libfringe/src/generator.rs

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// This file is part of libfringe, a low-level green threading library.
// Copyright (c) whitequark <whitequark@whitequark.org>
// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
// http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
// http://opensource.org/licenses/MIT>, at your option. This file may not be
// copied, modified, or distributed except according to those terms.
//! 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::{ptr, mem};
use stack;
use context::Context;
#[derive(Debug, Clone, Copy)]
pub enum State {
/// Generator can be resumed. This is the initial state.
Runnable,
/// Generator cannot be resumed. This is the state of the generator after
/// the generator function has returned or panicked.
Unavailable
}
/// Generator wraps a function and allows suspending its execution more than once, returning
/// a value each time.
///
/// The first time `resume(input0)` is called, the function is called as `f(yielder, input0)`.
/// It runs until it suspends its execution through `yielder.suspend(output0)`, after which
/// `resume(input0)` returns `output0`. The function can be resumed again using `resume(input1)`,
/// after which `yielder.suspend(output0)` returns `input1`, and so on. Once the function returns,
/// the `resume()` call will return `None`, and it will return `None` every time it is called
/// after that.
///
/// If the generator function panics, the panic is propagated through the `resume()` call as usual.
///
/// After the generator function returns or panics, 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::Runnable` after creation and suspension, and `State::Unavailable`
/// once the generator function returns or panics.
///
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/// When the input type is `()`, a generator implements the Iterator trait.
///
/// # Example
///
/// ```
/// use fringe::{OsStack, Generator};
///
/// let stack = OsStack::new(0).unwrap();
/// let mut add_one = Generator::new(stack, move |yielder, mut input| {
/// loop {
/// if input == 0 { break }
/// input = yielder.suspend(input + 1)
/// }
/// });
/// println!("{:?}", add_one.resume(2)); // prints Some(3)
/// println!("{:?}", add_one.resume(3)); // prints Some(4)
/// println!("{:?}", add_one.resume(0)); // prints None
/// ```
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///
/// # Iterator example
///
/// ```
/// use fringe::{OsStack, Generator};
///
/// let stack = OsStack::new(0).unwrap();
/// let mut nat = Generator::new(stack, move |yielder, ()| {
/// for i in 1.. { yielder.suspend(i) }
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/// });
/// println!("{:?}", nat.next()); // prints Some(0)
/// println!("{:?}", nat.next()); // prints Some(1)
/// println!("{:?}", nat.next()); // prints Some(2)
/// ```
#[derive(Debug)]
pub struct Generator<Input: Send, Output: Send, Stack: stack::Stack> {
state: State,
context: Context<Stack>,
phantom: (PhantomData<*const Input>, PhantomData<*const Output>)
}
impl<Input, Output, Stack> Generator<Input, Output, Stack>
where Input: Send, Output: Send, Stack: stack::Stack {
/// Creates a new generator.
pub fn new<F>(stack: Stack, f: F) -> Generator<Input, Output, Stack>
where Stack: stack::GuardedStack,
F: FnOnce(&mut Yielder<Input, Output, Stack>, Input) + 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<Input, Output, Stack>
where F: FnOnce(&mut Yielder<Input, Output, Stack>, Input) + Send {
unsafe extern "C" fn generator_wrapper<Input, Output, Stack, F>(env: usize) -> !
where Input: Send, Output: Send, Stack: stack::Stack,
F: FnOnce(&mut Yielder<Input, Output, Stack>, Input) {
// Retrieve our environment from the callee and return control to it.
let (mut yielder, f) = ptr::read(env as *mut (Yielder<Input, Output, Stack>, F));
let data = Context::swap(yielder.context, yielder.context, 0);
// See the second half of Yielder::suspend_bare.
let (new_context, input) = ptr::read(data as *mut (*mut Context<Stack>, Input));
yielder.context = new_context as *mut Context<Stack>;
// Run the body of the generator.
f(&mut yielder, input);
// Past this point, the generator has dropped everything it has held.
loop { yielder.suspend_bare(None); }
}
let mut generator = Generator {
state: State::Runnable,
context: Context::new(stack, generator_wrapper::<Input, Output, Stack, F>),
phantom: (PhantomData, PhantomData)
};
// Transfer environment to the callee.
let mut env = (Yielder::new(&mut generator.context), f);
Context::swap(&mut generator.context, &generator.context,
&mut env as *mut (Yielder<Input, Output, Stack>, F) as usize);
mem::forget(env);
generator
}
/// Resumes the generator and return the next value it yields.
/// If the generator function has returned, returns `None`.
#[inline]
pub fn resume(&mut self, input: Input) -> Option<Output> {
match self.state {
State::Runnable => {
// Set the state to Unavailable. Since we have exclusive access to the generator,
// the only case where this matters is the generator function panics, after which
// it must not be invocable again.
self.state = State::Unavailable;
// Switch to the generator function, and retrieve the yielded value.
let val = unsafe {
let mut data_in = (&mut self.context as *mut Context<Stack>, input);
let data_out =
ptr::read(Context::swap(&mut self.context, &self.context,
&mut data_in as *mut (*mut Context<Stack>, Input) as usize)
as *mut Option<Output>);
mem::forget(data_in);
data_out
};
// Unless the generator function has returned, it can be switched to again, so
// set the state to Runnable.
if val.is_some() { self.state = State::Runnable }
val
}
State::Unavailable => None
}
}
/// Returns the state of the generator.
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#[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::Runnable`), panics.
pub fn unwrap(self) -> Stack {
match self.state {
State::Runnable => panic!("Argh! Bastard! Don't touch that!"),
State::Unavailable => unsafe { self.context.unwrap() }
}
}
}
/// Yielder is an interface provided to every generator through which it
/// returns a value.
#[derive(Debug)]
pub struct Yielder<Input: Send, Output: Send, Stack: stack::Stack> {
context: *mut Context<Stack>,
phantom: (PhantomData<*const Input>, PhantomData<*const Output>)
}
impl<Input, Output, Stack> Yielder<Input, Output, Stack>
where Input: Send, Output: Send, Stack: stack::Stack {
fn new(context: *mut Context<Stack>) -> Yielder<Input, Output, Stack> {
Yielder {
context: context,
phantom: (PhantomData, PhantomData)
}
}
#[inline(always)]
fn suspend_bare(&mut self, mut val: Option<Output>) -> Input {
unsafe {
let data = Context::swap(self.context, self.context,
&mut val as *mut Option<Output> as usize);
mem::forget(val);
let (new_context, input) = ptr::read(data as *mut (*mut Context<Stack>, Input));
// 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;
// However, between this point and the next time we enter suspend_bare
// the generator cannot be moved, as a &mut Generator is necessary
// to resume the generator function.
input
}
}
/// Suspends the generator and returns `Some(item)` from the `resume()`
/// invocation that resumed the generator.
#[inline(always)]
pub fn suspend(&mut self, item: Output) -> Input {
self.suspend_bare(Some(item))
}
}
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impl<Output, Stack> Iterator for Generator<(), Output, Stack>
where Output: Send, Stack: stack::Stack {
type Item = Output;
fn next(&mut self) -> Option<Self::Item> { self.resume(()) }
}