11. Unsafe Rust — Controlled Danger 🔴

unsafe {
    // 1. Dereference a raw pointer
    let ptr: *const i32 = &42;
    let value = *ptr; // Could be a dangling/null pointer

    // 2. Call an unsafe function
    let layout = std::alloc::Layout::new::<u64>();
    let mem = std::alloc::alloc(layout);

    // 3. Access a mutable static variable
    static mut COUNTER: u32 = 0;
    COUNTER += 1; // Data race if multiple threads access

    // 4. Implement an unsafe trait
    // unsafe impl Send for MyType {}

    // 5. Access fields of a union
    // union IntOrFloat { i: i32, f: f32 }
    // let u = IntOrFloat { i: 42 };
    // let f = u.f; // Reinterpret bits — could be garbage
}

/// A fixed-capacity stack-allocated buffer.
/// All public methods are safe — the unsafe is encapsulated.
pub struct StackBuf<T, const N: usize> {
    data: [std::mem::MaybeUninit<T>; N],
    len: usize,
}

impl<T, const N: usize> StackBuf<T, N> {
    pub fn new() -> Self {
        StackBuf {
            // Each element is individually MaybeUninit — no unsafe needed.
            // `const { ... }` blocks (Rust 1.79+) let us repeat a non-Copy
            // const expression N times.
            data: [const { std::mem::MaybeUninit::uninit() }; N],
            len: 0,
        }
    }

    pub fn push(&mut self, value: T) -> Result<(), T> {
        if self.len >= N {
            return Err(value); // Buffer full — return value to caller
        }
        // SAFETY: len < N, so data[len] is within bounds.
        // We write a valid T into the MaybeUninit slot.
        self.data[self.len] = std::mem::MaybeUninit::new(value);
        self.len += 1;
        Ok(())
    }

    pub fn get(&self, index: usize) -> Option<&T> {
        if index < self.len {
            // SAFETY: index < len, and data[0..len] are all initialized.
            Some(unsafe { self.data[index].assume_init_ref() })
        } else {
            None
        }
    }
}

impl<T, const N: usize> Drop for StackBuf<T, N> {
    fn drop(&mut self) {
        // SAFETY: data[0..len] are initialized — drop them properly.
        for i in 0..self.len {
            unsafe { self.data[i].assume_init_drop(); }
        }
    }
}

// Declare the C function signature:
extern "C" {
    fn strlen(s: *const std::ffi::c_char) -> usize;
    fn printf(format: *const std::ffi::c_char, ...) -> std::ffi::c_int;
}

// Safe wrapper:
fn safe_strlen(s: &str) -> usize {
    let c_string = std::ffi::CString::new(s).expect("string contains null byte");
    // SAFETY: c_string is a valid null-terminated string, alive for the call.
    unsafe { strlen(c_string.as_ptr()) }
}

// Calling Rust from C (export a function):
#[no_mangle]
pub extern "C" fn rust_add(a: i32, b: i32) -> i32 {
    a + b
}

use bumpalo::Bump;

fn process_sensor_frame(raw_data: &[u8]) {
    // Create an arena for this frame's allocations
    let arena = Bump::new();

    // Allocate objects in the arena — ~2ns each (just a pointer bump)
    let header = arena.alloc(parse_header(raw_data));
    let readings: &mut [f32] = arena.alloc_slice_fill_default(header.sensor_count);

    for (i, chunk) in raw_data[header.payload_offset..].chunks(4).enumerate() {
        if i < readings.len() {
            readings[i] = f32::from_le_bytes(chunk.try_into().unwrap());
        }
    }

    // Use readings...
    let avg = readings.iter().sum::<f32>() / readings.len() as f32;
    println!("Frame avg: {avg:.2}");

    // `arena` drops here — ALL allocations freed at once in O(1)
    // No per-object destructor overhead, no fragmentation
}
# fn parse_header(_: &[u8]) -> Header { Header { sensor_count: 4, payload_offset: 8 } }
# struct Header { sensor_count: usize, payload_offset: usize }

use typed_arena::Arena;

struct AstNode<'a> {
    value: i32,
    children: Vec<&'a AstNode<'a>>,
}

fn build_tree() {
    let arena: Arena<AstNode<'_>> = Arena::new();

    // Allocate nodes — returns &AstNode tied to arena's lifetime
    let root = arena.alloc(AstNode { value: 1, children: vec![] });
    let left = arena.alloc(AstNode { value: 2, children: vec![] });
    let right = arena.alloc(AstNode { value: 3, children: vec![] });

    // Build the tree — all references valid as long as `arena` lives
    // (Mutable access requires interior mutability for truly mutable trees)

    println!("Root: {}, Left: {}, Right: {}", root.value, left.value, right.value);

    // `arena` drops here — all nodes freed at once
}

use slab::Slab;

struct Connection {
    id: u64,
    buffer: [u8; 1024],
    active: bool,
}

fn connection_pool_example() {
    // Pre-allocate a slab for connections
    let mut connections: Slab<Connection> = Slab::with_capacity(256);

    // Insert returns a key (usize index) — O(1)
    let key1 = connections.insert(Connection {
        id: 1001,
        buffer: [0; 1024],
        active: true,
    });

    let key2 = connections.insert(Connection {
        id: 1002,
        buffer: [0; 1024],
        active: true,
    });

    // Access by key — O(1)
    if let Some(conn) = connections.get_mut(key1) {
        conn.buffer[0..5].copy_from_slice(b"hello");
    }

    // Remove returns the value — O(1), slot is reused for next insert
    let removed = connections.remove(key2);
    assert_eq!(removed.id, 1002);

    // Next insert reuses the freed slot — no fragmentation
    let key3 = connections.insert(Connection {
        id: 1003,
        buffer: [0; 1024],
        active: true,
    });
    assert_eq!(key3, key2); // Same slot reused!
}

#![cfg_attr(not(test), no_std)]

use core::alloc::Layout;
use core::cell::{Cell, UnsafeCell};

/// A simple bump allocator backed by a fixed-size byte array.
/// Not thread-safe — use per-core or with a lock for multi-threaded contexts.
///
/// **Important**: Like `bumpalo`, this arena does NOT call destructors on
/// allocated items when the arena is dropped. Types with `Drop` impls will
/// leak their resources (file handles, sockets, etc.). Only allocate types
/// without meaningful `Drop` impls, or manually drop them before the arena.
pub struct FixedArena<const N: usize> {
    // UnsafeCell is REQUIRED here: we mutate `buf` through `&self`.
    // Without UnsafeCell, casting &self.buf to *mut u8 would be UB
    // (violates Rust's aliasing model — shared ref implies immutable).
    buf: UnsafeCell<[u8; N]>,
    offset: Cell<usize>, // Interior mutability for &self allocation
}

impl<const N: usize> FixedArena<N> {
    pub const fn new() -> Self {
        FixedArena {
            buf: UnsafeCell::new([0; N]),
            offset: Cell::new(0),
        }
    }

    /// Allocate a `T` in the arena. Returns `None` if out of space.
    pub fn alloc<T>(&self, value: T) -> Option<&mut T> {
        let layout = Layout::new::<T>();
        let current = self.offset.get();

        // Align up
        let aligned = (current + layout.align() - 1) & !(layout.align() - 1);
        let new_offset = aligned + layout.size();

        if new_offset > N {
            return None; // Arena full
        }

        self.offset.set(new_offset);

        // SAFETY:
        // - `aligned` is within `buf` bounds (checked above)
        // - Alignment is correct (aligned to T's requirement)
        // - No aliasing: each alloc returns a unique, non-overlapping region
        // - UnsafeCell grants permission to mutate through &self
        // - The arena outlives the returned reference (caller must ensure)
        let ptr = unsafe {
            let base = (self.buf.get() as *mut u8).add(aligned);
            let typed = base as *mut T;
            typed.write(value);
            &mut *typed
        };

        Some(ptr)
    }

    /// Reset the arena — invalidates all previous allocations.
    /// Caller must ensure no references to arena-allocated data exist.
    pub unsafe fn reset(&self) {
        self.offset.set(0);
    }

    pub fn used(&self) -> usize {
        self.offset.get()
    }

    pub fn remaining(&self) -> usize {
        N - self.offset.get()
    }
}

graph TD
    A["What's your allocation pattern?"] --> B{All same type?}
    A --> I{"Environment?"}
    B -->|Yes| C{Need individual free?}
    B -->|No| D{Need individual free?}
    C -->|Yes| E["<b>Slab</b><br/>slab crate<br/>O(1) alloc + free<br/>Index-based access"]
    C -->|No| F["<b>typed-arena</b><br/>Bulk alloc, bulk free<br/>Lifetime-scoped refs"]
    D -->|Yes| G["<b>Standard allocator</b><br/>Box, Vec, etc.<br/>General-purpose malloc"]
    D -->|No| H["<b>Bump arena</b><br/>bumpalo crate<br/>~2ns alloc, O(1) bulk free"]
    
    I -->|no_std| J["FixedArena (custom)<br/>or embedded-alloc"]
    I -->|std| K["bumpalo / typed-arena / slab"]
    
    style E fill:#91e5a3,color:#000
    style F fill:#91e5a3,color:#000
    style G fill:#89CFF0,color:#000
    style H fill:#91e5a3,color:#000
    style J fill:#ffa07a,color:#000
    style K fill:#91e5a3,color:#000

use std::mem::MaybeUninit;

pub struct FixedVec<T, const N: usize> {
    data: [MaybeUninit<T>; N],
    len: usize,
}

impl<T, const N: usize> FixedVec<T, N> {
    pub fn new() -> Self {
        FixedVec {
            data: [const { MaybeUninit::uninit() }; N],
            len: 0,
        }
    }

    pub fn push(&mut self, value: T) -> Result<(), T> {
        if self.len >= N { return Err(value); }
        // SAFETY: len < N, so data[len] is within bounds.
        self.data[self.len] = MaybeUninit::new(value);
        self.len += 1;
        Ok(())
    }

    pub fn pop(&mut self) -> Option<T> {
        if self.len == 0 { return None; }
        self.len -= 1;
        // SAFETY: data[len] was initialized (len was > 0 before decrement).
        Some(unsafe { self.data[self.len].assume_init_read() })
    }

    pub fn as_slice(&self) -> &[T] {
        // SAFETY: data[0..len] are all initialized, and MaybeUninit<T>
        // has the same layout as T.
        unsafe { std::slice::from_raw_parts(self.data.as_ptr() as *const T, self.len) }
    }

    pub fn len(&self) -> usize { self.len }
    pub fn is_empty(&self) -> bool { self.len == 0 }
}

impl<T, const N: usize> Drop for FixedVec<T, N> {
    fn drop(&mut self) {
        // SAFETY: data[0..len] are initialized — drop each one.
        for i in 0..self.len {
            unsafe { self.data[i].assume_init_drop(); }
        }
    }
}

fn main() {
    let mut v = FixedVec::<String, 4>::new();
    v.push("hello".into()).unwrap();
    v.push("world".into()).unwrap();
    assert_eq!(v.as_slice(), &["hello", "world"]);
    assert_eq!(v.pop(), Some("world".into()));
    assert_eq!(v.len(), 1);
}