AOC/5 (Rust)b

main.rs

#![feature(conservative_impl_trait)]

const INPUT: &str = include_str!("../input.txt");

fn main() {
    println!("{}", run_program(read_input(), |i| i + 1));
    println!("{}", run_program(read_input(), offset));
}

fn offset(i: i32) -> i32 {
    if i >= 3 { i - 1 } else { i + 1 }
}

fn run_program<I, F>(instructions: I, offset: F) -> usize
where
    I: IntoIterator<Item = i32>,
    F: Fn(i32) -> i32,
{
    let mut instructions: Vec<_> = instructions.into_iter().collect();
    let mut pointer = 0;
    let mut counter = 0;

    while pointer >= 0 && pointer < (instructions.len() as i32) {
        let next_pointer = pointer + instructions[pointer as usize];
        instructions[pointer as usize] = offset(instructions[pointer as usize]);
        pointer = next_pointer;
        counter += 1;
    }

    counter
}

// Impl trait feature employed here. Ordinarily, the return of any item whose type
// signature includes a closure (a unique, unnameable type) would require boxing.
fn read_input() -> impl Iterator<Item = i32> {
    INPUT.lines().filter_map(|n| n.parse().ok())
}