planus/builder.rs
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use core::{marker::PhantomData, mem::MaybeUninit};
use crate::{backvec::BackVec, Offset, Primitive, WriteAsOffset};
#[derive(Debug)]
/// Builder for serializing flatbuffers.
///
///
/// # Examples
/// ```
/// use planus::Builder;
/// use planus_example::monster_generated::my_game::sample::Weapon;
/// let mut builder = Builder::new();
/// let weapon = Weapon::create(&mut builder, "Axe", 24);
/// builder.finish(weapon, None);
/// ```
pub struct Builder {
pub(crate) inner: BackVec,
#[cfg(feature = "vtable-cache")]
vtable_cache: crate::builder_cache::Cache<crate::builder_cache::VTable>,
// byte slices and strings are sufficiently similar that they can share a
// cache type, but they cannot share the cache because strings need to be
// null byte terminated
#[cfg(feature = "string-cache")]
pub(crate) string_cache: crate::builder_cache::Cache<crate::builder_cache::ByteVec>,
#[cfg(feature = "bytes-cache")]
pub(crate) bytes_cache: crate::builder_cache::Cache<crate::builder_cache::ByteVec>,
// This is a bit complicated. The buffer has support for guaranteeing a
// specific write gets a specific alignment. It has many writes and thus
// many promises, so how does keep track of this this across those promises, even
// when writing from the back?
//
// The algorithm works by aggregating all of the promises into one big promise.
// Specifically, we promise that the remaining part of the buffer will always
// be of size `self.delayed_bytes + self.alignment() * K` where we are free to
// choose K as we want.
//
// Initially we set `delayed_bytes` to 0 and `alignment` to 1, i.e. we have
// only promised to write `0 + 1 * K` bytes, for any `K` we choose, which will
// be true no matter how many bytes we write.
//
// Whenever we get a new request `(req_size, req_alignment)`, then that
// `req_size` will be counted towards the previous promises, i.e. we need
// to decrease `self.delayed_bytes()` by `req_bytes` and calculate the new value
// of `req_size` modulo `alignment`. However we also need to fulfil this new
// promise.
//
// To do this, we do two things. 1) We insert sufficient padding, before the
// current request, to make sure that the current request is compatible with
// the previous ones. 2) We set `alignment = alignment.max(req_alignment)`.
//
// One small wrinkle is that we do not store `alignment` directly for performance
// reasons. Instead we store `alignment_mask = alignment - 1`, so we can do use
// binary and (`&`) instead of modulo (`%`).
delayed_bytes: usize,
alignment_mask: usize,
#[cfg(debug_assertions)]
// Bytes missing to be written by a call to prepare_write
missing_bytes: usize,
}
impl Default for Builder {
fn default() -> Self {
Self::with_capacity(0)
}
}
impl Builder {
/// Creates a new Builder.
pub fn new() -> Self {
Self::with_capacity(0)
}
/// Gets the length of the internal buffer in bytes.
pub fn len(&self) -> usize {
self.inner.len()
}
/// Returns true if the internal buffer is empty.
pub fn is_empty(&self) -> bool {
self.len() == 0
}
/// Creates a new builder with a specific internal capacity already allocated.
pub fn with_capacity(capacity: usize) -> Self {
Self {
inner: BackVec::with_capacity(capacity),
delayed_bytes: 0,
alignment_mask: 0,
#[cfg(feature = "vtable-cache")]
vtable_cache: crate::builder_cache::Cache::default(),
#[cfg(feature = "string-cache")]
string_cache: crate::builder_cache::Cache::default(),
#[cfg(feature = "bytes-cache")]
bytes_cache: crate::builder_cache::Cache::default(),
#[cfg(debug_assertions)]
missing_bytes: 0,
}
}
/// Serializes a string and returns the offset to it
pub fn create_string(&mut self, v: impl WriteAsOffset<str>) -> Offset<str> {
v.prepare(self)
}
/// Serializes a slice and returns the offset to it
pub fn create_vector<T>(&mut self, v: impl WriteAsOffset<[T]>) -> Offset<[T]> {
v.prepare(self)
}
/// Resets the builders internal state and clears the internal buffer.
pub fn clear(&mut self) {
self.inner.clear();
#[cfg(feature = "vtable-cache")]
self.vtable_cache.clear();
#[cfg(feature = "string-cache")]
self.string_cache.clear();
#[cfg(feature = "bytes-cache")]
self.bytes_cache.clear();
self.delayed_bytes = 0;
self.alignment_mask = 0;
#[cfg(debug_assertions)]
{
self.missing_bytes = 0;
}
}
pub(crate) fn prepare_write(&mut self, size: usize, alignment_mask: usize) -> usize {
debug_assert!((alignment_mask + 1) & alignment_mask == 0); // Check that the alignment is a power of two
#[cfg(debug_assertions)]
debug_assert_eq!(self.missing_bytes, 0);
let delayed_bytes = self.delayed_bytes.wrapping_sub(size) & self.alignment_mask;
let needed_padding = delayed_bytes & alignment_mask;
self.delayed_bytes = delayed_bytes.wrapping_sub(needed_padding);
self.alignment_mask |= alignment_mask;
self.inner.reserve(size.wrapping_add(needed_padding));
// TODO: investigate if it makes sense to use an extend_with_zeros_unchecked for performance, given
// that we know we have enough space
self.inner.extend_with_zeros(needed_padding);
debug_assert_eq!(self.delayed_bytes & alignment_mask, 0);
#[cfg(debug_assertions)]
{
self.missing_bytes = size;
}
self.len() + size
}
#[doc(hidden)]
pub fn current_offset<T: ?Sized>(&self) -> Offset<T> {
Offset {
offset: self.inner.len() as u32,
phantom: PhantomData,
}
}
pub(crate) fn write_vtable(&mut self, vtable: &[u8]) -> usize {
const VTABLE_ALIGNMENT: usize = 2;
const VTABLE_ALIGNMENT_MASK: usize = VTABLE_ALIGNMENT - 1;
#[cfg(feature = "vtable-cache")]
let hash = {
let hash = self.vtable_cache.hash(vtable);
if let Some(offset) = self.vtable_cache.get(self.inner.as_slice(), hash, vtable) {
return offset.into();
}
hash
};
let offset = self.prepare_write(vtable.len(), VTABLE_ALIGNMENT_MASK);
self.write(vtable);
#[cfg(feature = "vtable-cache")]
self.vtable_cache
.insert(hash, offset.try_into().unwrap(), self.inner.as_slice());
offset
}
pub(crate) fn write(&mut self, buffer: &[u8]) {
#[cfg(debug_assertions)]
{
self.missing_bytes = self.missing_bytes.checked_sub(buffer.len()).unwrap();
}
// TODO: investigate if it makes sense to use an extend_from_slice_unchecked for performance, given
// that we know we have enough space
self.inner.extend_from_slice(buffer);
}
#[doc(hidden)]
pub unsafe fn write_with(
&mut self,
size: usize,
alignment_mask: usize,
f: impl FnOnce(u32, &mut [MaybeUninit<u8>]),
) {
let offset = self.prepare_write(size, alignment_mask) as u32;
self.inner.extend_write(size, |bytes| f(offset, bytes));
#[cfg(debug_assertions)]
{
self.missing_bytes = self.missing_bytes.checked_sub(size).unwrap();
}
}
/// Finish writing the internal buffer and return a byte slice of it.
///
/// This will make sure all alignment requirements are fullfilled and that
/// the file identifier has been written if specified.
///
/// # Examples
/// ```
/// use planus::Builder;
/// use planus_example::monster_generated::my_game::sample::Weapon;
/// let mut builder = Builder::new();
/// let weapon = Weapon::create(&mut builder, "Axe", 24);
/// builder.finish(weapon, None);
/// ```
///
/// It can also be used to directly serialize an owned flatbuffers struct
/// ```
/// use planus::Builder;
/// use planus_example::monster_generated::my_game::sample::Weapon;
/// let mut builder = Builder::new();
/// let weapon = Weapon { name: Some("Sword".to_string()), damage: 12 };
/// let data = builder.finish(&weapon, None);
/// ```
pub fn finish<T>(
&mut self,
root: impl WriteAsOffset<T>,
file_identifier: Option<[u8; 4]>,
) -> &[u8] {
let root = root.prepare(self);
if let Some(file_identifier) = file_identifier {
// TODO: how does alignment interact with file identifiers? Is the alignment with out without the header?
let offset = self.prepare_write(
8,
<Offset<T> as Primitive>::ALIGNMENT_MASK.max(self.alignment_mask),
) as u32;
self.write(&(offset - 4 - root.offset).to_le_bytes());
self.write(&file_identifier);
} else {
let offset = self.prepare_write(
4,
<Offset<T> as Primitive>::ALIGNMENT_MASK.max(self.alignment_mask),
) as u32;
self.write(&(offset - root.offset).to_le_bytes());
}
debug_assert_eq!(self.delayed_bytes, 0);
self.as_slice()
}
/// Returns a reference to the current data buffer.
///
/// It will return the same slice as the one return by [`finish`], unless additional data has been appened afterwards.
///
/// [`finish`]: Self::finish
pub fn as_slice(&self) -> &[u8] {
self.inner.as_slice()
}
}
#[cfg(test)]
mod tests {
use alloc::vec::Vec;
use rand::{thread_rng, Rng};
use super::*;
#[test]
fn test_buffer_random() {
let mut slice = [0; 128];
let mut rng = thread_rng();
let mut back_offsets: Vec<(usize, usize, usize)> = Vec::new();
for _ in 0..50 {
let mut builder = Builder::new();
back_offsets.clear();
for byte in 1..50 {
let size: usize = rng.gen::<usize>() % slice.len();
let slice = &mut slice[..size];
for p in &mut *slice {
*p = byte;
}
let alignment: usize = 1 << (rng.gen::<u32>() % 5);
let alignment_mask = alignment - 1;
let offset = builder.prepare_write(size, alignment_mask);
let len_before = builder.inner.len();
builder.write(slice);
assert_eq!(offset, builder.len());
assert!(builder.inner.len() < len_before + slice.len() + alignment);
back_offsets.push((builder.inner.len(), size, alignment));
}
let random_padding: usize = rng.gen::<usize>() % slice.len();
let slice = &mut slice[..random_padding];
for p in &mut *slice {
*p = rng.gen();
}
builder.prepare_write(random_padding, 1);
builder.write(slice);
let buffer = builder.finish(builder.current_offset::<()>(), None);
for (i, (back_offset, size, alignment)) in back_offsets.iter().enumerate() {
let byte = (i + 1) as u8;
let offset = buffer.len() - back_offset;
assert_eq!(offset % alignment, 0);
assert!(buffer[offset..offset + size].iter().all(|&b| b == byte));
}
}
}
#[test]
fn test_buffer_align() {
let mut builder = Builder::new();
builder.prepare_write(3, 0);
builder.write(b"MNO");
assert_eq!(builder.delayed_bytes, 0);
builder.prepare_write(4, 1);
builder.write(b"IJKL");
assert_eq!(builder.delayed_bytes, 0);
builder.prepare_write(8, 3);
builder.write(b"ABCDEFGH");
assert_eq!(builder.delayed_bytes, 0);
builder.prepare_write(7, 0);
builder.write(b"0123456");
assert_eq!(
builder.finish(builder.current_offset::<()>(), None),
b"\x05\x00\x00\x00\x000123456ABCDEFGHIJKLMNO"
);
builder.clear();
builder.prepare_write(4, 3);
builder.write(b"IJKL");
assert_eq!(builder.delayed_bytes, 0);
builder.prepare_write(1, 0);
builder.write(b"X");
assert_eq!(builder.delayed_bytes, 3);
builder.prepare_write(1, 0);
builder.write(b"Y");
assert_eq!(builder.delayed_bytes, 2);
builder.prepare_write(8, 7);
builder.write(b"ABCDEFGH");
assert_eq!(builder.delayed_bytes, 0);
assert_eq!(
builder.finish(builder.current_offset::<()>(), None),
b"\x08\x00\x00\x00\x00\x00\x00\x00ABCDEFGH\x00\x00YXIJKL"
);
}
}