rust-fsm 0.8.0

/*!
[![Documentation][docs-badge]][docs-link]
[![Latest Version][crate-badge]][crate-link]

The `rust-fsm` crate provides a simple and universal framework for building
state machines in Rust with minimum effort.

The essential part of this crate is the
[`StateMachineImpl`](trait.StateMachineImpl.html) trait. This trait allows a
developer to provide a strict state machine definition, e.g. specify its:

* An input alphabet - a set of entities that the state machine takes as
  inputs and performs state transitions based on them.
* Possible states - a set of states this machine could be in.
* An output alphabet - a set of entities that the state machine may output
  as results of its work.
* A transition function - a function that changes the state of the state
  machine based on its current state and the provided input.
* An output function - a function that outputs something from the output
  alphabet based on the current state and the provided inputs.
* The initial state of the machine.

Note that on the implementation level such abstraction allows build any type
of state machines:

* A classical state machine by providing only an input alphabet, a set of
  states and a transition function.
* A Mealy machine by providing all entities listed above.
* A Moore machine by providing an output function that do not depend on the
  provided inputs.

## Feature flags

### Default

- `std` - implement features that require the `std` environment. See below.
- `dsl` - re-export `rust-fsm-dsl` from `rust-fsm`. Recommended to leave this on
  for the best development experience.

### Non-default

- `diagram` - generate Mermaid state diagrams in the doc strings. See below.

## Usage in `no_std` environments

This library has the feature named `std` which is enabled by default. You
may want to import this library as
`rust-fsm = { version = "0.8", default-features = false, features = ["dsl"] }`
to use it in a `no_std` environment. This only affects error types (the `Error`
trait is only available in `std`).

The DSL implementation re-export is gated by the feature named `dsl` which is
also enabled by default.

## Use

Initially this library was designed to build an easy to use DSL for defining
state machines on top of it. Using the DSL will require to connect an
additional crate `rust-fsm-dsl` (this is due to limitation of the procedural
macros system).

### Using the DSL for defining state machines

The DSL is parsed by the `state_machine` macro. Here is a little example.

```rust
use rust_fsm::*;

state_machine! {
    #[derive(Debug)]
    #[repr(C)]
    /// A Circuit Breaker state machine.
    circuit_breaker(Closed)

    Closed(Unsuccessful) => Open [SetupTimer],
    Open(TimerTriggered) => HalfOpen,
    HalfOpen => {
        Successful => Closed,
        Unsuccessful => Open [SetupTimer]
    }
}
```

This code sample:

* Defines a state machine called `circuit_breaker`;
* Derives the `Debug` trait for it. All attributes you use here (like
  `#[repr(C)]`) will be applied to all types generated by this macro. If you
  want to apply attributes or a docstring to the `mod` generated by this macro,
  just put it before the macro invocation.
* Sets the initial state of this state machine to `Closed`;
* Defines state transitions. For example: on receiving the `Successful`
  input when in the `HalfOpen` state, the machine must move to the `Closed`
  state;
* Defines outputs. For example: on receiving `Unsuccessful` in the
  `Closed` state, the machine must output `SetupTimer`.

This state machine can be used as follows:

```rust,ignore
// Initialize the state machine. The state is `Closed` now.
let mut machine = circuit_breaker::StateMachine::new();
// Consume the `Successful` input. No state transition is performed.
let _ = machine.consume(&circuit_breaker::Input::Successful);
// Consume the `Unsuccesful` input. The machine is moved to the `Open`
// state. The output is `SetupTimer`.
let output = machine.consume(&circuit_breaker::Input::Unsuccessful).unwrap();
// Check the output
if let Some(circuit_breaker::Output::SetupTimer) = output {
    // Set up the timer...
}
// Check the state
if let circuit_breaker::State::Open = machine.state() {
    // Do something...
}
```

The following entities are generated:

* An empty structure `circuit_breaker::Impl` that implements the
  `StateMachineImpl` trait.
* Enums `circuit_breaker::State`, `circuit_breaker::Input` and
  `circuit_breaker::Output` that represent the state, the input alphabet and the
  output alphabet respectively.
* Type alias `circuit_breaker::StateMachine` that expands to
  `StateMachine<circuit_breaker::Impl>`.

Note that if there is no outputs in the specification, the output alphabet is an
empty enum and due to technical limitations of many Rust attributes, no
attributes (e.g. `derive`, `repr`) are applied to it.

Within the `state_machine` macro you must define at least one state
transition.

#### Visibility

You can specify visibility like this:

```rust
use rust_fsm::*;

state_machine! {
    pub CircuitBreaker(Closed)

    Closed(Unsuccessful) => Open [SetupTimer],
    Open(TimerTriggered) => HalfOpen,
    HalfOpen => {
        Successful => Closed,
        Unsuccessful => Open [SetupTimer],
    }
}
```

The default visibility is private.

#### Custom alphabet types

You can supply your own types to use as input, output or state. All of them are
optional: you can use only one of them or all of them at once if you want to.
The current limitation is that you have to supply a fully qualified type path.

```rust,ignore
use rust_fsm::*;

pub enum Input {
    Successful,
    Unsuccessful,
    TimerTriggered,
}

pub enum State {
    Closed,
    HalfOpen,
    Open,
}

pub enum Output {
    SetupTimer,
}

state_machine! {
    #[state_machine(input(crate::Input), state(crate::State), output(crate::Output))]
    circuit_breaker(Closed)

    Closed(Unsuccessful) => Open [SetupTimer],
    Open(TimerTriggered) => HalfOpen,
    HalfOpen => {
        Successful => Closed,
        Unsuccessful => Open [SetupTimer]
    }
}
```

#### Diagrams

`state_machine` macro can document your state machines with diagrams. This is
controlled by the `diagram` feature, which is non-default. The diagrams are
generated in the [Mermaid][mermaid] format. This feature includes the Mermaid
script into the documentation page.

To see this in action, download the repository and run:

```bash
cargo doc -p doc-example --open
```

![image](doc-diagram-example.png)

### Without DSL

The `state_machine` macro has limited capabilities (for example, a state
cannot carry any additional data), so in certain complex cases a user might
want to write a more complex state machine by hand.

All you need to do to build a state machine is to implement the
`StateMachineImpl` trait and use it in conjuctions with some of the provided
wrappers (for now there is only `StateMachine`).

You can see an example of the Circuit Breaker state machine in the
[project repository][repo].

[repo]: https://github.com/eugene-babichenko/rust-fsm
[docs-badge]: https://docs.rs/rust-fsm/badge.svg
[docs-link]: https://docs.rs/rust-fsm
[crate-badge]: https://img.shields.io/crates/v/rust-fsm.svg
[crate-link]: https://crates.io/crates/rust-fsm
[mermaid]: https://mermaid.js.org/
*/

#![cfg_attr(not(feature = "std"), no_std)]

use core::fmt;
#[cfg(feature = "std")]
use std::error::Error;

#[cfg(feature = "dsl")]
pub use rust_fsm_dsl::state_machine;

#[cfg(feature = "diagram")]
pub use aquamarine::aquamarine;

/// This trait is designed to describe any possible deterministic finite state
/// machine/transducer. This is just a formal definition that may be
/// inconvenient to be used in practical programming, but it is used throughout
/// this library for more practical things.
pub trait StateMachineImpl {
    /// The input alphabet.
    type Input;
    /// The set of possible states.
    type State;
    /// The output alphabet.
    type Output;
    /// The initial state of the machine.
    // allow since there is usually no interior mutability because states are enums
    #[allow(clippy::declare_interior_mutable_const)]
    const INITIAL_STATE: Self::State;
    /// The transition fuction that outputs a new state based on the current
    /// state and the provided input. Outputs `None` when there is no transition
    /// for a given combination of the input and the state.
    fn transition(state: &Self::State, input: &Self::Input) -> Option<Self::State>;
    /// The output function that outputs some value from the output alphabet
    /// based on the current state and the given input. Outputs `None` when
    /// there is no output for a given combination of the input and the state.
    fn output(state: &Self::State, input: &Self::Input) -> Option<Self::Output>;
}

/// A convenience wrapper around the `StateMachine` trait that encapsulates the
/// state and transition and output function calls.
#[derive(Debug, Clone)]
pub struct StateMachine<T: StateMachineImpl> {
    state: T::State,
}

#[derive(Debug, Clone)]
/// An error type that represents that the state transition is impossible given
/// the current combination of state and input.
pub struct TransitionImpossibleError;

impl<T> StateMachine<T>
where
    T: StateMachineImpl,
{
    /// Create a new instance of this wrapper which encapsulates the initial
    /// state.
    pub fn new() -> Self {
        Self::from_state(T::INITIAL_STATE)
    }

    /// Create a new instance of this wrapper which encapsulates the given
    /// state.
    pub fn from_state(state: T::State) -> Self {
        Self { state }
    }

    /// Consumes the provided input, gives an output and performs a state
    /// transition. If a state transition with the current state and the
    /// provided input is not allowed, returns an error.
    pub fn consume(
        &mut self,
        input: &T::Input,
    ) -> Result<Option<T::Output>, TransitionImpossibleError> {
        if let Some(state) = T::transition(&self.state, input) {
            let output = T::output(&self.state, input);
            self.state = state;
            Ok(output)
        } else {
            Err(TransitionImpossibleError)
        }
    }

    /// Returns the current state.
    pub fn state(&self) -> &T::State {
        &self.state
    }
}

impl<T> Default for StateMachine<T>
where
    T: StateMachineImpl,
{
    fn default() -> Self {
        Self::new()
    }
}

impl fmt::Display for TransitionImpossibleError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(
            f,
            "cannot perform a state transition from the current state with the provided input"
        )
    }
}

#[cfg(feature = "std")]
impl Error for TransitionImpossibleError {
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        None
    }
}