mode 0.4.1

A behavioral state machine library written in Rust
Documentation 
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// Copyright 2019 Andrew Thomas Christensen
//
// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or http://www.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.

use mode::{Automaton, Family, Mode};

const HEAD : u16 = 8;
const MASK : u16 = 1 << HEAD;

struct StateFamily;

impl Family for StateFamily {
    type Base = State;
    type Mode = State;
}

#[derive(Copy, Clone, Debug)]
enum PrintOp { Clear, Print }

#[derive(Copy, Clone, Debug)]
enum ShiftOp { Left,  Right }

#[derive(Copy, Clone, Debug, Eq, PartialEq)]
enum State { A, B, C, D, E, H }

impl Mode for State {
    type Family = StateFamily;
}

fn step(state : State, tape : &mut u16) -> (State, bool) {
    use State::*;
    use PrintOp::*;
    use ShiftOp::*;

    let bit = (*tape & MASK) >> HEAD;

    let (next, op) =
        match (state, bit) {
            (A, 0) => (H, None),
            (A, 1) => (B, Some((Clear, Right))),
            (B, 0) => (C, Some((Clear, Right))),
            (B, 1) => (B, Some((Print, Right))),
            (C, 0) => (D, Some((Print,  Left))),
            (C, 1) => (C, Some((Print, Right))),
            (D, 0) => (E, Some((Clear,  Left))),
            (D, 1) => (D, Some((Print,  Left))),
            (E, 0) => (A, Some((Print, Right))),
            (E, 1) => (E, Some((Print,  Left))),
            (H, _) => (H, None),
            (_, _) => unreachable!(),
        };

    print!("{:016b} {:?} => {:?}, ", *tape, state, next);

    if let Some(op) = op {
        println!("{:?}, {:?}", op.0, op.1);
    }
    else {
        println!("Halt");
    }

    if let Some((print_op, shift_op)) = op {
        match print_op {
            Print => { *tape = *tape |  (1 << HEAD) },
            Clear => { *tape = *tape & !(1 << HEAD) },
        }

        match shift_op {
            Left  => { *tape = *tape << 1 },
            Right => { *tape = *tape >> 1 },
        }
    }

    // The first tuple element will be interpreted as the next Mode to swap in. The second will become the return value
    // of the Automaton::next_with_result() function.
    (next, next != State::H)
}

fn main() {
    let mut tape : u16 = 0b111 << HEAD;
    let mut automaton = StateFamily::automaton_with_mode(State::A);

    // NOTE: We can do this because step() returns false in the "result" parameter if the machine has halted.
    while Automaton::next_with_result(&mut automaton, |current_state| step(current_state, &mut tape)) { }
}