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//! Executor implementation for Brainfuck.
use std::io::Read;
use miette::Diagnostic;
use thiserror::Error;
use crate::{constants::TapeInner, parser::Instruction};
/// Runtime errors that can occur in brainfuck executor.
#[derive(Debug, Diagnostic, Error)]
pub enum Error {
/// Brainfuck code performed an out of bounds index on the tape during
/// runtime.
#[error("tape indexed out of bounds, attempted index at `{0}`")]
IndexOutOfBounds(usize),
/// Executor was unable to read an input byte from stdin.
#[error("could not read input from stdin")]
ReadInput(#[from] std::io::Error),
}
/// Executes the provided instruction set, utilising the provided tape..
///
/// # Errors
///
/// This function will return an error if the Brainfuck code indexes out of
/// bounds of the tape, or if the executor cannot read an input byte from stdin.
pub fn execute(
instructions: &[Instruction],
tape: &mut [TapeInner],
data_pointer: &mut usize,
) -> Result<(), Error> {
for instruction in instructions {
match *instruction {
Instruction::IncrementPointer => {
let tape_len: usize = tape.len() - 1;
if *data_pointer == tape_len {
*data_pointer = 0;
} else {
*data_pointer += 1;
}
}
Instruction::DecrementPointer => {
*data_pointer = match *data_pointer {
0 => tape.len() - 1,
_ => *data_pointer - 1,
};
}
Instruction::IncrementByte => match tape.get_mut(*data_pointer) {
Some(value) => *value = value.overflowing_add(1).0,
None => return Err(Error::IndexOutOfBounds(*data_pointer)),
},
Instruction::DecrementByte => match tape.get_mut(*data_pointer) {
Some(value) => *value = value.overflowing_sub(1).0,
None => return Err(Error::IndexOutOfBounds(*data_pointer)),
},
Instruction::OutputByte => print!(
"{}",
char::from(match tape.get(*data_pointer) {
Some(value) => *value,
None => return Err(Error::IndexOutOfBounds(*data_pointer)),
})
),
Instruction::InputByte => {
let mut input: [TapeInner; 1] = [0; 1];
std::io::stdin().read_exact(&mut input)?;
match tape.get_mut(*data_pointer) {
Some(value) => *value = input[0],
None => return Err(Error::IndexOutOfBounds(*data_pointer)),
};
}
Instruction::Loop(ref instructions) => {
while match tape.get(*data_pointer) {
Some(value) => *value,
None => return Err(Error::IndexOutOfBounds(*data_pointer)),
} != 0
{
execute(instructions, tape, data_pointer)?;
}
}
}
}
Ok(())
}
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