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 core::cell::Cell;
use stack;
use debug;
use arch::{self, StackPointer};
#[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,
stack: Stack,
stack_id: debug::StackId,
stack_ptr: arch::StackPointer,
phantom: PhantomData<(*const Input, *const Output)>
}
impl<Input, Output, Stack> Generator<Input, Output, Stack>
where Input: Send, Output: Send, Stack: stack::Stack {
/// Creates a new generator.
///
/// See also the [contract](../trait.GuardedStack.html) that needs to be fulfilled by `stack`.
pub fn new<F>(stack: Stack, f: F) -> Generator<Input, Output, Stack>
where Stack: stack::GuardedStack,
F: FnOnce(&mut Yielder<Input, Output>, 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.
///
/// See also the [contract](../trait.Stack.html) that needs to be fulfilled by `stack`.
pub unsafe fn unsafe_new<F>(stack: Stack, f: F) -> Generator<Input, Output, Stack>
where F: FnOnce(&mut Yielder<Input, Output>, Input) + Send {
unsafe extern "C" fn generator_wrapper<Input, Output, Stack, F>(env: usize, stack_ptr: StackPointer) -> !
where Input: Send, Output: Send, Stack: stack::Stack,
F: FnOnce(&mut Yielder<Input, Output>, Input) {
// Retrieve our environment from the callee and return control to it.
let f = ptr::read(env as *const F);
let (data, stack_ptr) = arch::swap(0, stack_ptr, None);
// See the second half of Yielder::suspend_bare.
let input = ptr::read(data as *const Input);
// Run the body of the generator.
let mut yielder = Yielder::new(stack_ptr);
f(&mut yielder, input);
// Past this point, the generator has dropped everything it has held.
loop { yielder.suspend_bare(None); }
}
let stack_id = debug::StackId::register(&stack);
let stack_ptr = arch::init(&stack, generator_wrapper::<Input, Output, Stack, F>);
// Transfer environment to the callee.
let stack_ptr = arch::swap(&f as *const F as usize, stack_ptr, Some(&stack)).1;
mem::forget(f);
Generator {
state: State::Runnable,
stack: stack,
stack_id: stack_id,
stack_ptr: stack_ptr,
phantom: PhantomData
}
}
/// 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 (data_out, stack_ptr) = arch::swap(&input as *const Input as usize, self.stack_ptr, Some(&self.stack));
self.stack_ptr = stack_ptr;
mem::forget(input);
ptr::read(data_out as *const Option<Output>)
};
// 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 => self.stack
}
}
}
/// Yielder is an interface provided to every generator through which it
/// returns a value.
#[derive(Debug)]
pub struct Yielder<Input: Send, Output: Send> {
stack_ptr: Cell<StackPointer>,
phantom: PhantomData<(*const Input, *const Output)>
}
impl<Input, Output> Yielder<Input, Output>
where Input: Send, Output: Send {
fn new(stack_ptr: StackPointer) -> Yielder<Input, Output> {
Yielder {
stack_ptr: Cell::new(stack_ptr),
phantom: PhantomData
}
}
#[inline(always)]
fn suspend_bare(&self, val: Option<Output>) -> Input {
unsafe {
let (data, stack_ptr) = arch::swap(&val as *const Option<Output> as usize, self.stack_ptr.get(), None);
self.stack_ptr.set(stack_ptr);
mem::forget(val);
ptr::read(data as *const Input)
}
}
/// Suspends the generator and returns `Some(item)` from the `resume()`
/// invocation that resumed the generator.
#[inline(always)]
pub fn suspend(&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(()) }
}