cjc_runtime/
buffer.rs

1//! COW (Copy-on-Write) buffer -- the fundamental memory primitive under [`Tensor`].
2//!
3//! [`Buffer<T>`] provides reference-counted storage with lazy deep-copy
4//! semantics. Cloning a buffer increments the refcount (O(1)); mutation
5//! via [`Buffer::set`] or [`Buffer::make_unique`] triggers a deep copy
6//! only when the buffer is shared (`refcount > 1`).
7//!
8//! # Determinism
9//!
10//! Buffer operations are fully deterministic. No randomized hashing,
11//! no platform-dependent allocation strategies. The COW mechanism
12//! ensures that mutation of shared data always produces a fresh,
13//! independent copy.
14//!
15//! [`Tensor`]: crate::tensor::Tensor
16
17use std::cell::{Ref, RefCell, RefMut};
18use std::rc::Rc;
19
20use crate::error::RuntimeError;
21
22// ---------------------------------------------------------------------------
23// 1. Buffer<T> -- Deterministic memory allocation with COW semantics
24// ---------------------------------------------------------------------------
25
26/// A reference-counted buffer with copy-on-write semantics.
27///
28/// Internally backed by `Rc<RefCell<Vec<T>>>`. When multiple `Buffer`s share
29/// the same underlying storage, a mutation via [`Buffer::set`] or
30/// [`Buffer::make_unique`] triggers a deep copy so that other holders are
31/// unaffected.
32#[derive(Debug)]
33pub struct Buffer<T: Clone> {
34    inner: Rc<RefCell<Vec<T>>>,
35}
36
37impl<T: Clone> Buffer<T> {
38    /// Allocate a buffer of `len` elements, each initialized to `default`.
39    pub fn alloc(len: usize, default: T) -> Self {
40        Buffer {
41            inner: Rc::new(RefCell::new(vec![default; len])),
42        }
43    }
44
45    /// Create a buffer from an existing `Vec<T>`.
46    pub fn from_vec(data: Vec<T>) -> Self {
47        Buffer {
48            inner: Rc::new(RefCell::new(data)),
49        }
50    }
51
52    /// Number of elements in the buffer.
53    pub fn len(&self) -> usize {
54        self.inner.borrow().len()
55    }
56
57    /// Whether the buffer is empty.
58    pub fn is_empty(&self) -> bool {
59        self.inner.borrow().is_empty()
60    }
61
62    /// Read the value at `idx`. Returns `None` if out of bounds.
63    pub fn get(&self, idx: usize) -> Option<T> {
64        self.inner.borrow().get(idx).cloned()
65    }
66
67    /// Write `val` at `idx`. If the buffer is shared (refcount > 1), a deep
68    /// copy is performed first (COW). Returns `Err` if `idx` is out of bounds.
69    pub fn set(&mut self, idx: usize, val: T) -> Result<(), RuntimeError> {
70        self.make_unique();
71        let mut vec = self.inner.borrow_mut();
72        if idx >= vec.len() {
73            return Err(RuntimeError::IndexOutOfBounds {
74                index: idx,
75                length: vec.len(),
76            });
77        }
78        vec[idx] = val;
79        Ok(())
80    }
81
82    /// Return a snapshot of the data as a `Vec<T>`.
83    pub fn as_slice(&self) -> Vec<T> {
84        self.inner.borrow().clone()
85    }
86
87    /// Borrow the underlying Vec without cloning.
88    /// The returned `Ref` guard keeps the borrow alive.
89    pub fn borrow_data(&self) -> Ref<Vec<T>> {
90        self.inner.borrow()
91    }
92
93    /// Mutably borrow the underlying Vec without cloning.
94    /// Call `make_unique()` first if the buffer may be shared.
95    pub fn borrow_data_mut(&self) -> RefMut<Vec<T>> {
96        self.inner.borrow_mut()
97    }
98
99    /// Force a deep copy, returning a new `Buffer` that does not share
100    /// storage with `self`.
101    pub fn clone_buffer(&self) -> Buffer<T> {
102        Buffer {
103            inner: Rc::new(RefCell::new(self.inner.borrow().clone())),
104        }
105    }
106
107    /// Ensure this `Buffer` has exclusive ownership of the underlying data.
108    /// If the refcount is > 1, the data is deep-copied and this `Buffer` is
109    /// re-pointed to the fresh copy.
110    pub fn make_unique(&mut self) {
111        if Rc::strong_count(&self.inner) > 1 {
112            let data = self.inner.borrow().clone();
113            self.inner = Rc::new(RefCell::new(data));
114        }
115    }
116
117    /// Number of live references to the underlying storage.
118    pub fn refcount(&self) -> usize {
119        Rc::strong_count(&self.inner)
120    }
121}
122
123impl<T: Clone> Clone for Buffer<T> {
124    /// Cloning a `Buffer` increments the refcount — it does NOT copy data.
125    fn clone(&self) -> Self {
126        Buffer {
127            inner: Rc::clone(&self.inner),
128        }
129    }
130}
131
cjc_runtime/buffer.rs

cjc_runtime/
buffer.rs
