1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161

//! Brainfuck interpreter written in Rust, using Logos.
//!
//! Usage:
//!     cargo run --example brainfuck <path/to/file>
//!
//! Example:
//!     cargo run --example brainfuck examples/hello_word.bf
//!
//! Brainfuck is an esoteric programming language that only
//! uses 8 single-character commands:
//! - '>';
//! - '<';
//! - '+';
//! - '-';
//! - '.';
//! - ',';
//! - '[';
//! - and ']'.
//!
//! Despite being very hard to use in practice, this makes
//! this language very simple to interpret. The following code
//! defines an [`execute`] function that runs Brainfuck code.
//!
//! Logos is used here to directly transform the code stream
//! into meaningful `Op` operations (or commands).
//! Errors, i.e., unknown tokens, are discarded using `filter_map`.
//!
//! More details can be found on Wikipedia:
//! <https://en.wikipedia.org/wiki/Brainfuck>.
//!
//! or on <http://brainfuck.org/>.

/* ANCHOR: all */
use logos::Logos;
use std::collections::HashMap;
use std::env;
use std::fs;
use std::io::{self, Read};

/* ANCHOR: tokens */
/// Each [`Op`] variant is a single character.
#[derive(Debug, Logos)]
enum Op {
    /// Increment pointer.
    #[token(">")]
    IncPointer,
    /// Decrement pointer.
    #[token("<")]
    DecPointer,
    /// Increment data at pointer.
    #[token("+")]
    IncData,
    /// Decrement data at pointer.
    #[token("-")]
    DecData,
    /// Output data at pointer.
    #[token(".")]
    OutData,
    /// Input (read) to data at pointer.
    #[token(",")]
    InpData,
    /// Conditionally jump to matching `']'`.
    #[token("[")]
    CondJumpForward,
    /// Conditionally jump to matching `'['`.
    #[token("]")]
    CondJumpBackward,
}
/* ANCHOR_END: tokens */

/// Print one byte to the terminal.
#[inline(always)]
fn print_byte(byte: u8) {
    print!("{}", byte as char);
}

/// Read one byte from the terminal.
#[inline(always)]
fn read_byte() -> u8 {
    let mut input = [0u8; 1];
    io::stdin()
        .read_exact(&mut input)
        .expect("An error occurred while reading byte!");
    input[0]
}

/// Execute Brainfuck code from a string slice.
pub fn execute(code: &str) {
    let operations: Vec<_> = Op::lexer(code).filter_map(|op| op.ok()).collect();
    let mut data = [0u8; 30_000]; // Minimum recommended size
    let mut pointer: usize = 0;
    let len = operations.len();

    // We pre-process matching jump commands, and we create
    // a mapping between them.
    let mut queue = Vec::new();
    let mut pairs = HashMap::new();
    let mut pairs_reverse = HashMap::new();

    for (i, op) in operations.iter().enumerate() {
        match op {
            Op::CondJumpForward => queue.push(i),
            Op::CondJumpBackward => {
                if let Some(start) = queue.pop() {
                    pairs.insert(start, i);
                    pairs_reverse.insert(i, start);
                } else {
                    panic!(
                        "Unexpected conditional backward jump at position {}, does not match any '['",
                        i
                    );
                }
            }
            _ => (),
        }
    }

    if !queue.is_empty() {
        panic!("Unmatched conditional forward jump at positions {:?}, expecting a closing ']' for each of them", queue);
    }

    /* ANCHOR: fsm */
    let mut i: usize = 0;
    // True program execution.
    loop {
        match operations[i] {
            Op::IncPointer => pointer += 1,
            Op::DecPointer => pointer -= 1,
            Op::IncData => data[pointer] = data[pointer].wrapping_add(1),
            Op::DecData => data[pointer] = data[pointer].wrapping_sub(1),
            Op::OutData => print_byte(data[pointer]),
            Op::InpData => data[pointer] = read_byte(),
            Op::CondJumpForward => {
                if data[pointer] == 0 {
                    // Skip until matching end.
                    i = *pairs.get(&i).unwrap();
                }
            }
            Op::CondJumpBackward => {
                if data[pointer] != 0 {
                    // Go back to matching start.
                    i = *pairs_reverse.get(&i).unwrap();
                }
            }
        }
        i += 1;

        if i >= len {
            break;
        }
    }
    /* ANCHOR_END: fsm */
}

fn main() {
    let src = fs::read_to_string(env::args().nth(1).expect("Expected file argument"))
        .expect("Failed to read file");

    execute(src.as_str());
}
/* ANCHOR_END: all */