libfringe/src/generator.rs

187 lines
6.6 KiB
Rust

// This file is part of libfringe, a low-level green threading library.
// Copyright (c) whitequark <whitequark@whitequark.org>
// See the LICENSE file included in this distribution.
//! Generators.
//!
//! Generators allow repeatedly suspending the execution of a function,
//! returning a value to the caller, and resuming the suspended function
//! afterwards.
use core::marker::PhantomData;
use core::iter::Iterator;
use core::{ptr, mem};
use stack;
use context;
#[derive(Debug, Clone, Copy)]
pub enum State {
/// Generator can be resumed. This is the initial state.
Suspended,
/// Generator cannot be resumed. This is the state of the generator after
/// the generator function has returned.
Finished
}
/// Generator wraps a function and allows suspending its execution more than
/// once, return a value each time.
///
/// It implements the Iterator trait. The first time `next()` is called,
/// the function is called as `f(yielder)`; every time `next()` is called afterwards,
/// the function is resumed. In either case, it runs until it suspends its execution
/// through `yielder.generate(val)`), in which case `next()` returns `Some(val)`, or
/// returns, in which case `next()` returns `None`. `next()` will return `None`
/// every time after that.
///
/// After the generator function returns, it is safe to reclaim the generator
/// stack using `unwrap()`.
///
/// `state()` can be used to determine whether the generator function has returned;
/// the state is `State::Suspended` after creation and suspension, and `State::Finished`
/// once the generator function returns.
///
/// # Example
///
/// ```
/// use fringe::{OsStack, Generator};
///
/// let stack = OsStack::new(0).unwrap();
/// let mut gen = Generator::new(stack, move |yielder| {
/// for i in 1..4 {
/// yielder.generate(i);
/// }
/// });
/// println!("{:?}", gen.next()); // prints Some(1)
/// println!("{:?}", gen.next()); // prints Some(2)
/// println!("{:?}", gen.next()); // prints Some(3)
/// println!("{:?}", gen.next()); // prints None
/// ```
#[derive(Debug)]
pub struct Generator<Item: Send, Stack: stack::Stack> {
state: State,
context: context::Context<Stack>,
phantom: PhantomData<Item>
}
impl<Item, Stack> Generator<Item, Stack>
where Item: Send, Stack: stack::Stack {
/// Creates a new generator.
pub fn new<F>(stack: Stack, f: F) -> Generator<Item, Stack>
where Stack: stack::GuardedStack, F: FnOnce(&mut Yielder<Item, Stack>) + Send {
unsafe { Generator::unsafe_new(stack, f) }
}
/// Same as `new`, but does not require `stack` to have a guard page.
///
/// This function is unsafe because the generator function can easily violate
/// memory safety by overflowing the stack. It is useful in environments where
/// guarded stacks do not exist, e.g. in absence of an MMU.
pub unsafe fn unsafe_new<F>(stack: Stack, f: F) -> Generator<Item, Stack>
where F: FnOnce(&mut Yielder<Item, Stack>) + Send {
unsafe extern "C" fn generator_wrapper<Item, Stack, F>(info: usize) -> !
where Item: Send, Stack: stack::Stack, F: FnOnce(&mut Yielder<Item, Stack>) {
// Retrieve our environment from the callee and return control to it.
let (mut yielder, f) = ptr::read(info as *mut (Yielder<Item, Stack>, F));
let new_context = context::Context::swap(yielder.context, yielder.context, 0);
// See Yielder::return_.
yielder.context = new_context as *mut context::Context<Stack>;
// Run the body of the generator.
f(&mut yielder);
// Past this point, the generator has dropped everything it has held.
loop { yielder.return_(None) }
}
let mut generator = Generator {
state: State::Suspended,
context: context::Context::new(stack, generator_wrapper::<Item, Stack, F>),
phantom: PhantomData
};
// Transfer environment to the callee.
let mut data = (Yielder::new(&mut generator.context), f);
context::Context::swap(&mut generator.context, &generator.context,
&mut data as *mut (Yielder<Item, Stack>, F) as usize);
mem::forget(data);
generator
}
/// Returns the state of the generator.
#[inline]
pub fn state(&self) -> State { self.state }
/// Extracts the stack from a generator when the generator function has returned.
/// If the generator function has not returned
/// (i.e. `self.state() == State::Suspended`), panics.
pub fn unwrap(self) -> Stack {
match self.state {
State::Suspended => panic!("Argh! Bastard! Don't touch that!"),
State::Finished => unsafe { self.context.unwrap() }
}
}
}
/// Yielder is an interface provided to every generator through which it
/// returns a value.
#[derive(Debug)]
pub struct Yielder<Item: Send, Stack: stack::Stack> {
context: *mut context::Context<Stack>,
phantom: PhantomData<Item>
}
impl<Item, Stack> Yielder<Item, Stack>
where Item: Send, Stack: stack::Stack {
fn new(context: *mut context::Context<Stack>) -> Yielder<Item, Stack> {
Yielder {
context: context,
phantom: PhantomData
}
}
#[inline(always)]
fn return_(&mut self, mut val: Option<Item>) {
unsafe {
let new_context = context::Context::swap(self.context, self.context,
&mut val as *mut Option<Item> as usize);
// The generator can be moved (and with it, the context).
// This changes the address of the context.
// Thus, we update it after each swap.
self.context = new_context as *mut context::Context<Stack>;
// However, between this point and the next time we enter return_
// the generator cannot be moved, as a &mut Generator is necessary
// to resume the generator function.
}
}
/// Suspends the generator and returns `Some(item)` from the `next()`
/// invocation that resumed the generator.
#[inline(always)]
pub fn generate(&mut self, item: Item) {
self.return_(Some(item))
}
}
impl<Item, Stack> Iterator for Generator<Item, Stack>
where Item: Send, Stack: stack::Stack {
type Item = Item;
/// Resumes the generator and return the next value it yields.
/// If the generator function has returned, returns `None`.
#[inline]
fn next(&mut self) -> Option<Self::Item> {
match self.state {
State::Suspended => {
let new_context = &mut self.context as *mut context::Context<Stack> as usize;
let val = unsafe {
ptr::read(context::Context::swap(&mut self.context, &self.context, new_context)
as *mut Option<Item>)
};
if let None = val { self.state = State::Finished }
val
}
State::Finished => None
}
}
}