
Self::new16(pages)
} else if cfg!(target_pointer_width = "32") {
Self::new32(pages)
} else if cfg!(target_pointer_width = "64") {
Self::new64(pages)
} else {
None
}
}
/// Creates [`Bytes`] from the given amount of [`Pages`] as if
/// on a 16-bit platform if possible.
///
/// Returns `None` otherwise.
///
/// # Note
///
/// This API exists in isolation for cross-platform testing purposes.
fn new16(pages: Pages) -> Option<Bytes> {
Self::new_impl(pages, Bytes::max16())
}
/// Creates [`Bytes`] from the given amount of [`Pages`] as if
/// on a 32-bit platform if possible.
///
/// Returns `None` otherwise.
///
/// # Note
///
/// This API exists in isolation for cross-platform testing purposes.
fn new32(pages: Pages) -> Option<Bytes> {
Self::new_impl(pages, Bytes::max32())
}
/// Creates [`Bytes`] from the given amount of [`Pages`] as if
/// on a 64-bit platform if possible.
///
/// Returns `None` otherwise.
///
/// # Note
///
/// This API exists in isolation for cross-platform testing purposes.
fn new64(pages: Pages) -> Option<Bytes> {
Self::new_impl(pages, Bytes::max64())
}
/// Actual underlying implementation of [`Bytes::new`].
fn new_impl(pages: Pages, max: u64) -> Option<Bytes> {
let pages = u64::from(u32::from(pages));
let bytes_per_page = usize::from(Self::per_page()) as u64;
let bytes = pages
.checked_mul(bytes_per_page)
.filter(|&amount| amount <= max)?;
Some(Self(bytes as usize))
}
}
impl From<Bytes> for usize {
#[inline]
fn from(bytes: Bytes) -> Self {
bytes.0
}
}
#[cfg(test)]
mod tests {
use super::*;
fn pages(amount: u32) -> Pages {
Pages::new(amount).unwrap()
}
fn bytes(amount: usize) -> Bytes {
Bytes(amount)
}
#[test]
fn pages_max() {
assert_eq!(Pages::max(), pages(u32::from(u16::MAX) + 1));
}
#[test]
fn pages_new() {
assert_eq!(Pages::new(0), Some(Pages(0)));
assert_eq!(Pages::new(1), Some(Pages(1)));
assert_eq!(Pages::new(1000), Some(Pages(1000)));
assert_eq!(
Pages::new(u32::from(u16::MAX)),
Some(Pages(u32::from(u16::MAX)))
);
assert_eq!(Pages::new(u32::from(u16::MAX) + 1), Some(Pages::max()));
assert_eq!(Pages::new(u32::from(u16::MAX) + 2), None);
assert_eq!(Pages::new(u32::MAX), None);
}
#[test]
fn pages_checked_add() {
let max_pages = u32::from(Pages::max());
assert_eq!(pages(0).checked_add(0u32), Some(pages(0)));
assert_eq!(pages(0).checked_add(1u32), Some(pages(1)));
assert_eq!(pages(1).checked_add(0u32), Some(pages(1)));
assert_eq!(pages(0).checked_add(max_pages), Some(Pages::max()));
assert_eq!(pages(0).checked_add(Pages::max()), Some(Pages::max()));
assert_eq!(pages(1).checked_add(max_pages), None);
assert_eq!(pages(1).checked_add(Pages::max()), None);
assert_eq!(Pages::max().checked_add(0u32), Some(Pages::max()));
assert_eq!(Pages::max().checked_add(1u32), None);
assert_eq!(pages(0).checked_add(u32::MAX), None);
for i in 0..100 {
for j in 0..100 {
assert_eq!(pages(i).checked_add(pages(j)), Some(pages(i + j)));
}
}
}
#[test]
fn pages_checked_sub() {
let max_pages = u32::from(Pages::max());
assert_eq!(pages(0).checked_sub(0u32), Some(pages(0)));
assert_eq!(pages(0).checked_sub(1u32), None);
assert_eq!(pages(1).checked_sub(0u32), Some(pages(1)));
assert_eq!(pages(1).checked_sub(1u32), Some(pages(0)));
assert_eq!(Pages::max().checked_sub(Pages::max()), Some(pages(0)));
assert_eq!(Pages::max().checked_sub(u32::MAX), None);
assert_eq!(Pages::max().checked_sub(1u32), Some(pages(max_pages - 1)));
for i in 0..100 {
for j in 0..100 {
assert_eq!(pages(i).checked_sub(pages(j)), i.checked_sub(j).map(pages));
}
}
}
#[test]
fn pages_to_bytes() {
assert_eq!(pages(0).to_bytes(), Some(0));
if cfg!(target_pointer_width = "16") {
assert_eq!(pages(1).to_bytes(), None);
}
if cfg!(target_pointer_width = "32") || cfg!(target_pointer_width = "64") {
let bytes_per_page = usize::from(Bytes::per_page());
for n in 1..10 {
assert_eq!(pages(n as u32).to_bytes(), Some(n * bytes_per_page));
}
}
}
#[test]
fn bytes_new16() {
assert_eq!(Bytes::new16(pages(0)), Some(bytes(0)));
assert_eq!(Bytes::new16(pages(1)), None);
assert!(Bytes::new16(Pages::max()).is_none());
}
#[test]
fn bytes_new32() {
assert_eq!(Bytes::new32(pages(0)), Some(bytes(0)));
assert_eq!(Bytes::new32(pages(1)), Some(Bytes::per_page()));
let bytes_per_page = usize::from(Bytes::per_page());
for n in 2..10 {
assert_eq!(
Bytes::new32(pages(n as u32)),
Some(bytes(n * bytes_per_page))
);
}
assert!(Bytes::new32(pages(i16::MAX as u32 + 1)).is_some());
assert!(Bytes::new32(pages(i16::MAX as u32 + 2)).is_none());
assert!(Bytes::new32(Pages::max()).is_none());
}
#[test]
fn bytes_new64() {
assert_eq!(Bytes::new64(pages(0)), Some(bytes(0)));
assert_eq!(Bytes::new64(pages(1)), Some(Bytes::per_page()));
let bytes_per_page = usize::from(Bytes::per_page());
for n in 2..10 {
assert_eq!(
Bytes::new64(pages(n as u32)),
Some(bytes(n * bytes_per_page))
);
}
assert!(Bytes::new64(Pages(u32::from(u16::MAX) + 1)).is_some());
assert!(Bytes::new64(Pages(u32::from(u16::MAX) + 2)).is_none());
assert!(Bytes::new64(Pages::max()).is_some());
}
}